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Kinorhynchamud dragons(Also: kinorhynchs)

By Jeremy Wright

Diversity

Phylum Kinorhyncha is divided into two orders, Cyclorhagida and Homalorhagida, with 21 genera and over 200 species, some of which have only been described from juvenile stages. These animals are very small, generally less than 1 mm in length, segmented, and limbless. They are motile, marine (occasionally brackish), benthic invertebrates found worldwide, from intertidal to abyssal zones, on sediment layers, sandy beaches, in algae, or associated with other, larger invertebrates. Their bodies are covered with multiple spines, which are movable extensions of the body wall. The spines around the head are used to help pull these animals along the benthos, where they deposit feed on diatoms, bacteria, and other organic matter in the substrate. The head and pharynx can be retracted and covered with plates (called placids) on the second body segment, differences in the number and closing mechanisms of these plates are often used to distinguish species and higher taxonomic groups. (Brusca and Brusca, 1990; Nehaus, 2012; Neuhas and Higgins, 2002; Schmidt-Rhaesa, 2012; Zhang, 2011)

Geographic Range

Species belonging to this phylum have been described from marine and brackish waters around the world. They have been found at depths from 8 to 8,000 meters. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002)

Habitat

Kinorhynchs are generally found interstitially in upper, well oxygenated sediment layers. They may also be found on sandy beaches, in algae, or associated with other invertebrates such as sponges, hydroids and ectroprocts. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002)

Systematic and Taxonomic History

The evolutionary relationships within phylum Kinorhyncha are still poorly resolved. Kinorhynchs are thought to be most closely related to loriciferans and priapulids, with whom they form superphylum Scalidophora. The inter-relationships of these phyla are still a matter of debate (although the majority of evidence points to a sister relationship between kinorhynchs and priapulids), as is the membership of this clade in the group Ecdysozoa (molting animals). The latter ambiguity is due to inadequate sampling of scalidophoran species in molecular examinations of possible ecdysozoans, as well as the lack of evidence that ecdysteroid hormones induce molting in this and other groups. (Aleshin, et al., 1998; Halanych, 2004; Nehaus, 2012)

Some authors have additionally included nematomorphans as evolutionary relatives to scalidophoran phyla, in a clade called Cephaloryncha. Genetic evidence has provided support for a sister relationship between loriciferans and nematomorphans, which would render Scalidophora paraphyletic and support the future use of the name Cephaloryncha, as modern taxonomic systems of classification endeavor to give names only to monophyletic groups. (Aleshin, et al., 1998; Halanych, 2004; Sørensen, et al., 2008)

  • Synonyms
  • Synapomorphies
    • thirteen segmented body
    • movable, paired lateral and unpaired mid-dorsal spine in segments 1 to 10
    • mid-terminal, paired lateral terminal, and lateral terminal accessory spine in segment 11
    • mouth cone as ring-like epidermal fold around the pharynx and protrusibule
    • nine outer oral styles
    • frontal circular muscle, composed of few cells
    • basal nerve ring
    • pharyngeal cuticle with fine-granular basal layer and multilamellar epicuticle
    • gland cells monociliary, with extremely thin microvilli in the atrium
    • hindgut lumen transverse slit-like
    • one pair of frontal, caudal, and dorsal dilatators and one unpaired circular dilatator in the hindgut

Physical Description

Kinorhynchs are yellowish brown and typically range from 0.13 to 1.04 mm, with bodies that can be divided into 13 zonites (segments). The first segment is the head, with a retractable oral cone, surrounded by anteriorly directed oral stylets (9 outer stylets and 3 to 4 rings of inner stylets, each ring with 5 stylets) and a mouth. Behind the oral cone are 5 to 7 rings of posteriorly directed scalids (sensory/locomotive spines). Each ring is made up of 10 to 20 circles, and each circle has 54 to 93 scalids. The neck is the second zonite, made of up to 16 placids, which can fold over the head when it is retracted. The remaining 11 zonites make up the animal’s trunk; each of these zonites is covered with a tergal (dorsal) and pair of sternal (ventral) plates, comprised of a chitinous basal layer and a membrane-like epicuticle. These form a rigid exoskeleton; articulation between the zonites as well as a flexible cuticle between the dorsal and ventral plates makes movement possible. There are no external cilia, but the entire body (including the organs) is lined in a thin cuticle with a chitinous basal layer. Some species are somewhat sexually dimorphic; males have outer spicules associated with their gonads. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002; Schmidt-Rhaesa, 2012)

Underneath the tergal and sternal plates, kinorhynchs have an epidermis which is non-ciliated and closely associated with the nervous system. Bands of dorsolateral and ventrolateral intersegmental, cross-striated muscles attach to the cuticle; strong fibers connect the sternal and tergal plates. Kinorhynchs have head retractor muscles as well as head cone retractor muscles, stretching between the base of the head scalids/mouth cone and posterior trunk segments. Additionally, there are circular muscles associated with the neck and mouth cone placids, but no circular body wall muscles. These animals are pseudocoelemates, and have a spacious anterior body cavity to allow for head retraction, their digestive and reproductive organs exist within blastocoelomic spaces. (Brusca and Brusca, 1990; Margulis and Chapman, 2009; Neuhas and Higgins, 2002)

The mouth leads into a buccal cavity that is followed by a sucking pharynx and esophagus, all of which are lined with cuticle. The esophagus is connected to a long, straight midgut, which is made of over 95% epithelial cells; these are interlocked and presumptively absorb nutrients. The midgut leads to a short hindgut, also lined with cuticle, terminating in an anus on the last zonite. Kinorhynchs excrete wastes via a pair of protonephridia located in the 8th and 9th trunk zonites, dorsolateral to the gut. These open through pores in the body cuticle and may have ciliate cells and/or microvilli. Gas diffusion takes place across the body wall and circulates by diffusion through the body cavity. (Brusca and Brusca, 1990; Margulis and Chapman, 2009; Ruppert, et al., 2004)

  • Sexual Dimorphism
  • sexes alike
  • sexes shaped differently

Development

Each ovary produces one large oocyte, fertilization is assumed to occur internally. Fertilized eggs are deposited in egg cases and embryos undergo direct development to juvenile stages, emerging with 11 of their 13 body segments fully formed. The final two segments, as well as a number of head scalids, are added during a series of six molts, as kinorhynchs progress to adulthood. Molting has only been observed in adults of two species. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002)

Reproduction

Both males and females have paired gonads and short gonoducts. Males have two or three hollow, often rigid, spicules associated with their gonopores. When present, placement of these spicules varies from species to species. Females have both germ cells and nutritive cells in their gonads, and oviducts with seminal receptacles; their gonopores open between the 12th and 13th zonites. Females have been found with a brownish mucosal mass comprised of spermatozoa and spermatids, which is considered to be a spermatophore surrounding their posteriors. Mature spermatozoa may be as large as 1/4 of the total body length of the adult male. Mating has only been observed in one species, Pycnophyes kielensis. In this case, a male and female positioned themselves so their ventral posterior ends faced each other, with the mucosal mass surrounding their posterior ends. (Brusca and Brusca, 1990; Neuhas and Higgins, 2002)

Kinorhynchs are dioecious but males and females are generally indistinguishable visually. It is assumed that they can reproduce year-round and there is some evidence that 3 to 6 eggs are released at a time. (Brusca and Brusca, 1990; Higgins, 1982; Neuhas and Higgins, 2002)

Little is known about egg production and early development in this group. Females provide yolk and an egg case for their eggs. Further parental investment is unknown. (Brusca and Brusca, 1990)

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • female

Lifespan/Longevity

There is currently no information available regarding the typical lifespan of kinorhynchs. (Brusca and Brusca, 1990)

Behavior

Kinorhynchs cannot swim. They move by extending the head and using anterior scalids to move the body forward to burrow in sediment or move through the water. Although they can sometimes be found in relatively high densities (up to 45 animals per 10 cm^2), these animals are considered solitary. (Brusca and Brusca, 1990; Higgins, 1982)

Communication and Perception

These animals have simple nervous systems, with a series of ten ganglia connected and arranged in a ring around the pharynx. The epithelium is highly innervated. Scalids contain up to 10 monociliary sensory cells, which are connected to an exterior distal pore; these are likely chemo-and mechanoreceptive. All species in this phylum have species-specific patterns of sensory spots on their trunks, consisting of one or two pores surrounded by micropapillae. These sensory spots may have a cilium. Microvillar eyespots may be located on the pharyngeal nerve ring and some species have red-pigmented, light sensitive ocelli behind the mouth cone. (Brusca and Brusca, 1990; Margulis and Chapman, 2009; Neuhas and Higgins, 2002)

Food Habits

Kinorhynchs are likely direct deposit feeders, consuming benthic diatoms and organic material, including bacteria and other unicellular algae in the substrate. Some species have longitudinal cuticular rods, which may aid in filtering bacteria. Those that feed on diatoms may collect the algae on their head scalids to be ingested later, or directly suck them in through the pharynx; the pharynx may be protruded from the mouth for this purpose by pharynx protractor muscles. In these species, the oral stylets may break up the diatom's frustules (cell walls). (Brusca and Brusca, 1990; Margulis and Chapman, 2009; Neuhas and Higgins, 2002; Schmidt-Rhaesa, 2012)

Predation

Kinorhynchs are likely included in the diets of most large marine invertebrates such as shrimp, snails, and other benthic feeders. (Margulis and Chapman, 2009)

Ecosystem Roles

As meiobenthic animals, kinorhynchs are an important link in the food chain as prey for larger animals. Kinorhynchs may be parasitized by fungi, and some species may be commensal with bryozoans, hydrozoans, and sponges. They have been found in the gill chambers of intertidal bivalves, but the nature of this relationship is unclear. (Higgins, 1978; Higgins, 2007; Manylov, 1999; Margulis and Chapman, 2009)

Species Used as Host
Commensal/Parasitic Species
  • Kinorhynchospora japonica (Division Microsporidia, Kingdom Fungi)

Economic Importance for Humans: Positive

There are no known positive effects of kinorhynchs on humans beyond the place they occupy in the marine food chain and their use in scientific research.

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of kinorhynchs on humans.

Conservation Status

As a broadly cosmopolitan phylum, kinorhynchs in general are not considered endangered or threatened, nor are there any conservation efforts directed at any individual species.

  • IUCN Red List [Link]
    Not Evaluated

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

Glossary

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

abyssal

on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

crepuscular

active at dawn and dusk

detritivore

an animal that mainly eats decomposed plants and/or animals

diurnal
  1. active during the day, 2. lasting for one day.
fertilization

union of egg and spermatozoan

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

planktivore

an animal that mainly eats plankton

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

year-round breeding

breeding takes place throughout the year

References

Aleshin, V., I. Milyutina, O. Kedrova, N. Vladychenskaya, N. Petrov. 1998. Phylogeny of Nematoda and Cephalorhyncha derived from 18S rDNA. Journal of Molecular Evolution, 47: 597-605.

Brusca, R., G. Brusca. 1990. Invertebrates (2nd Edition). Sunderland, MA: Sinauer Associates.

Halanych, K. 2004. The new view of animal phylogeny. Annual Review of Ecology, Evolution, and Systematics, 35: 229-256.

Higgins, R. 1982. "Phylum Kinorhycha or Echinodera" (On-line). Online Zoologists. Accessed March 15, 2013 at http://www.onlinezoologists.com/topics/taxa/kinorhyncha.html.

Higgins, R. 1978. Echinoderes gerardi n.sp. and E. reidli (Kinorhyncha) from the Gulf of Tunis. Transactions of the American Microscopical Society, 97/2: 171-180.

Higgins, R. 2007. Kinorhyncha, Loricifera, and Priapulida. Pp. 267-273 in J Carlton, ed. The Light and Smith manual: Intertidal Invertebrates from Central California to Oregon, 4th Ed. Berkeley, CA: University of California Press.

Manylov, O. 1999. First finding of a microsporidian parasite in the gastrotrich, Turbanella lutheri (Gastrotricha: Macrodasyida). Protistology, 1: 17-19. Accessed March 15, 2013 at http://protistology.ifmo.ru/num1_1/manylov.pdf.

Margulis, L., M. Chapman. 2009. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. Boston, MA: Academic Press.

Nehaus, B. 2012. "Kinorhyncha" (On-line). World Register of Marine Species. Accessed February 23, 2013 at http://www.marinespecies.org/.

Neuhas, B., R. Higgins. 2002. Ultrastructure, biology, and phylogenetic relationships of Kinorhyncha. Integrative and Comparative Biology, 42/3: 619-632.

Ruppert, E., R. Fox, R. Barnes. 2004. Invertebrate zoology: A functional evolutionary approach (7th Edition). Belmont, CA: Thomson-Brooks/Cole.

Schmidt-Rhaesa, A. 2012. Handbook of Zoology: Nematomorpha, Pripulida, Kinorhyncha, Loricifera. Hamburg, Germany: de Gruyter.

Sørensen, M., M. Hebsgaard, I. Heiner, H. Glenner, E. Willerslev, R. Kristensen. 2008. New data from an enigmatic phylum: evidence from molecular sequence data supports a sister-group relationship between Loricifera and Nematomorpha. Journal of Zoological Systematics and Evolutionary Research, 46/3: 231-239.

Zhang, Z. 2011. Animal biodiversity: an introduction to higher-level classification and taxonomic richness. Zootaxa, 3148: 7-12.

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