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Psephenidaewater-penny beetles

By George Hammond

Diversity

This small family of beetles gets its common name ("water pennies") from the unusual shape of the larvae, which have widely expanded dorsal plates that give the animals a flat round shape, like a coin. The classification of the family is uncertain, some authorities recognize some subgroups as separate families. There are only about 263 species in 31 genera are known (Lee et alia, 2007) but this family has not received intensive study, and there are probably more species still undescribed. (Brown, 1991; Lee, et al., 2007; Shepard, 2002)

Geographic Range

Members of the Psephenidae are found on all continents except Antarctica. (Brown, 1991)

Habitat

Psephenid larvae cling to hard substrates (rocks, woody debris) in well-oxygenated water. They are most often found in fast-moving streams, especially riffle beds, but some species are found on rocky lake shores with wave action. Most pupae and all adults are air-breathing, but stay near the water. Adults are often found on the underside of logs and other objects overhanging streams. (Brown, 1991; McCafferty, 1983; White and Brigham, 1996)

Physical Description

Larvae are 3-10 mm long, ranging in color from pale amber to nearly black (usually matching the substrate) and have a distinctive shape. They dorso-ventrally flattened, with expansions of the dorsal plates that extend out so far that they are oblong or nearly round when viewed from above -- the head and legs completely covered. This flat round shape, plus their brownish color, earns them the name "water pennies". Larvae have clawed legs for clinging to rocks in moving water, and chewing mouthparts for scraping food off of rock and other hard substrate.

Adults are dark-colored, 4-6 mm long, with somewhat flattened bodies, short thread-like antennae, and mandibles not visible from above.

Most species in this family have amphipneustic larvae: they can extract oxygen both from air and directly from the water. They have a few functional spiracles for air breathing. Most also have filamentous gills that extract oxygen from the water, either paired structures on abdomenal sterna, or a single caudal structure near the anus. The genus Psephenoides is more fully aquatic, neither larvae nor pupae have functional spiracles, and both stages have gills. In the rest of the family, pupae and adults are air-breathers, with functional spiracles. (Brown, 1991; McCafferty, 1983; White and Brigham, 1996; Wichard, et al., 2002)

Development

Female water penny beetles lay their eggs above or at the edge of the stream or shore habitat where their larvae will live. Aquatic larvae hatch from the eggs and drop or crawl into the water. They cling to gravel and other hard debris in the water, grazing on the algae and other micro-organisms that grow there. In this stage they grow and molt several times during warm months. If the warm season is short, they may need more than one year to complete the larval stage. When read to transform, larvae small chamber where they enter the pupal stage and metamorphose into an adult. Some species create an air-filled chamber under water, but most pupate on land, in moist soil. Pupation is completed inside the larval skin. Adults don't live long. (White and Brigham, 1996)

Reproduction

Reproduction information only known for one or a few species. Psephenus herricki females lay hundreds or thousands of small, bright yellow eggs on submerged and emergent objects in stream riffles with fast current. Probably semelparous, adults only live a few weeks. Believed to all be sexual dioecious species (each individual either male or female), with internal fertilization. (White and Brigham, 1996)

Behavior

Water penny larvae tend to move away from light, cling to whatever they touch, and move into any water current they encounter. This helps them keep their place in streams, and not be washed into the water. Larvae of some species seem to be attracted to each other, but some are solitary and it is rare to find more than one. (White and Brigham, 1996)

Food Habits

Water penny larvae feed on periphyton, the thin layer of algae and micro-organisms that forms on stones and other objects in moving freshwater. Adults may not feed. (White and Brigham, 1996)

Predation

Water pennies hide from predators during the day, and are most active at night.

Freshwater trout are reported to be able to pluck the larvae off of stones. (McCafferty, 1983)

Contributors

George Hammond (author), Animal Diversity Web.

Glossary

Australian

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

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Ethiopian

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

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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.

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Neotropical

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

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Palearctic

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

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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.

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.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

freshwater

mainly lives in water that is not salty.

herbivore

An animal that eats mainly plants or parts of plants.

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.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

internal fertilization

fertilization takes place within the female's body

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

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

oriental

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

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oviparous

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

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

semelparous

offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.

sexual

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

social

associates with others of its species; forms social groups.

solitary

lives alone

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).

terrestrial

Living on the ground.

References

Brown, H. 1991. Psephenidae (Dryopoidea) (including Eubriidae, Psephenoididae). Pp. 395-397 in F Stehr, ed. Immature Insects, Vol. 2. Dubuque, Iowa, USA: Kendall/Hunt Publishing Company.

Lee, C., M. Satô, W. Shepard, M. Jäch. 2007. Phylogeny of Psephenidae (Coleoptera: Byrrhoidea) based on larval, pupal and adult characters. Systematic Entomology, 32(3): 502-538. Accessed April 30, 2009 at http://www3.interscience.wiley.com/journal/117964692/abstract.

McCafferty, W. 1983. Aquatic Entomology: The Fishermen's and Ecologists' Illustrated Guide to Insect and Their Relatives. Boston, Massachusetts, USA: Jones and Bartlett Publishers, Inc..

Shepard, W. 2002. Pesphenidae Lacordaire 1854. Pp. 133-134 in R Arnett, M Thomas, P Skelley, J Frank, eds. American Beetles, Vol. 2. Boca Raton, Florida, USA: CRC Press.

White, D., W. Brigham. 1996. Aquatic Coleoptera. Pp. 399-473 in R Merritt, K Cummins, eds. An Introduction to the Aquatic Insects of North America. Dubuque, Iowa, USA: Kendall/Hunt Publishing Company.

Wichard, W., W. Arens, G. Eisenbeis. 2002. Biological Atlas of Aquatic Insects. Stenstrup, Denmark: Apollo Books.

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