Clam shrimp
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Clam shrimps comprise three orders of small branchiopod crustaceans. They were originally classified in a single order Conchostraca, which later proved to be artificial (paraphyletic). They have a two-part shell similar to that of a bivalve mollusc.
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[edit] Characteristics
Both valves of the shell are held together by a strong closing muscle. The crustaceans 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.
Opening the valves reveals the crustacean's structure. In most species the head is dorsoventrally compressed. The sessile compound eyes are close together and located on the forehead; in the genus Cyclestheria they are truly fused. In front of them is a simple naupliar eye. The first pair of antennae 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. Only in the common genus Lynceus, that can open its spherical valves wide, the thoracic legs move in an oarlike 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 of tadpole shrimps, 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 carrying food forward to the mouth and respiration. The gills are basically the outer lobes of all thoracic legs that are closest to the base of the leg. In order to carry out these functions, the legs are in constant movement, and the water between the valves of the carapace is quickly renewed. The body ends in a large chitinized telson, that is either laterally compressed and bears a pair of large hooks, or dorsoventrally compressed, with short hooks.
[edit] Ecology
Most clam shrimps are benthic animals. The largest member, Limnadia, which may reach 17 mm in length, is usually found lying on its side, while Cyzicus digs in the sediment, scraping the silt aside using its abdomen, and partly the legs. Members of these and other genera only swim during the mating period. Only the comparatively small (4-5 mm) Lynceus brachyurus usually swims with its back facing upwards and the shell opened, and rarely goes down to the bottom. Lynceus gracilicornis eats mostly planktonic organisms, while Cyzicus eats sediment along with the organisms inside it, and Limnadia grazes on microsopic algae that grow on the bottom and aquatic plants. Limnadia glues large, inedible particles together with a special secretion from special leg glands, and expels through the hind slit of its shell.
[edit] Reproduction and development
[edit] Reproduction
Clam shrimps have different reproductive strategies. All members of the family Limnadiidae reproduce parthenogenetically, no males among them are known. In other species the males are found occasionally (known as androdioecy). As one travels south, the males increase in numbers. During mating the male holds the female with hooks on his antennae and inserts his tail into the hind slit of her shell. The eggs are laid into a space between the female's back and her shell. In Cyclestheria certain hind thoracic segments are broadened, to form a brood pouch. After carrying the eggs in the pouch for a while, the female moults and the eggs sink to the bottom of the pool.
[edit] Life cycle
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 out as long as 7 years after the release.
When the egg arrives in a suitable pool, a larva hatches out at the nauplius stage. Clam shrimp nauplius is distinguished by very small front antennae. At the second stage - the metanauplius - the larva develops the small shell. They develop very quickly. For instance, Cyzicus reaches sexual maturity in 19 days after hatching.
[edit] Taxonomy
Clam shrimps belong to three orders, divided into five families and 19 genera.
- Cyclestherida
- Cyclestheriidae
- Cyclestheria
- Paracyclestheria
- Cyclestheriidae
- Laevicaudata
- Lynceidae
- Lynceiopsis
- Lynceus
- Paralimnetis
- Lynceidae
- Spinicaudata
- Cyzicidae
- Caenestheria
- Cyzicus
- Eocyzicus
- Leptestheriidae
- Eoleptestheria
- Leptestheria
- Leptestheriella
- Maghrebestheria
- Sewellestheria
- Limnadiidae
- Eulimnadia
- Imnadia
- Limnadia
- Limnadiopsis
- Limnadiopsidum
- Metalimnadia
- Cyzicidae
[edit] Geological history
Modern clam shrimps have no significance to humans. However, extinct species of these crustaceans are often studied by geologists. In freshwater deposits, generally poor in fossils, the well-preserved clam shrimp shells are found quite often. They help identify the age of the corresponding strata.
During the past geological periods clam shrimps were apparently more numerous and common than they are now. 300 extinct species are known, and twice as little living species. The oldest clam shrimps, such as Asmussia, had been found in Devonian deposits. What is remarkable is that many extinct species, especially Triassic ones, lived in the sea, where no clam shrimps remain today.
[edit] External links
[edit] References
L. A. Zenkevich. The Animal Life (Zhizn' Zhivotnykh), Volume 2. Chapter 7 - Phylum Arthropoda.
