Animal Diversity Web

Diversity

Actinopterygians, or ‘ray-finned fishes,’ are the largest and most successful group of fishes and make up half of all living vertebrates. While actinopterygians appeared in the fossil record during the Devonian period, between 400-350 million years ago (Ma), it was not until the Carboniferous period (360 Ma) that they had become dominant in freshwaters and started to invade the seas. At present, approximately 42 orders, 431 families, and nearly 24,000 species are recognized within this class but there are bound to be taxonomic revisions as research progresses. Teleosts comprise approximately 23,000 of the 24,000 species within the actinopterygians, and 96 percent of all living fish species (see Systematic/Taxonomic History). The latter estimates, however, will probably never be accurate because actinopterygian species are becoming extinct faster than they can be discovered in some areas, such as the Amazon and Congo Basins. Unfortunately, habitat destruction, pollution and international trade, among other human impacts, have contributed to the endangerment of many actinopterygians (see Conservation Status). (Grande, 1998; Helfman, et al., 1997; Moyle and Cech, 2004)

Clearly, given the enormous diversity of this class, entire books could be (and are) written for each of the categories below, so this account does not attempt an exhaustive summary of the diversity of habitats, body forms, behaviors, reproductive habits, etc. of actinopterygians. Instead, each section introduces important ichthyological concepts and terminology, as well as numerous examples from a diverse range of ray-finned fish families. A section of particular interest is Systematic/Taxonomic History because salient features of the evolutionary history of actinopterygians are discussed. The phylogenetic trends within early actinopterygians provide a basis for understanding why this group has been so successful, as more derived forms (i.e. neopterygians and teleosts), which make up nearly all existing ray-finned fishes, have repeated and extended early trends. Many of the sections, such as Physical Description, Reproduction, Behavior and Ecosystem Roles merely scratch the surface, but there are numerous links to family-level ray-finned fish accounts. (‘Fishes’ is used interchangeably with ‘ray-finned fishes’ and 'actinopterygians' from this point forward). (Helfman, et al., 1997; Liem, 1998; Moyle and Cech, 2004; Nelson, 1994; Wheeler, 1985)

Geographic Range

Ray-finned fishes inhabit a variety of extreme environments. These include high altitude lakes and streams, desert springs (e.g. pupfishes), subterranean caves (e.g. cavefishes), ephemeral pools, polar seas, and the depths of the ocean (e.g. deepsea anglerfishes). Across these habitats water temperatures may range from -1.8˚C to nearly 40˚C, pH levels from 4 to 10+, dissolved oxygen levels from zero to saturation, salinities from 0 to 90 parts per million and depths ranging from 0 to 7,000 m (Davenport and Sayer 1993 in Moyle and Cech 2004:1)! Some fish even spend considerable time outside of water: mudskippers prey on the invertebrates of mudflat habitats, while airbreathing catfishes and gouramies live in stagnant, low oxygen ponds (among other habitats) or migrate over land to colonize new areas. Another extreme example of habitat adaptation is found in hillstream loaches , which live in the steep, torrential watercourses of Asiatic hillstreams. Hillstream loaches have flattened bodies and utilize suckers, permanently clinging to rock faces so they are not swept downstream. Lanternfishes , hatchetfishes , dragonfishes , deep-sea codfishes , halosaurs and spiny eels all have lights (flashing or constant), created by luminescent bacteria or special glandular cells, to find prey, communicate with other individuals, or for defense in the blackness of their deepsea habitats (see Communication, Food Habits, and Predation). (Moyle and Cech, 2004; Paxton, 1998; Wheeler, 1985)

Disparate localities may have similar geographic conditions, yet fish species composition varies widely across similar regions. In other words, patterns of fish distribution are not simply related to how well a fish is adapted to a particular type of environment, which is why invasive species can be so devastating (see Conservation). The study of zoogeography attempts to answer questions about how and why fish (and other animal) faunas differ across geographic regions. Zoogeography integrates a variety of disciplines within ichthyology (ecology, physiology, systematics , paleontology, geology and biogeography) to explain patterns of fish distribution. While ichthyologists certainly have incomplete knowledge in many of these areas, advances in plate tectonics and phylogenetic systematics have allowed them to define various zoogeographic (or biogeographic) regions (also subregions) and types. (Helfman, et al., 1997; Moyle and Cech, 2004; Paxton, 1998; Wheeler, 1985)

Fresh water covers only a tiny fraction of the earth’s surface (.0093 percent), yet it is home to approximately 41 percent of all fish species. Most of these are concentrated in the tropics (1,500 different species in the Amazon Basin alone), and Southeast Asia probably has the most diverse assemblage of freshwater species. In marine areas, species concentrations are highest around coral reefs, where butterflyfishes and angelfishes , wrasses , parrotfishes and triggerfishes are common. In the arctic seas five notothenoid families dominate: thornfishes , plunderfishes, Antarctic dragonfishes , and notothens. (Helfman, et al., 1997; Paxton, 1998; Wheeler, 1985)

• Biogeographic Regions
• nearctic
  • native
• palearctic
  • native
• oriental
  • introduced
  • native
• ethiopian
  • introduced
  • native
• neotropical
  • introduced
  • native
• australian
  • introduced
  • native
• antarctica
  • native
• oceanic islands
  • introduced
  • native
• arctic ocean
  • native
• indian ocean
  • native
• atlantic ocean
  • native
• pacific ocean
  • native
• mediterranean sea
  • introduced
  • native

• Other Geographic Terms
• holarctic
• cosmopolitan
• island endemic

Habitat

Ray-finned fishes inhabit a variety of extreme environments. These include high altitude lakes and streams, desert springs (e.g. pupfishes), subterranean caves (e.g. cavefishes), ephemeral pools, polar seas, and the depths of the ocean (e.g. deepsea anglerfishes). Across these habitats water temperatures may range from -1.8˚C to nearly 40˚C, pH levels from 4 to 10+, dissolved oxygen levels from zero to saturation, salinities from 0 to 90 parts per million and depths ranging from 0 to 7,000 m (Davenport and Sayer 1993 in Moyle and Cech 2004:1)! Some fish even spend considerable time outside of water: mudskippers prey on the invertebrates of mudflat habitats, while airbreathing catfishes and gouramies live in stagnant, low oxygen ponds (among other habitats) or migrate over land to colonize new areas. Another extreme example of habitat adaptation is found in hillstream loaches , which live in the steep, torrential watercourses of Asiatic hillstreams. Hillstream loaches have flattened bodies and utilize suckers, permanently clinging to rock faces so they are not swept downstream. Lanternfishes , hatchetfishes , dragonfishes , deep-sea codfishes , halosaurs and spiny eels all have lights (flashing or constant), created by luminescent bacteria or special glandular cells, to find prey, communicate with other individuals, or for defense in the blackness of their deepsea habitats (see Communication, Food Habits, and Predation). (Moyle and Cech, 2004; Paxton, 1998; Wheeler, 1985)

Researchers have long divided freshwater and saltwater habitats. However, habitat boundaries are often crossed by migratory species, some of which are diadromous – meaning they migrate between fresh water and the sea. Depending on the type of migration, they can be anadromous (migrate up rivers to spawn), with a pattern of freshwater-ocean-freshwater (typical of salmon and lampreys), or catadromous (migrate from freshwater to the sea to spawn), which is characteristic of freshwater eels . In the latter family juveniles, carried to river mouths by ocean currents, migrate upstream and live for up to 10 years before returning to spawning grounds in the ocean and dying shortly after (see Behavior as well). (Moyle and Cech, 2004; Paxton, 1998; Wheeler, 1985)

Fresh water covers only a tiny fraction of the earth’s surface (.0093 percent), yet it is home to approximately 41 percent of all fish species. Most of these are concentrated in the tropics (1,500 different species in the Amazon Basin alone), and Southeast Asia probably has the most diverse assemblage of freshwater species. In marine areas, species concentrations are highest around coral reefs, where butterflyfishes and angelfishes , wrasses , parrotfishes and triggerfishes are common. In the arctic seas five notothenoid families dominate: thornfishes , plunderfishes, Antarctic dragonfishes , and notothens. (Helfman, et al., 1997; Paxton, 1998; Wheeler, 1985)

• Habitat Regions
• temperate
• tropical
• polar
• saltwater or marine
• freshwater

• Terrestrial Biomes
• forest
• rainforest

• Aquatic Biomes
• pelagic
• benthic
• reef
• lakes and ponds
• rivers and streams
• temporary pools
• coastal
• abyssal
• brackish water

• Wetlands
• marsh
• swamp
• bog

• Other Habitat Features
• urban
• suburban
• agricultural
• riparian
• estuarine
• intertidal or littoral

Systematic and Taxonomic History

Even though there is not a strong set of derived characters (characteristics not present in ancestral species) for actinopterygians, they are believed to be monophyletic. The known ancestral features include lepidotrichia (“like scales, [they] are of dermal origin and probably are derived from scales. They form the soft rays of the fins, which are segmented and dumbbell-shaped in cross-section.” Moyle and Cech, 2004:231), heavy ganoid scales , structurally distinctive pelvic and pectoral girdles, fins attached to the body by the fin rays (rather than with a fleshy lobe), branchiostegal rays (a series of long, curved bones supporting the gill membrane), and no internal nares (or choana – separate internal openings to the lungs). (Nelson, 1994)

Two subclasses are recognized, the neopterygians (new-finned) and chondrosteans , the latter being non-monophyletic. Within the Chondrostei, only sturgeons , bichirs and paddlefishes survive today, and many are threatened (see Conservation). The rest of the actinopterygians, which includes the vast majority of species, are in the subclass Neopterygii: “In their great numbers and degree of anatomical diversity, the modern ray-finned fishes may be considered the most successful of all vertebrates” (Caroll, 1986:136 in Helfman et. al. 1997:162). Further, modern teleosts represent the culmination of continuous ‘improvements’ on the basic fish design, within the Neopterygii. Investigators have traced these changes in the basic fish design with the aid of the fossil record, and have identified important trends in actinopterygian evolution. Refinements in the structure of scales, branchiostegal rays, swimbladder, jaws, tail and fins have all contributed to the diverse radiation (increase in number of species) of actinopterygian fishes. The trend among actinopterygians has been toward lighter, more flexible bones and scales, an internal muscular-tendonous system, neutral buoyancy via the swimbladder, greater maneuverability, mobility and speed via changes in the tail and fins, and improvements in mouth structure (as described in detail below). (Helfman, et al., 1997; Moyle and Cech, 2004)

Scales: Heavy and complex scales composed of three layers (ganoid) served as a relatively inflexible suit of armor for ancestral species but the general trend in actinopterygians has been to reduce weight and complexity and increase flexibility. The result is the elasmoid scale (ctenoid and cycloid), found in teleosts , which is thin, light, flexible and composed of only two layers (an external fibrous layer and an internal bony layer). In fact, many teleosts and all the surviving chondrosteans (sturgeons and paddlefishes) have taken the final step and shed their scales entirely. (Helfman, et al., 1997; Moyle and Cech, 2004)

Branchiostegal rays: Branchiostegal rays developed from the bones at the base of the branchial cavity, and increased the efficiency with which water was pumped across the gills (“two pump” respiratory system) . The interopercular bones, which were formed by the modification of branchiostegal rays at the bottom of each gill cover (operculum), further improved the pumping efficiency by enlarging the opercular cavity, thus increasing the volume of water that could flow through the gills. Branchiostegal rays have also allowed actinopterygians to utilize suction feeding methods, rather than simply grabbing – a function equally significant as improved respiration. (Helfman, et al., 1997; Liem, 1998; Moyle and Cech, 2004)

Swimbladder: The swimbladder probably functioned as a lung in the earliest actinopterygians, but in more derived ray-finned fishes it is used primarily as a hydrostatic organ (to maintain buoyancy). With neutral buoyancy achieved by controlling the amount of gas in the swimbladder, the pectoral fins no longer needed to function as hydroplanes, so they evolved to aid in greater maneuverability. Finally, in some actinopterygians, such as gouramies (and others), the swimbladder is utilized for non-respiratory functions such as hearing and sound production. (Helfman, et al., 1997; Liem, 1998; Moyle and Cech, 2004)

Jaws: Jaws developed in conjunction with the branchiostegal rays and show a trend towards more flexibility. (See an illustration of jaw anatomy). Two major bones of the upper jaw, the maxilla, and the premaxilla, were previously firmly attached to the skull and had teeth. However, in recently derived actinopterygians, there are fewer attachments and teeth are rarely present. This has allowed for the upper jaw to extend, making it protrusible (dramatically illustrated by some wrasses), and permitted a variety of feeding specializations to develop, such as plankton straining (usually zooplankton). One result of increased flexibility of the upper jaw has been that processing could not easily occur at the rim of the mouth any longer. Therefore, many fish with protrusible jaws have a second set of jaws in the throat, termed pharyngeal jaws , that process food and free the outer jaw to continue feeding. (Helfman, et al., 1997; Moyle and Cech, 2004)

Tail: Changes in the lobes of the tail, from heterocercal (upper lobe longer than lower lobe) to homocercal (upper and lower lobes same size) in recently derived actinopterygians, are also related to the achievement of neutral buoyancy. Heterocercal tails (still found in sharks) provide lift, which is unnecessary for neutrally buoyant fish. Homocercal tails provide uniform thrust and allow for precise movements (each ray can be controlled individually), which are quite important for fast-swimming, pelagic fish and small, maneuverable fish, respectively. (See an illustration of locomotion in fish). (Helfman, et al., 1997; Liem, 1998; Moyle and Cech, 2004)

Fins: With the loss of heavy, armoring scales, actinopterygians developed spines, which are used as anti-predator devices when individuals are unable to use speed to escape (see Predation). The positioning of the pelvic and pectoral fins also changed along with the achievement of neutral buoyancy. Instead of having pelvic fins located well behind the pectoral fins, in more derived actinopterygians the pelvic fins are located just below or even slightly in front of the pectoral fins; here they aid in increased maneuverability instead of being used simply as stabilizers, as in earlier actinopterygians. (See an illustration of fin function). (Helfman, et al., 1997; Moyle and Cech, 2004)

• Synapomorphies

  • Branchiostegal rays and interopercular bone
  • Swimbladder reduced in size and specialized for uses other than breathing, and primarily as a hydrostatic organ
  • Distinctive jaw structure – maxillae and premaxillae often lack teeth and disconnected from skull (dePinna, 1996 in Moyle and Cech, Jr., 2004)
  • Homocercal tail
  • Distinctive structure of the pectoral girdle (dePinna, 1996 in Moyle and Cech, Jr., 2004)

Physical Description

The truly spectacular array of body forms within this class can only be appreciated by familiarizing oneself with some of the more than 25,000 species of actinopterygians – the largest and most diverse of all vertebrate classes – that exist today. Consider the fact that actinopterygians may fly, walk, or remain immobile (in addition to 'swimming'), exist in virtually all types of habitats except constantly dry land (though some can walk over land), feed on nearly every type of organic matter, utilize several types of sensory systems (including chemoreception, electroreception, magnetic reception and a “distance-touch” sensation – see Communication), and some even produce their own light or electricity. In addition, color diversity in ray-finned fishes is “essentially unlimited, ranging from uniformly dark black or red in many deepsea forms, to silvery in pelagic and water-column fishes, to countershaded in nearshore fishes of most littoral [near-shore] communities, to the strikingly contrasted colors of tropical freshwater and marine fishes” (Helfman et al. 1997:367). Of course, extravagant coloration is not helpful for fish at risk of being eaten, yet bright coloration is environment-specific (see Helfman et al. 1997:367) and bright colors at one depth are cryptic at others due to light attenuation (see Communication). Further, color change is common in brightly colored (as well as many other) fishes and occurs under a variety of circumstances. Pigments are responsible for a many types of color change, but there are also structural colors, resulting from light reflecting off of crystalline molecules housed in special chromatophores (cells located mainly in the outer layer of skin). The silvery sheen displayed by many pelagic fishes is an example of structural color. Numerous actinopterygians are also sexually dimorphic (males and females look different), and body form changes drastically during development, so there is significant diversity within, as well as among, species. (Berra, 2001; Bertelson and Pietsch, 1998; Helfman, et al., 1997; Moyle and Cech, 2004; Paxton, 1998)

Among the largest actinopterygians are the pirarucu (also known as giant arapaima , up to 2.5m in length) in freshwater and the black marlin (up to 900kg) in saltwater; the longest is the oarfish , Lampris guttatus, which averages between 5 and 8m in length; and the smallest, a variety of diminutive gobies in saltwater and minnows , catfishes and characins in freshwater. At various points in this account, there is further discussion of physical characteristics as they relate to particular topics (i.e. Systematic/Taxonomic History, Communication, Food Habits and Predation), but for a technical description of actinopterygians, see below. (View an illustration of external fish parts or a fish skeleton). (Helfman, et al., 1997; Moyle and Cech, 2004; Paxton, 1998)

Actinopterygians may have ganoid, cycloid, or ctenoid scales, or no scales at all in many groups. With the exception of Polypteriformes, the pectoral radials are attached to the scapulo-coracoid, a region of the pectoral girdle skeleton. (The pectoral radials are one of a series of endochondral - growing or developing within cartilage - bones in the pectoral and pelvic girdle on which the fin rays insert). Most have an interopercle and branchiostegal rays and the nostrils are positioned relatively high on the head. Finally, the spiracle (respiratory opening between the eye and the first gill slit – connects with the gill cavity) and gular plate (behind the chin and between the sides of the lower jaw) are usually absent, and internal nostrils are absent. (Froese and Pauly, 2004; Nelson, 1994)

• Other Physical Features
• heterothermic
• bilateral symmetry
• polymorphic
• poisonous
• venomous

• Sexual Dimorphism
• sexes alike
• female larger
• male larger
• sexes colored or patterned differently
• female more colorful
• male more colorful
• sexes shaped differently
• ornamentation

Development

In general, five major developmental periods are recognized in fish: embryonic, larval, juvenile, adult, and senescent. Fish development is known for its confounding terminology, so there are many gray areas within these major categories, and, as with many other animals, many species tend to defy classification into discrete categories. For instance, species in several teleostean families bear live young (viviparous) – Poeciliidae, Scorpaenidae, and Embiotocidae (to name a few), and the young in some families (Salmonidae) seem to emerge as juveniles after hatching (externally) from the egg. (Moyle and Cech, 2004)

There are two important developmental characteristics that separate fish from most vertebrates: indeterminate growth (growing throughout life) and a larval stage. The fact that most fish (although there are always exceptions) are always growing means they constantly change in terms of anatomy, ecological requirements, and reproduction (i.e. larger size means larger clutches, more mates, better defense, etc. in most species). Increased age is also associated with better survivability, As physiological tolerances and sensitivity improve, familiarity with the local environment accrues, and behavior continues to develop. The larval stage is usually associated with a period of dispersal from the parental habitat. Also, the disappearance of the yolk sac (the beginning of the larval stage according to most researchers) marks a critical period in which most individuals die from starvation or predation. (Helfman, et al., 1997)

Recently, researchers of coral reef fishes (mostly of the order Perciformes) have made significant advances concerning the life history of larvae. Nearly all bony coral reef fishes produce pelagic young (meaning they live in the water column for a period of time before settling on reefs), and the length of the stage is highly variable, from only a week in some damselfishes to greater than 64 weeks in some porcupine fishes . Initially, researchers made relatively simplistic assumptions about the pelagic phase, "portray[ing] larvae as little more than passive tracers of water movement that 'go with the flow,' doing nothing much until they bump into a reef by chance and settle at once" (Lies and McCormick 2002:171). Actually, the larvae of most coral reef fishes are endowed with good swimming abilities, good sensory systems, and sophisticated behavior that is quite flexible. And, while mortality rates are quite high at this stage (as with many other actinopterygian larvae), many larvae are able to detect predators at a considerable distance, and they are often transparent (usually larvae) or cryptically colored (many juveniles). (Deloach, 1999; Lies and McCormick, 2002)

It is important to note that the young of reef fishes develop quite differently from most temperate fishes that have been studied. While the eggs of most temperate fishes hatch from 3 to 20 days after laying, the eggs of most coral reef species hatch within only a day. Also, at any given size, the larvae of reef fishes are more developed than most temperate, non-perciform fish: they have "more complete fins, develop scales at smaller size, [have] seemingly better sensory apparatus at any size, and are morphologically equipped for effective feeding within a few days of hatching" (173). Finally, the settling habitat for reef fishes (coral reefs) tends to be relatively fragmented and, therefore, much more difficult to locate, unlike the habitat of temperate fishes, which tends to have large expanses suitable for settling. This brief glimpse into the pelagic stage of reef fishes reveals the diversity and complexity of development in actinopterygians. (Lies and McCormick, 2002)

• Development - Life Cycle
• neotenic/paedomorphic
• metamorphosis
• temperature sex determination
• indeterminate growth

Reproduction

Ray-finned fishes exhibit quite a variety of mating systems. The four major types, along with a few examples, are: monogamy - maintains the same partner for an extended period or spawns repeatedly with one partner (damselfishes , hawkfishes , blennies); polygyny - male has multiple partners over each breeding season (sculpins , sea basses , sunfishes , darters); polyandry - female has multiple partners over each breeding season (anemonefishes); and polygynandry or promiscuity - both males and females have multiple partners during the breeding season (herrings , sticklebacks , wrasses , surgeonfishes). Polygyny is much more common than polyandry, and usually involves territorial males organized into harems (males breed exclusively with a group of females), as in numerous cichlid species and several families of reef fishes (parrotfishes , wrasses and damselfishes , tilefishes , surgeonfishes and triggerfishes). (Helfman, et al., 1997; Moyle and Cech, 2004)

There are also "alternative mating systems," which include alternative male strategies, hermaphroditism, and unisexuality (Moyle and Cech 2004:161). Alternative male strategies usually occur in species with large males dominating spawning, such as salmon , parrotfishes and wrasses . In this situation, smaller males attempt to 'sneak' fertilize the eggs of females as peak spawning is occurring; the smaller males release gametes simultaneously in the vicinity of the spawning pair. Hermaphroditism in ray-finned fishes involves individuals containing ovarian and testicular tissue (synchronous or simultaneous), as in the black hamlet, as well as individuals that change from one sex to another (sequential). Sequential hermaphrodites most commonly change from being female to male (protogynous), as in parrotfishes , wrasses and groupers . A much smaller number of actinopterygians, such as anemonefishes and some moray eels , change from being male to female (protandrous). Finally, unisexuality (egg development occurring with or without fertilization) can also occur in a variety of forms, and usually involves some male involvement, although at least ones species (Texas silverside) appears to utilize true parthenogenesis – females produce only female offspring with no participation from males. In most cases, however, there is at least some male involvement, either simply to commence fertilization (gynogenesis) or to produce true female hybrids (hybridogenesis). (Helfman, et al., 1997; Moyle and Cech, 2004)

The mating systems above do not necessarily represent discrete categories and, as with development, the discussion ignores much of the complexity and variety within each system. For instance, one unisexual species, which is actually part of a "species complex" (Mexican mollies), the Amazon molly , uses the sperm from two other bisexual species within the complex (shortfin molly and sailfin molly) to activate development of the eggs; only genetic material from the female lineage is retained (Moyle and Cech, 2004:162; Helfman et. al. 1997:352). This means that the unisexual females are actually parasitizing bisexual males of these other species. Also, many species exhibit a combination of major and alternative mating systems. For instance, hermaphroditism is known among some polygynous wrasses and parrotfishes (among others). (Helfman, et al., 1997; Moyle and Cech, 2004)

• Mating System
• monogamous
• polyandrous
• polygynous
• polygynandrous (promiscuous)
• cooperative breeder

Most ray-finned fishes reproduce continually throughout their lifetime (iteroparity), although some (e.g. Pacific salmon and lampreys) spawn only once and die shortly thereafter (semelparity). Fertilization occurs externally in the great majority of species, however in some mouthbrooding species (incubation occurs inside mouth for the purpose of protection, mostly among cichlids), fertilization occurs inside the mouth. In a few families, such as clinids , surfperches , scorpionfishes , liverbearers , eggs are fertilized internally. (Helfman, et al., 1997; Moyle and Cech, 2004)

During courtship ray-finned fishes exhibit a wide range of complex behaviors, reflecting their evolutionary heritage and the particular environments they inhabit. For instance, pelagic spawners tend to have more elaborate courtship rituals than benthic spawners. Some of the behaviors include sound production, nest building, rapid swimming patterns, the formation of large schools, and many others. In addition, ray-finned fishes frequently change color at specific points in their reproductive cycle, either intensifying or darkening depending on the species, release pheromones, or grow tubercles (tiny bumps of keratin) on the fins, head or body. (Helfman, et al., 1997; Moyle and Cech, 2004)

One of the more peculiar mating behaviors among actinopterygians is found in deepsea anglerfishes (superfamily Ceratioidea). Many female deepsea anglerfishes are essentially "passively floating food traps"; quite a useful adaptation in the dark, barren waters of the deep sea (Bertelson and Pietsch 1998:140). However, this makes it quite difficult to locate a mate. Finding a female, therefore, is the sole purpose of many males, which are dramatically smaller than females (from 3 to 13 times shorter) and unable to feed as they lack teeth and jaws. With good swimming capabilities and olfactory organs, they are guided to females by pheromones (a unique chemical odor). After finding their mate, males attach themselves to females with hooked denticles, and in some species (Haplophryne mollis) the tissue between the two fuses; the males become permanently attached and receive nourishment from the female while the testes develop. (Bertelson and Pietsch, 1998)

• Key Reproductive Features
• semelparous
• iteroparous
• seasonal breeding
• year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• simultaneous hermaphrodite
• sequential hermaphrodite
  • protandrous
  • protogynous
• parthenogenic
• sexual
• asexual
• fertilization
  • external
  • internal
• viviparous
• ovoviviparous
• oviparous

While a surprising number of actinopterygian families exhibit parental care, it is not common, occurring only in approximately 22 percent. Unlike mammals , most parental care is the responsibility of males (11 percent), with 7 percent the sole responsibility of females and the rest carried out by both sexes. Not surprisingly, virtually no pelagic spawners, which release their gametes into the water column, exhibit parental care. However, among the fishes that do exhibit parental care, there is considerable diversity. (Helfman, et al., 1997; Moyle and Cech, 2004)

Some of the most extensive parental behaviors are found in cichlids. Many cichlids brood the eggs in the mouth and, although rare, the free-swimming young of some species also rush into the parent’s mouth for protection. Quite an elaborate form of parental care is found in spraying characin. At peak spawning, males and females of this species make simultaneous leaps out of the water, touching and briefly adhering to the underside of overlying vegetation (a leaf). Each time, a fertilized egg is stuck to the underside of the leaf, usually a dozen or so. Then, to keep the leaf moistened, the male, correcting for the refraction of the water surface, sprays the eggs at one- to two-minute intervals by splashing with his tail. After keeping this up for two to three days (!), the newly hatched young fall into the water. Several tidal species utilize similar methods to keep eggs from desiccating as the tide goes out. Two such methods include coiling the body around the eggs (pricklebacks and gunnels) and covering the eggs with algae (temperate sculpins and wrasses). (Helfman, et al., 1997)

• Parental Investment
• no parental involvement
• precocial
• male parental care
• female parental care

Lifespan/Longevity

Not surprisingly, the lifespan of ray-finned fishes varies widely. In general, smaller fish have shorter lives and vice versa. For instance, many smaller species live for only a year or less, such as North American minnows in the genus Pimephales, a few galaxiids from Tasmania and New Zealand, Sundaland noodlefishes , a silverside , a stickleback , and a few gobies . However, researchers of coral reef fishes are beginning to find that this correlation does not hold for some families. While many people, especially in the business of fisheries, assumed short lifespans for many fish, researchers are starting to find that many live much longer than previously expected. For example, common species, such as the European perch (aka river perch) and largemouth bass can live 25 and 15 to 24 years respectively. Even more impressive, some sturgeons (which are severely threatened) can live between 80 to 150 years. Several species of rockfish (deepwater rockfish , silvergray rockfish and rougheye rockfish) live from 90 to 140 years! These long lifespans have quickly become a serious issue for some fisheries because populations can be decimated if individuals that naturally accumulate in older age classes are removed (see Conservation). (Choat and Robertson, 2002; Helfman, et al., 1997)

Behavior

Many ray-finned fishes exhibit migratory behavior; daily migrations are usually related to feeding or predator avoidance while longer migrations are usually for reproduction purposes. Some fishes stay within saltwater (oceanodromous) or fresh water (potamodromous) their entire lives, while others migrate between the salt and fresh water as part of their life cycle (e.g. to reproduce) or to feed (diadromous). Diadromous species can be broken down into three types: those in which growth occurs primarily in saltwater but move into freshwater to spawn (termed anadromous) – e.g. salmon; those in which growth occurs primarily in freshwater but move into saltwater to spawn (termed catadromous) – e.g. anguillid eels; and those that migrate between salt and fresh water for purposes other than spawning, such as feeding (termed amphidromous) – e.g. various gobies , sleepers and galaxiids. While many ray-finned fishes migrate well outside their home range – in many cases hundreds of kilometers, against current and even up waterfalls – they have remarkable abilities to find their way back. For instance, salmon can remember the odor of the rivers they originated from, as well as the odor of other rivers they have passed during migration. In addition, salmon (among other actinopterygians) use currents, salinity and temperature gradients, and topographic cues (buoys or islands) for orientation. Tidepool sculpins separated from their home pool by 100 m can also find their way back using olfactory and visual cues. While younger fish rely on visual or olfactory cues, some older fish, even if removed from their original locale for multiple years, only require visual cues, utilizing a cognitive map to navigate. (Helfman, et al., 1997; Moyle and Cech, 2004; Paxton, 1998)

When ray-finned fishes group together, either for spawning migration, feeding or protection, they sometimes form shoals. While in some cases fishes simply form aggregates (no social interaction but a mutual attraction to resources), shoaling represents a continuum of fascinating social behaviors. Schooling, in which individuals form a synchronized, polarized group, is actually an extreme form of shoaling and represents one of many types of shoal formation. The formation changes shape depending on whether the group is resting, foraging, traveling, spawning or avoiding predators. Approximately 25 percent of fishes shoal throughout life (e.g. herrings , anchovies , minnows , silversides) and about half form shoals at some point during their lifetime. (Helfman, et al., 1997)

Another common characteristic of ray-finned fishes is aggressive behavior, which results from competition for valuable resources, such as feeding, refuge and mating territories, mates, eggs, and young. One form of aggressive behavior is dominance hierarchies, which are found in many groups (e.g. catfishes , minnows , cods , ricefishes , topminnows , cichlids , wrasses , blennies , and boxfishes). The hierarchy is determined through a variety of factors, including size, sex, age, previous residency, and previous experience. In most actinopterygian species males dominate females, subordinate individuals are relegated to suboptimal sites in terms of cover availability, current velocity and prey densities, and dominant individuals have favorable habitats, higher feeding rates and tend to remain dominant. Another aggressive behavior is territoriality, which is found in numerous ray-finned fishes and spread across a wide variety of groups, such as freshwater eels , cyprinids , knifefishes , salmonidsfrogfishes , rockfishes , sculpins , sunfishes and black basses , butterflyfishes , cichlids , damselfishes , barracuda , blennies , gobies , surgeonfishes and labyrinthfishes. Territorial interactions primarily occur along territorial boundaries and usually involve displays, vocalizations, chasing, and biting as a last resort. As with dominance hierarchies, prior experience, previous residency, and individual size are all important in determining the outcome of an altercation. (Behavioral characteristics relating directly to Reproduction, Food Habits, defense (Predation), or Ecosystem Roles can be found in their respective sections). (Helfman, et al., 1997; Moyle and Cech, 2004; Paxton, 1998)

• Key Behaviors
• fossorial
• natatorial
• diurnal
• nocturnal
• crepuscular
• parasite
• motile
• nomadic
• migratory
• sedentary
• aestivation
• daily torpor
• solitary
• territorial
• social
• colonial
• dominance hierarchies

Communication and Perception

Ray-finned fishes perceive the external environment in five major ways – vision, mechanoreception, chemoreception, electroreception and magnetic reception, and to humans several of these sensory systems are entirely alien. Many types of perception are also used by ray-finned fishes to communicate with individuals of the same (conspecifics) or other species (heterospecifics). (Helfman, et al., 1997; Moyle and Cech, 2004)

Vision is the most important means of communication and foraging for many ray-finned fishes. The eyes of fish are very similar to terrestrial vertebrates so they are able to recognize a broad range of wavelengths. A species’ ability to perceive various wavelengths corresponds to the depth at which it lives since different wavelengths attenuate (become weaker) with depth. In addition to the normal spectrum perceived by most vertebrates, several shallow-water species are able to see ultraviolet light; others, such as anchovies , cyprinids , salmonids and cichlids , can even detect polarized light! Many fishes also have specially modified eyes adapted for sight in light-poor environments and even outside of water (e.g. mudskippers). For example, several families of deepsea fishes (deepsea hatchetfishes , pearleyes , giganturids , barreleyes) have elongate (long and narrow), upward-pointing, tubular eyes that enhance light gathering and binocular vision, providing better depth perception. Also, several deepwater, midwater and a few shallow species actually have internally generated lights around the eyes to find and attract prey and communicate with other species (see below). Light is usually produced in two ways: by special glandular cells embedded in the skin or by harnessing cultures of symbiotic luminous bacteria in special organs. (Helfman, et al., 1997; Moyle and Cech, 2004; Nelson, 1994; Parrish, 1998; Wheeler, 1985)

One way fishes communicate visually is simply through their static color pattern and body form. For instance, juveniles progress through a range of color and shape patterns as they mature, and sexes are often colored differently (sexual dimorphism). In addition, some fishes are quite good at identifying other species; the Beau Gregory damselfish is apparently able to distinguish 50 different reef fish species that occur within its territory. A second way fishes communicate visually is through dynamic display, which involves color change and rapid, often highly stereotyped movements of the body, fins, operculae, and mouth. Such displays are often associated with changes in behavioral state, such as aggressive interactions, breeding interactions, pursuit and defense. A third form of visual communication is light production, found among numerous fishes in deepsea habitats. Midwater species, such as lanternfishes , hatchetfishes and dragonfishes have rows of lights along the underside of the body, probably for mating and identification as well as foraging. Even some shallow-water species, such as pineconefishes , cardinalfishes and flashlight fish (family Anomalopidae) of the Red Sea utilize internal light sources to form nighttime feeding shoals or for other behavioral interactions. (Helfman, et al., 1997; Moyle and Cech, 2004; Nelson, 1994; Parrish, 1998; Wheeler, 1985)

Mechanoreception includes equilibrium and balance, hearing, tactile sensation, and a ‘distance-touch-sense’ provided by the lateral line (Wheeler, Alwyne 1985:viii). Detecting sound in water can be difficult because waves pass through objects of similar density. Therefore, ray-finned fishes have otoliths, which have greater density than the rest of the fish, in the inner ear attached to sensory hair cells. Since gas bubbles increase sensitivity to sound, many ray-finned fish (e.g. herrings , elephantfishes and squirrelfishes) have modified gas bladders and swimbladders adjacent to the inner ear. Most ray-finned fishes have keen hearing ability and sound production is common but not universal. In groups that do utilize sound for communication, the most common purpose is territorial defense (e.g. damselfishes and European croakers) or prey defense (e.g. herrings , characins , catfishes , cods , squirrelfishes and porcupinefishes). Sound production is also used in mating (for attraction, arousal, approach or coordination) and communication between shoal mates. Stridulation, which involves rubbing together hard surfaces such as teeth (e.g. filefishes) or fins (e.g. sea catfishes), or the vibration of muscles (e.g. drums), is the most common way sound is produced. Often the latter structures have a muscular connection to the swimbladder to amplify sound. Accordingly, the swimbladder itself is the source of the most complex forms of sound production in many groups (e.g. toadfishes , searobins and flying gurnards). The lateral line is composed of a collection of sensory cells beneath the scales and is able to detect turbulence, vibrations and pressure in the water, acting as a close-quarters radar. This sensation is particularly important in the formation of schools (see Behavior) because consistent positioning is essential for turbulence reduction and smooth hydrodynamic functioning. Consequently, individuals are “so sensitive to the movements of companions that thousands of individuals can wheel and turn like a single organism” (Moyle and Cech 2004:206). Experiments have shown that the lateral line sensation can even compensate for loss of sight in some species, such as trout. The fact that several naturally sightless fish occupy caves (e.g. cavefishes) and other subterranean environments, making extensive use of distance-touch sensation, provides further evidence. (Helfman, et al., 1997; Moyle and Cech, 2004; Nelson, 1994; Parrish, 1998; Wheeler, 1985)

Chemoreception involves both smell (olfaction) and taste (gustation), but, as in terrestrial vertebrates , olfaction is much more sensitive and chemically specific than gustation, and each has a specific location and processing center in the brain. Many fishes use chemical cues to find food. Taste buds are scattered widely around the lips, mouth, pharynx, and even the gill arches; and barbels are used for taste reception in many families (most carps , catfishes and cod). However, the use of nares (like nostrils, located on the top of the head) to detect pheromones is probably the most important type of chemoreception in fishes. Pheromones are chemicals secreted by one fish and detected by conspecifics, and sometimes closely related species, producing a specific behavioral response. Pheromones allow fish to recognize specific habitats (such as natal streams in salmon), members of the same species, members of the opposite sex, individuals in a group or hierarchy, young, predators, etc. Some groups in dominance hierarchies even associate the scents of individuals with their particular ranking. Also, groups of closely related species, such as cyprinids , are able to detect ‘fear scents,’ which are pheromones released when the skin is broken (i.e. a predator has attacked), prompting others to adopt some type of predator avoidance behavior. (Helfman, et al., 1997; Moyle and Cech, 2004)

Some ray-finned fishes, usually inhabiting turbid environments, have specialized organs for electroreception. Several groups can detect weak electrical currents emitted by organs, such as the heart and respiratory muscles, and locate prey buried in sediment (catfish) or in extremely turbid waters (elephantfishes). Elephantfishes and naked-back knifefishes actually produce a constant, weak electrical field around their bodies that functions like radar, allowing them to navigate through their environment, find food, and communicate with mates. In fact, a diverse range of actinopterygian orders have developed the ability to use electricity for communication: Mormyriformes (elephantfishes and Gymnarchidae), Gymnotiformes (six families) , Siluriformes (electric catfishes), and Perciformes (stargazers). The key to electrical communication is not simply the ability to detect electrical fields, but to produce a mild electrical discharge and modify the amplitude, frequency, and pulse length of the signal. This makes electrical signals individually specific, in addition to being sex and species-specific. Consequently, “electrical discharges can have all the functions that visual and auditory signals have in other fishes, including courtship, agonistic behavior and individual recognition” (Moyle and Cech 2004:206). Finally, a few highly migratory ray-finned fishes can apparently detect earth-strength magnetic fields directly, in much the same way sensation occurs with the lateral line. While the specific mechanisms of magnetic reception are unknown, researchers have found magnetite in the heads of some tunas (e.g. yellowfin tuna) and in the nares of some anadromous salmon (subfamily Salmoninae). Presumably, magnetic perception helps fish locate long distance migration routes for both feeding and reproduction. (Berra, 2001; Helfman, et al., 1997; Moyle and Cech, 2004; Nelson, 1994; Parrish, 1998; Wheeler, 1985)

Clearly ray-finned fishes display considerable complexity in their ability to perceive their environment and communicate with other individuals, yet until recently it was assumed that fish had negligible cognitive ability. Current research, however, indicates that learning and memory are integral parts of fish development and rely on processes very similar to those of terrestrial vertebrates. Experiments have shown, for instance, that individuals can remember the exact location of holes in fishing net years after exposure, and that fish in schools learn faster by following the lead other individuals. Some researcher believe that the cognitive ability of some fishes is even comparable to that of non-human primates. (Brown, 2003)

• Communication Channels
• visual
• tactile
• acoustic
• chemical
• electric

• Other Communication Modes
• photic/bioluminescent
• mimicry
• duets
• choruses
• pheromones
• scent marks
• vibrations

• Perception Channels
• visual
• infrared/heat
• ultraviolet
• polarized light
• tactile
• acoustic
• vibrations
• chemical
• electric
• magnetic

Food Habits

Based on feeding habits, researchers broadly classify ray-finned fishes as herbivores, carnivores, omnivores, zooplanktivores and detrivores. There is considerable nuance within each of these categories because many fish are opportunistic feeders – they tend to consume whatever is around, especially when food is scarce. However, primary feeding habits are often associated with body form, mouth type and digestive apparatus, as well as teeth. For instance, gars , pike-characids , pike , needlefish , pike killifish and barracuda represent a diverse range of taxa, yet they all have elongate (long and narrow) bodies, long snouts, and sharp teeth with the fins placed toward the back of the body; this is the design of a fast-start predator, which often lurks motionless in the water column, slightly camouflaged and ready to lunge quickly at unsuspecting prey. These fishes are not made for sustained speed and maneuverability, whereas tunas and billfishes (suborder Scombroidei), with their rounded and highly tapered bodies, are streamlined pelagic chasers capable of very high speeds over long periods. These two fishes are termed ram feeders. Other predators avoid the extra energy expenditure of chasing prey, and instead wait passively, depending largely on good vision, explosive thrust and large mouths capable of forming strong vacuums and effectively inhaling prey (the latter method is termed suction feeding). These sit-in-wait predators are often completely hidden with elaborate camouflage or by burying themselves beneath sediment with only the eyes exposed. Fishes of this type include many scorpionfishes , flatheads , hawkfishes , sea basses , stonefishes , stargazers, flatfishes , frogfishes, and lizardfishes. (Bertelson and Pietsch, 1998; Ferraris, 1998; Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

Herbivorous fishes posses specialized organs, such as extended guts, pharyngeal mills and gizzards, that allow them to exploit various reef plants and algae. Some of the most successful freshwater families (e.g. minnows , catfishes , cichlids), and most abundant coral reef families (e.g. halfbeaks , parrotfishes , blennies , surgeonfishes , rabbitfishes), include many species of herbivorous fishes. Several groups of herbivorous coral reef species defend territories or form feeding shoals (freshwater cichlids have many of the same behaviors). Some parrotfishes and surgeonfishes utilize shoals to overwhelm the defenses of territorial species, thus gaining access to areas with higher concentrations of plant material. (Bertelson and Pietsch, 1998; Ferraris, 1998; Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

Zooplanktivores, which feed on small crustaceans like water fleas and copepods floating in the water column (termed zooplankton), abound in oceans throughout the world. Groups such as silversides , herrings and anchovies often congregate in feeding shoals numbering in the millions. Smaller shoals of zooplanktivores, such as rabbitfishes and the juvenile forms of many other reef species, are also found hovering above and around coral reefs. The characteristic features of zooplanktivorous fishes are small size, streamlined and compressed bodies, forked tails, few teeth, and a protrusible mouth that forms a circle when open. When patches of zooplankton are particularly high, many pelagic zooplanktivores keep their mouths agape, and when patches are low they pick animals out individually (the latter are also termed suction feeders). (Bertelson and Pietsch, 1998; Ferraris, 1998; Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

As discussed in Communication, several groups of ray-finned fishes have quite peculiar methods of capturing prey. Deepsea anglerfishes , among many others in the Stomiiformes and Lophiiformes orders, have developed a luminous bait to attract prey in the deep, dark waters they inhabit. Turbid habitats are home to many fishes that utilize electroreception to find prey, and some predators (e.g. knifefishes and the electric eel) use intense electrical shocks of as much as 350 volts to stun prey before consuming them. Archerfishes exploit a food source that is unavailable to most other fishes: terrestrial insects in overlying vegetation. By shooting jets or bullets of water, and correcting for light refraction, archerfishes knock insects down to the water surface and quickly consume them. Finally, some boxfishes and triggerfishes use an equally novel technique for capturing prey. Both groups expel jets of water from their mouths to uncover buried animals, while triggerfishes use jets and their snouts to flip over and consume otherwise inedible prey, such as spiny sea urchins. (Bertelson and Pietsch, 1998; Ferraris, 1998; Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

• Primary Diet
• carnivore
  • eats terrestrial vertebrates
  • piscivore
  • eats eggs
  • sanguivore
  • eats body fluids
  • insectivore
  • eats non-insect arthropods
  • molluscivore
  • scavenger
• herbivore
  • frugivore
  • granivore
• omnivore
• planktivore
• detritivore

• Foraging Behavior
• stores or caches food
• filter-feeding

Predation

Ray-finned fishes generally avoid predators in two ways, through behavioral adaptation and physical structures, such as spines, camouflage and scents. Usually, several behavioral and structural tactics are integrated because it is advantageous for fishes to break the predation cycle (1-4) in as many places as possible, and the earlier the better. For instance, (1) the primary goal of most fish is to avoid detection, or avoid being exposed during certain times of the day. If detected, (2) a fish might try to hide very quickly, blend in with the surroundings, or school; (3) if the fish is about to be attacked then it must try to deflect the attack, and if attack is unavoidable (4) the fish will try to avoid being handled and possibly escape. Therefore, many fishes avoid even the chance of attack through particular cycles of activity, shading (or lighting, see below) and camouflage, mimicking, and warning coloration. (Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

For example, fishes usually avoid dusk because predators often take advantage of quickly changing light conditions that make it difficult for prey to see predators. (Species that feed at dusk are termed crepuscular and include jackssnappers , tarpon , cornetfishes and groupers). Most ray-finned fishes feed during daylight hours (diurnal), when they can see predators. Zooplanktivores, cleaner fishes, and many herbivores are abundant and conspicuous by day but hide within the reef at night. Several wrasses and parrotfishes even secrete a foul-smelling mucous tent or bury themselves in the sediment for protection. Shoaling, which is common among many groups (found in sticklebacks , bluegills , gobies and many others), provides many benefits as a daytime defense. Some predators actually mistake shoals for large fish and avoid attacking. Also, when shoals detect predators they form a tight, polarized group, or school, that is able make synchronous motions. Attacking predators may find it difficult to isolate individuals as the school morphs around them, and some groups (snappers , goatfishes , butterflyfishes , damselfishes , etc.) even mob the predator, nipping and displaying, to thwart an attack. (Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

Because many larger species of zooplankton and other invertebrates come out at night, several groups have developed nighttime feeding patterns (nocturnal) and associated defense mechanisms. Many of these groups, including flashlight fishes , ponyfishes , pineapple fishes and some cardinalfishes , have luminescent organs. While luminescence is likely used for communication (shoaling and mating) and catching prey (via luminescent eyes, which can be turned on and off (!), and baits), several species use luminescence for defense. Rows of lights along the bottom of the body make these fishes indistinguishable to benthic (living at the bottom) predators because they match the intensity of moonlight or dim sunlight shining down. This peculiar method of invisibility is similar to countershading, which is common in several other pelagic ray-finned fishes (as well as sharks and rays). Countershaded fishes are graded in color from dark on top to light on bottom, rendering them invisible from nearly any angle because their coloring is opposite that of downwelling light; the light reflected is equivalent to the background (as above). Two other methods by which pelagic fishes remain invisible are by having a shiny coating (mirror-sided), as in anchovies , minnows , smelts , herrings and silversides ; or by having transparent bodies, like glassfishes , African glass catfishes and Asian glass catfishes. (Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

Benthic ray-finned fishes also utilize numerous methods of camouflage (for both hunting and predator avoidance). A common and elaborate method in tropical seas is mimicking the background of the habitat (protective resemblance), which involves variable color patterns as well as peculiar growths of the skin that may resemble pieces of dead vegetation, corals , and a variety of bottom types (e.g. flatfishes). There are numerous examples of this type of crypticity, from sargassumfishes and leafy seadragons that mimic the seaweed among which they hover, to clingfishes , shrimp fishes and cardinalfishes that have black stripes resembling the sea urchins they use for cover. Another method of camouflage is to look and behave like something inedible, but remain conspicuous. Juvenile sweetlips and batfishes mimic certain types of flatworms and nudibranchs that have toxins in their skin and associated bright coloration, making possible predators wary. (Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

Bold or bright coloration in ray-finned fishes (termed aposematic) usually means that the species posses a structural or chemical defense, such as poisonous spines, or toxic chemicals in the skin and internal organs. Surgeonfishes and lionfishes , for instance, have bold coloration to match scalpel-like and poisonous spines, respectively. Aposematic fishes also advertise their inedibility by moving slowly, instead of darting away when predators are present. However, displays of aggression back up this behavior. When disturbed, weevers erect a dark-colored and highly venomous dorsal spine, while pufferfishes , also poisonous, puff up into a ball of spikes. (Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998)

• Anti-predator Adaptations
• mimic
• aposematic
• cryptic

• Known Predators

  • fish (Actinopterygii)
  • sharks Chondrichthyes
  • aquatic invertebrates
  • birds (Aves)
  • mammals (Mammalia)
  • reptiles (Lepidosauria)
  • amphibians (Amphibia)

Ecosystem Roles

Ray-finned fishes are essential components of most ecosystems in which they occur. While many ray-finned fishes prey on each other, they can also have significant impacts on nearly all other animals in their habitats. Zooplanktivorous fishes, for instance, select for specific types and sizes of zooplankton when they feed, thus influencing the type and quantity of zooplankton, and, by extension, phytoplankton present in surface waters (zooplankton consume algae; together they are simply termed plankton). When non-native species invade new habitats (usually through human intervention), the fragility of this balance is dramatically illustrated. For instance, when alewives (family Clupeidae) invaded Lake Michigan, they decimated two larger species of zooplankton and dramatically reduced two midsize species, resulting in the increase of ten smaller species and higher algal content. Later, Pacific salmon (genus Oncorhynchus) were introduced into the lake and dramatically reduced alewife populations and the larger zooplankton species recovered. Because the larger species grazed on algae more efficiently, phytoplankton density decreased dramatically and the lake cleared. This is an example of a trophic cascade, and although the ecosystem achieved relative balance in this example, this is not always the case. For instance, the introduction of Nile perch , a voracious predator, into Lake Victoria (Africa) caused a precipitous decline of many small, planktivorous cichlids. These cichlid species exerted considerable predation pressure on zooplankton, and after they were eliminated the zooplankton community changed drastically, to the point that a new and very large cladoceran species appeared in the lake, Daphnia magna. Unfortunately, this introduction resulted in one of the largest mass extinctions of endemic species in modern times, and the repercussions did not stop with the perch introduction. Many local people consumed the smaller cichlid species and hung them in the sun to dry and preserve them. When Nile perch began to impact local cichlid fisheries, locals started to consume Nile perch, but this fish required firewood for drying and preservation because it is much larger. Consequently, deforestation started to occur around Lake Victoria, leading to increased runoff and siltation during rainy periods, and consequently, decreasing water quality. Decreasing water quality further endangered endemic cichlids, resulting in even more extinctions. The latter example illustrates the complexity of ecological interactions and the fact that ecological interactions are not confined to aquatic organisms. Because ray-finned fishes are often important food source to terrestrial organisms (see below), including humans (see Economic Importance and Conservation), changes in ray-finned fish communities can have significant ecological implications. (Berra, 2001; Jonna and Lehman, 2002; Moyle and Cech, 2004)

A variety of terrestrial vertebrates, such as mammals , amphibians , reptiles , and many marine and freshwater birds depend on ray-finned fishes as a primary source of food. Piscivorous ray-finned fishes compete with many of the organisms above and in some cases are involved in symbiotic relationships with them. A simultaneous competitive and commensal (one benefits and the other is unaffected) relationship is found between bluefish and common terns. These two species interact at a critical period of the terns’ feeding cycle, just after mating when there are chicks to feed. At this time, bluefish migrate to feed on anchovies, concentrating and driving them up in the water column, where terns can catch sight of the anchovies (commensalism). However, bluefish reduce anchovies’ populations considerably, and terns that breed after the bluefish migration are usually unsuccessful (competition). There are numerous other examples of symbiosis, mutualism, commensalism and parasitism between ray-finned fishes and other groups. For example, gobies share burrows with several shrimp-like crustaceans (mutualism) or live among sponges and corals (commensalism). Cardinalfishes and pearlfishes live inside large gastropods and mollusks , respectively (inquilism-sheltering inside living invertebrates). Recently, researchers have begun to appreciate the importance of fish in linking terrestrial and aquatic ecosystems. This is especially true of anadromous species, which grow primarily in the sea but return to aquatic areas before they, spreading nutrients from the ocean up and down rivers. During rainy periods in tropical watersheds, ray-finned fish forage in flooded areas, consuming seeds and dispersing them throughout the floodplain. (Helfman, et al., 1997; Moyle and Cech, 2004)

Several groups of invertebrates (mostly marine), such as cone shells , crabs , anemones , squids and siphonophores (colonies of organisms, e.g. man-o-war), also regularly consume various ray-finned fish. There is even some unlikely predators like dinoflagellates , that can cause large fish kills, known as “red tides”. Some dinoflagellates consume the scales of the dead fish as they sink. Ray-finned fishes also have significant impacts on a variety of plant species. The trophic cascade example (above) illustrated an indirect connection between microscopic plants (phytoplankton) and fish, but fish also excrete soluble nutrients into the water, such as phosphorus. Phosphorus is essential for phytoplankton growth, and fish secretions may provide significant amounts of nutrients in some lakes. A more direct connection is simply the consumption of numerous plant species (see Food Habits). Finally, fish may significantly alter the geological dynamics of their habitats. Many ray-finned fish build nests or burrows (e.g. several minnows , trout and salmon and tilefishes), while others break down substrates, such as dead coral, into sand (e.g. parrotfishes , wrasses , surgeonfishes , triggerfishes and pufferfishes). (Helfman, et al., 1997; Moyle and Cech, 2004)

• Ecosystem Impact
• disperses seeds
• creates habitat
• biodegradation
• keystone species
• parasite

Economic Importance for Humans: Positive

Fishes are obviously of enormous economic import to humans. Primarily, humans consume fish through fishing and aquaculture, and fish are an essential form of protein for millions of people around the world. The farmed salmon industry alone is valued at over 2 billion dollars a year, but unfortunately, aquaculture operations can have serious ecological consequences. Similarly, ray-finned fishes are quite popular in the aquarium trade, and those with high cash value, such as many tropical fishes, are removed in highly damaging (i.e. using poisons) and exploitive ways (see Conservation and Other Comments). Televised sport fishing events are also popular on rivers, lakes, coastal areas and reefs around the world. The fast-growing scuba industry relies heavily on thriving coral reefs with diverse and abundant communities of ray-finned fishes. Finally, of less direct (and severely underappreciated) economic importance are the ecological roles that fishes fill, like controlling insect populations (e.g. many still-water groups like gouramies) and ensuring healthy-functioning aquatic systems, which helps to ensure clean water and reduces the spread of disease. (Almeda-Villela, 1998; Moyle and Cech, 2004)

• Positive Impacts
• pet trade
• food
• body parts are source of valuable material
• ecotourism
• source of medicine or drug
• research and education
• produces fertilizer
• controls pest population

Economic Importance for Humans: Negative

There was no specific information found on negative impacts to humans. However, many fish are poisonous and venomous, and when disturbed, like many other animals, they can inflict serious wounds and death in some cases. This is also true of predatory fishes that are attracted to shiny objects. Humans willingly eat poisonous species considered delicacy, such as pufferfishes. In some cases, people die from consuming poisonous fish. In the great majority of cases, however, fishes have positive or negligible impacts on humans. (Froese and Pauly, 2004; Helfman, et al., 1997; Moyle and Cech, 2004)

• Negative Impacts
• injures humans
  • bites or stings
  • causes disease in humans
  • poisonous
  • venomous

Conservation Status

The threat to aquatic habitats has grown steadily over the course of the twentieth century and continues today for a variety of reasons, most of which involve human intervention via overexploitation, introduced species, habitat alterations, pollution, and international trade. However, until recently, researchers did not fathom the scope of the problem among marine species because they assumed that broad distributions, the method of reproduction (pelagic dispersal), and the vastness of the marine environment might create a buffer to threats such as overexploitation and ecological decline. Unfortunately, there are worrying signs, such as collapses in many of the world’s fisheries and drastic declines in many large, mobile species (e.g. tunas). Additionally, researchers are finding that some species live quite long and have low reproduction and growth rates, meaning that removal of larger individuals can have significant impacts on populations. Another trade-related threat is excessive removal of exotic reef species using harsh chemicals, such as cyanide, for the aquarium trade. (Almeda-Villela, 1998; IUCN, 2003; Moyle and Cech, 2004)

Freshwater groups, however, account for the vast majority of actual extinctions in ray-finned fishes. The most significant threats are to families with restricted distribution (i.e. endemic) because localized threats can easily eliminate all individuals of a species. Introduced species, such as Nile perch and mosquitofish (genus Gambusia), combined with pollution and habitat alteration have proven particularly disastrous for groups of endemic ray-finned fishes (i.e. cichlids and many cyprinids). At this point, approximately 90 species of ray-finned fishes are known to be extinct or only survive in aquaria, 279 are critically endangered or endangered, and another 506 are listed as vulnerable or near threatened. Families of particular concern (in descending order) are cyprinids , cichlids , silversides , pupfishes , and especially sturgeons and paddlefishes since every member in the latter two families are threatened. (Almeda-Villela, 1998; IUCN, 2003; Moyle and Cech, 2004)

• IUCN Red List [Link]

  Not Evaluated

Other Comments

Actinopterygian fossils first appeared in deposits from the late Silurian (425 to 405 Ma) or early Devonian (405 to 345 Ma) period. While there is a need for more research to understand the evolutionary relationships among the earliest actinopterygians, ichthyologists have found that actinopterygians did not begin to dominate the fish fauna until the beginning of the Carboniferous period, 360 million years ago (Ma). The most derived forms (i.e. teleosts) were uncommon until the late Cretaceous (144 to 65 Mz) period. It was at this time that major diversification began and has continued to this day, as actinopterygians dominate the world’s fish fauna. (Grande, 1998; Helfman, et al., 1997; Moyle and Cech, 2004)

The earliest actinopterygians are grouped in the subclass Chondrostei, of which only sturgeons , bichirs and paddlefishes survive today. The rest of the actinopterygians, which includes the vast majority of species, are in the subclass Neopterygii, meaning ‘new fins’. Further, the large majority of neopterygians are placed in the group Teleostei (infraclass). The bowfin is the only surviving species of the halecomorphs, the largest group outside of the teleosts and gars (order Lepisosteiformes – also known as Semionotiformes), with seven species, are the only other surviving non-teleosts. (Grande, 1998; Helfman, et al., 1997; Moyle and Cech, 2004)

Ray-finned fishes have significant aesthetic, cultural, scientific and transformative value to humans. To many native people, especially in the United States, fish are symbols of cultural tradition and the subject of works of art. Snorkeling, scuba diving, and sport fishing are increasingly popular around the world and, of course, ray-finned fishes have significant scientific and educational value. (Almeda-Villela, 1998; Moyle and Cech, 2004)

Contributors

Tanya Dewey (editor), University of Michigan-Ann Arbor.

R. Jamil Jonna (author), Animal Diversity Web.

References

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Bertelson, E., T. Pietsch. 1998. Anglerfishes. Pp. 137-141 in J Paxton, W Eschmeyer, eds. Encyclopedia of Fishes. San Diego, CA: Academic Press.

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Grande, L. 1998. Fishes Through the Ages. Pp. 27-31 in J Paxton, W Eschmeyer, eds. Encyclopedia of Fishes. San Diego, CA: Academic Press.

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IUCN, 2003. "2003 IUCN Red List of Threatened Species" (On-line). Accessed August 16, 2004 at http://www.redlist.org.

Jonna, R., J. Lehman. 2002. The Invasion of Lake Victoria by the Large Bodied Herbivorous Cladoceran Daphnia magna. Pp. 321-333 in E Odada, D Olago, eds. The East African Great Lakes: Limnology, Paleolimnology and Biodiversity. Boston: Kluwer Academic Publishers.

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Sargent, R., M. Gross. 1993. Williams’ Principle: An Explanation of Parental Care in Teleost Fishes. Pp. 333-361 in T Pitcher, ed. Behavior of Teleost Fishes - second edition. London; New York: Chapman and Hall.

Wheeler, A. 1985. The World Encyclopedia of Fishes. London: Macdonald.