Clam shrimp are a group of bivalved branchiopod

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s that resemble the unrelated bivalved molluscs. They are extant and also known from the fossil record, from at least the Devonian period and perhaps before. They were originally classified in the former order Conchostraca, which later proved to be

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, because water fleas are nested within clam shrimps. Clam shrimp are now divided into three orders, Cyclestherida, Laevicaudata, and Spinicaudata, in addition to the fossil family Leaiidae.

Characteristics

Both valves of the shell are held together by a strong closing

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

. 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 eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

s 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 The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main ...

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 A carapace is a dorsal (upper) 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 unde ...

is quickly renewed. The body ends in a large

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

ised telson, which is either laterally compressed and bears a pair of large hooks, or dorsoventrally compressed, with short hooks.

Reproduction and development

Reproduction

Clam shrimp have different reproductive strategies. For example, within the family Limnadiidae are found

dioecious Dioecy ( ; ; adj. dioecious, ) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is ...

(male-female), hermaphroditic (only hermaphrodites), and androdioecious (male-hermaphrodite) species.

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 after as long as 7 years. When the egg arrives in a suitable pool, a larva hatches out at the nauplius stage (the nauplius stage is absent in Cyclestherida). 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.

Taxonomy

Extant clam shrimp belong to three orders, divided into five families; some notable genera and

prehistoric Prehistory, also called pre-literary history, is the period of human history between the first known use of stone tools by hominins million years ago and the beginning of recorded history with the invention of writing systems. The use o ...

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

are also listed: Spinicaudata Linder, 1945 * Cyzicidae Stebbing, 1910 ** '' Caenestheria'' ** '' Caenestheriella'' ** ''

Cyzicus Cyzicus ( ; ; ) was an ancient Greek town in Mysia in Anatolia in the current Balıkesir Province of Turkey. It was located on the shoreward side of the present Kapıdağ Peninsula (the classical Arctonnesus), a tombolo which is said to have or ...

'' ** '' Eocyzicus'' * Eosestheriidae Zhang & Chen, 1976 (

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

) ** '' Bairdestheria'' ** '' Menucoestheria'' Gallego & Covacevich, 1998 * Euestheridae Defretin, 1965 (

) ** '' Laxitextella'' Kozur, 1982 * Leptestheriidae Daday, 1923 ** '' Eoleptestheria'' ** '' Leptestheria'' ** '' Leptestheriella'' ** '' Maghrebestheria'' ** '' Sewellestheria'' * Limnadiidae Baird, 1849 (including Imnadiidae) ** '' Afrolimnadia'' ** '' Calalimnadia'' ** '' Eulimnadia'' ** '' Imnadia'' ** '' Limnadia'' ** '' Limnadiopsis'' ** '' Limnadiopsidum'' ** '' Metalimnadia'' * Palaeolimnadiidae Tasch, 1956 (

) ** '' Krasiestheria'' Olempska, 2004 (tentatively placed here) Cyclestherida Sars, 1899 * Cyclestheriidae Sars, 1899 ** '' Cyclestheria'' ** '' Paracyclestheria'' Laevicaudata Linder, 1945 * Lynceidae Baird, 1845 ** '' Leptestheriella'' ** '' Limnetis'' ** '' Lynceiopsis'' ** ''

Lynceus In Greek mythology, Lynceus (; Ancient Greek: Λυγκεύς "lynx-eyed") may refer to the following personages. * Lynceus, one of the 50 Sons of Aegyptus. * Lynceus, king in Thrace and husband of Lathusa, friend of Procne. Tereus gave Philomel ...

'' ** '' Paralimnetis''

Geological history

Modern clam shrimp have little significance to humans. However, extinct species of these crustaceans are often studied by

geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

s. In

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

deposits, generally poor in fossils, the well-preserved clam shrimp shells are found quite often. They help identify the age of the corresponding

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

. 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 as '' Asmussia murchisoniana'', were found in

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

deposits. Many extinct species, mostly Triassic specimens, once lived in marine environments, where no extant clam shrimp inhabit today.

References

External links

Introduction to the Branchiopoda
* * * {{Taxonbar, from=Q500917 Branchiopoda Extant Devonian first appearances Arthropod common names