Phylum Arthropoda
Subphylum Crustacea
Class Branchiopoda
Orders Cyclestherida, Laevicaudata and Spinicaudata
Common names: conchostracans, clam shrimps, shelled fairy shrimps
Overview
Clam shrimps are small, distinctive freshwater crustaceans whose body is enclosed within a clam-like carapace or shell. The carapace can be opened and closed using a ligament and typically exhibits concentric growth rings, however these are not always obvious. Within its carapace the body is comprised of a small head and a large abdominal region. The head has a pair of compound eyes, a pair of short antennules, and a much longer pair of antennae covered with numerous bristles that, in addition to their legs, are used for swimming. The thorax is divided into 10�32 segments each with a pair of paddle-like legs, which are used for carrying food forward to the mouth and respiration by moving water over the gills. In females, the outer lobes of several middle legs are long and curved, modified to hold the eggs on the dorsal side of the body under the shell. The abdomen terminates in a telson (tail) that bears a pair of claw- or spine-like caudal rami. Clam shrimps range in size from 2�17 mm and are typically translucent to pale, although some species can be reddish due to haemoglobin in the haemolymph (body fluid). Their carapace can range from translucent to dark brown in colour. Originally classified in a single order, Conchostraca, which was proved to be paraphyletic, there are now three orders, five families and 19 genera. Cyclestherida, Laevicaudata and Spinicaudata are sometimes regarded as suborders, along with the Cladocera, in the order Diplostraca.
Distribution and diversity
Clam shrimp are a relatively small group of crustaceans of about 200 species found only in freshwater, inhabiting mostly temporary water bodies on all continents except for the polar regions. Geological records show that many now extinct species also inhabited marine environments. They occur Australia-wide with around 26 described species from four families; all but one species (Cyclestheria hislopi) are endemic.
Life cycle
Some species of clam shrimp reproduce sexually, some only parthenogenetically, while other species reproduce by both means and males are only infrequently found. During mating males attach to the ventral edge of the female�s carapace and extend their abdomen into her carapace to transfer a spermatophore. A female can lay up to several hundred eggs into an area between her back and carapace (brood pouch). In most species, eggs are released with each molt of the female where they sink to the sediment to hatch. Breeding is constant throughout the adult life of clam shrimps, and the female sheds eggs with every molt thus several broods are typically produced throughout the lifespan of the female, which can be several months or until the water is gone. With the exception of the Cyclestheriidae, where the eggs hatch in the brood pouch and develop into replicas of the adult before being released without a naupliar stage, the eggs of most species hatch as a nauplius larva that attains a carapace and quickly develops into an adult via several naupliar stages. Thoughout the season, �summer� eggs are laid which may develop parthenogenetically or be fertilised, possess a thin shell and develop rapidly. The �dry-season� or �winter� eggs which are usually laid at the end of the season are sexually produced and possess a thick shell (cysts), are capable of withstanding desiccation and remain dormant in the dried mud or sediment and can be dispersed by water or wind. These eggs are generated in smaller numbers and their production is stimulated by factors such as population density, temperature, and photoperiod. Nauplii from these eggs hatch in response to rain and temperature.
Feeding
Clam shrimp eat algae, detritus, bacteria, protozoa and rotifers. By rapidly moving their legs to produce a current, conchostracans draw tiny particles of food towards their mouth. In addition to filter feeding, they also graze, scrape and tear their food and will opportunistically scavenge on just about any other organism in their environment.
Ecology
Clam shrimp typically inhabit temporary water that fills seasonally such as ephemeral lakes, ponds and even livestock water troughs. However, a few species have also been recorded from permanent streams. Most are benthic animals, spending the majority of their time at the lowest levels of these water bodies and can frequently be found burrowed in the sediment at the bottom, only swimming during the mating period. They often swim in an upside-down position, in spiral or staggered movements. They are particularly important as a food source for migrating birds. The well-preserved carapaces of extinct species are often important to help identify the age of the corresponding strata in freshwater deposits that are otherwise typically poor in fossils.
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Conchostraca
Image credit: � Peter Serov,
Stygoecologia used with permission.
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Spinicaudata: Eocyzicus parooensis
Image credit: � Brian Timms, Honorary Research Associate,
Australian Museum, Sydney. Used with permission.
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Spinicaudata: Eoleptestheria ticinensis
Image credit: � Jean-Fran�ois Cart
used with permission
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Spinicaudata: Limandia badia
Image credit: � Brian Timms, Honorary Research Associate,
Australian Museum, Sydney. Used with permission.
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