| Clam shrimp | |
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| California clam shrimp,Cyzicus californicus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Branchiopoda |
| Subclass: | Phyllopoda |
| Superorder: | Diplostraca |
| Orders of clam shrimp | |
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Clam shrimp are a group of bivalvedbranchiopodcrustaceans that resemble the unrelatedbivalved molluscs.[1] They are extant and also known from the fossil record, from at least the Devonian period and perhaps before.[2] They were originally classified in the formerorderConchostraca, which later proved to beparaphyletic, becausewater fleas are nested within clam shrimps. Clam shrimp are now divided into three orders,Cyclestherida,Laevicaudata, andSpinicaudata, in addition to the fossil family Leaiidae.[3][4]
Both valves of the shell are held together by a strong closingmuscle. The animals react to danger by contracting the muscle, so that the valves close tightly and the crustacean, as if dead, lies motionlessly at the bottom of the pool.
In most species the head isdorsoventrally compressed. Thesessilecompound eyes are close together and located on the forehead; in the genusCyclestheria they are truly fused. In front of them is a simplenaupliar eye. The first pair ofantennae is reduced and unsegmented. The second pair of antennae, however, is long and biramous. Both branches are covered with numerous bristles. The crustaceans swim primarily by swooping the antennae. In the common genusLynceus, which can open its spherical valves wide, thethoracic legs move in an oar-like manner along with the antennae.
The number of segments constituting the thorax varies from 10 to 32, and the number of legs varies accordingly. They are similar in structure to the legs oftadpole shrimp, and similarly, their size decreases from front to back. In females, the outer lobes of several middle legs are modified into long, upward-bending threadlike outgrowths, used to hold the eggs on the dorsal side of the body under the shell. However, the main functions of the thoracic legs are respiration and carrying food forward to the mouth. The gills are basically the outer lobes of all thoracic legs that are closest to the base of the leg. The legs are in constant movement, and the water between the valves of thecarapace is quickly renewed. The body ends in a largechitinisedtelson, which is eitherlaterally compressed and bears a pair of large hooks, or dorsoventrally compressed, with short hooks.
Clam shrimp have different reproductive strategies. For example, within the familyLimnadiidae are founddioecious (male-female),hermaphroditic (only hermaphrodites), andandrodioecious (male-hermaphrodite) species.
The eggs are surrounded by a tough shell and can withstand drying out, freezing and other hostile conditions. In some species these eggs can hatch after as long as 7 years.
When the egg arrives in a suitable pool, a larva hatches out at thenauplius stage (the nauplius stage is absent in Cyclestherida).[5] Clam shrimp nauplii are distinguished by very small front antennae. At the second stage (metanauplius), the larva develops the small shell. They develop very quickly. For instance,Cyzicus reaches sexual maturity in 19 days after hatching.
Extant clam shrimp belong to three orders, divided into five families;[6] some notable genera andprehistorictaxa are also listed:
SpinicaudataLinder, 1945
| CyclestheridaSars, 1899
LaevicaudataLinder, 1945
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Modern clam shrimp have little significance to humans. However, extinct species of these crustaceans are often studied bygeologists. Infreshwater deposits, generally poor in fossils, the well-preserved clam shrimp shells are found quite often. They help identify the age of the correspondingstrata.[7]
During the past geological periods clam shrimp were apparently more numerous and diverse than they are now. 300 extinct species are known, and half as many living species. The oldest clam shrimp, such asAsmussia murchisoniana, were found inDevonian deposits. Many extinct species, mostly Triassic specimens, once lived in marine environments, where no extant clam shrimp inhabit today.
Data related toCyclestherida at Wikispecies Data related toLaevicaudata at Wikispecies Data related toSpinicaudata at WikispeciesFamilies of classBranchiopoda | ||
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| Anostraca |  | |
| Notostraca | ||
| Laevicaudata | ||
| Spinicaudata | ||
| Cyclestherida | ||
| Cladocera | ||
