Conchostraca
Encyclopedia
Clam shrimp are a taxon of bivalved branchiopod crustacean
s that resemble the unrelated bivalved molluscs
. They are extant, and known from the fossil record, from at least the Devonian period and perhaps before. They were originally classified in a single order Conchostraca, which later proved to be paraphyletic
(artificial).
. 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 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. In the common genus Lynceus, which can open its spherical valves wide, the thoracic 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 of tadpole 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 the carapace
is quickly renewed. The body ends in a large chitin
ised telson
, which is either laterally compressed and bears a pair of large hooks, or dorsoventrally compressed, with short hooks.
When the egg arrives in a suitable pool, a larva hatches out at the nauplius stage. 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.
s. 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 shrimp were apparently more numerous and common than they are now. 300 extinct species are known, and half as many living species. The oldest clam shrimp, such as Asmussia murchisoniana, were found in Devonian
deposits. Many extinct species, especially Triassic ones, lived in the sea, where no clam shrimp remain today.
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s that resemble the unrelated bivalved molluscs
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
. They are extant, and known from the fossil record, from at least the Devonian period and perhaps before. They were originally classified in a single order Conchostraca, which later proved to be paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
(artificial).
Characteristics
Both valves of the shell are held together by a strong closing muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
. 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 is dorsoventrally compressed. The sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
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
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
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 genus Lynceus, which can open its spherical valves wide, the thoracic 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 of tadpole 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 the carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
is quickly renewed. The body ends in a large chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ised telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
, which is either laterally compressed and bears a pair of large hooks, or dorsoventrally compressed, with short hooks.
Reproduction
Clam shrimp have different reproductive strategies. For example, within the family Limnadiidae are found dioecious (male-female), hermaphroditic (only hermaphrodites), and androdioecious (male-hermaphrodite) speciesLife 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 after as long as 7 years.When the egg arrives in a suitable pool, a larva hatches out at the nauplius stage. Clam shrimp nauplii are distinguished by very small front antennae. At the second stage (metanauplius
Metanauplius
Metanauplius is an early larval stage of some crustaceans such as krill. It follows the nauplius stage.In sac-spawning krill, there is an intermediary phase called pseudometanauplius, a newly hatched form distinguished from older metanauplii by its extremely short abdomen...
), the larva develops the small shell. They develop very quickly. For instance, Cyzicus reaches sexual maturity in 19 days after hatching.
Taxonomy
Extant clam shrimp belong to three orders, divided into five families and 19 genera.- Cyclestherida Sars, 1899
- Cyclestheriidae Sars, 1899
- Cyclestheria
- Paracyclestheria
- Cyclestheriidae Sars, 1899
- Laevicaudata Linder, 1945
- Lynceidae Baird, 1845
- Lynceiopsis
- Lynceus
- Paralimnetis
- Lynceidae Baird, 1845
- Spinicaudata Linder, 1945
- Cyzicidae Stebbing, 1910
- Caenestheria
- Cyzicus
- Eocyzicus
- Leptestheriidae Daday, 1923
- EoleptestheriaEoleptestheriaEoleptestheria ticinensis is a species of clam shrimp in the family Leptestheriidae. Although up to eight species have formerly been recognised in the genus Eoleptestheria, they are more usually all considered part of one species with a cosmopolitan distribution....
- Leptestheria
- Leptestheriella
- Maghrebestheria
- Sewellestheria
- Eoleptestheria
- Limnadiidae Baird, 1849
- Eulimnadia
- ImnadiaImnadiaImnadia is a genus of crustacean in family Limnadiidae. It contains at least the following species:* Imnadia banatica* Imnadia cristata* Imnadia panonica...
- Limnadia
- Limnadiopsis
- Limnadiopsidum
- Metalimnadia
- Cyzicidae Stebbing, 1910
Geological history
Modern clam shrimp have no significance to humans. However, extinct species of these crustaceans are often studied by geologistGeologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
s. In freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
deposits, generally poor in fossils, the well-preserved clam shrimp shells are found quite often. They help identify the age of the corresponding strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
.
During the past geological periods clam shrimp were apparently more numerous and common than they are now. 300 extinct species are known, and half as many living species. The oldest clam shrimp, such as Asmussia murchisoniana, were found in Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
deposits. Many extinct species, especially Triassic ones, lived in the sea, where no clam shrimp remain today.