Conchostraca

(invertebrate zoology) An order of mussellike crustaceans of moderate size belonging to the subclass Branchiopoda.

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(Clam shrimps)

Phylum: Arthropoda

Subphylum: Crustacea

Class: Branchiopoda

Order: Conchostraca

Number of families: 5

Thumbnail description
Shrimps that possess a bivalved shell with a hinge that envelops the entire body and limbs; they are found exclusively in freshwater

Evolution and systematics

The fossil record of the Conchostraca is one of the oldest among the Branchiopoda, extending from the Lower Devonian. This record consists mostly of fossils of carapaces, and since the major diagnostic features of the clam shrimps relate to this carapace, these fossils are easily classified as conchostracans. Nevertheless, these characteristics do not provide details of the evolution within the group.

The Conchostraca and Cladocera belong to the subclass Diplostraca, which, together with the subclasses Calamanostraca and Sarsostraca, form the three living groups of the class Branchiopoda; two fossil groups, Kazacharthra and Lipostraca, also belong to this class. The term Branchiopoda literally means "gilled feet," and branchiopods are consequently said to breathe through their feet, which are leaflike and divided into lobes, each containing a gill plate. The presence of gills on the feet of the Branchiopoda is almost the only common characteristic among the diverse members of this group, although minor similarities can be found in the organization of the trunk segments and trunk limbs.

In 1986 the conchostracans were divided into five families of bivalve crustaceans: Lynceidae, Limnadiidae, Cyclestheriidae, Cyzicidae, and Leptestheriidae. However, the relationships among these families are not well understood and many authors believe that these relationships may not even exist. In fact, the validity of the term Conchostraca as a taxonomic name is debatable, since no evidence has been found that supports a monophyletic origin for clam shrimps. Nevertheless, up to the present, no author has presented a phylogenetically supported or generally accepted reclassification for this group.

Physical characteristics

Conchostracans are commonly known as clam shrimp due to their outward resemblance to bivalve mollusks. All conchostracans are laterally flattened and possess a bivalve carapace, joined by a dorsal hinge or fold with the two halves connected and controlled by a strong adductor muscle. This carapace covers the entire body and limbs (or almost), and, in most cases, is marked by concentric lines of growth. The trunk is divided into 10–32 segments, each with a pair of appendages. The second pair of antennae is well developed and biramous, and the compound eyes are sessile. Conchostracans can be distinguished from other groups of the Branchiopoda because their bodies are completely enclosed within the carapace and because they possess a comparatively reduced abdomen. They range in size from a few millimeters to up to 0.7 in (1.7 cm). Their color is usually translucent to pale, although some species might present a pink, reddish, or orange coloration because of the presence of hemoglobin in the haemolymph; the shell color varies from translucent to dark brown.

Distribution

Modern conchostracans occur exclusively in freshwater bodies on all continents, except Antarctica and northern polar regions. Some extinct species apparently inhabited marine habitats during the Devonian and Carboniferous.

Habitat

Clam shrimps are commonly found in temporary water bodies such as ephemeral ponds and troughs. Nevertheless, there have been reports of members of the family Lynceidae in prairie streams, while the family Cyclestheriidae mainly inhabits permanent bodies of water that are always associated with a thick algal mat. Clam shrimps spend most of their time on the bottom of these water bodies filtering nutritious particles, and they can frequently be found burrowed in the mud.

Behavior

Clam shrimps use their second antennae in addition to their legs for swimming, sometimes in an upside-down position, performing spiral or staggered movements. They can reproduce sexually, asexually (via parthenogenesis), or by both means. The females carry several hundred eggs attached to a specialized appendix; these eggs are usually shed when the female molts, although in some species, the eggs hatch in a brood pouch attached to the carapace. Some species produce drought-resistant eggs that can be dispersed by water or wind and are widely distributed.

Feeding ecology and diet

Conchostracans are generally acknowledged as filter feeders, but they can also scrape and tear at their food, and will scavenge almost any organism in their environment. Some species also scrape materials from rocks or other substrates. Conchostracans use their forefeet to collect food, while the hind appendages are modified as mandibles for grinding large food particles.

Reproductive biology

During copulation, the male clam shrimp grasps the ventral edge of the female shell and deposits a spermatophore inside the female's carapace by extending his abdomen. The eggs are brooded inside the female's shell and are shed at the time of the female molt. Breeding is constant throughout the adult life of clam shrimps, and the female sheds eggs with every molt. Conchostracans usually produce two kinds of eggs: vegetative and asexual eggs, known as summer eggs, and more resistant, dry-season or winter eggs that are usually sexually produced. The summer eggs possess a thin shell and they might develop parthenogenetically or be fertilized; their development is rapid and they may hatch while still attached to the female. The winter eggs possess a thick shell and, in some species, they can remain dormant for long periods of time. Winter eggs are produced in lesser numbers and their production might be stimulated by external factors such as population density, temperature, and photoperiod. With the exception of the family Cyclestheriidae, the eggs of clam shrimps hatch as a nauplius larva that develops quickly into an adult without undergoing metamorphosis, although the acquisition of the carapace gives a false impression of metamorphosis. The family Cyclestheriidae reproduces mostly by parthenogenesis, with the eggs hatching in a brood pouch attached to the female and developing into replicas of the adult before being released without a naupliar stage.

Conservation status

No species of clam shrimp are listed by the IUCN. Some endemic species have limited distributions, but none have been considered endangered so far, probably because this group has been poorly studied.

Significance to humans

There is no known significance to humans.

Species accounts

Resources

Bliss, Dorothy E. Biology of the Crustacea. New York: Academic Press, 1982–1985.

Schram, Frederick R. Crustacea. New York: Oxford University Press, 1986.

Martin, J. W., B. E. Felgenhauer, and L. G. Abele. "Redescription of the Clam Shrimp Lynceus gracilicornis (Packard) (Branchiopoda, Conchostraca, Lynceidae) from Florida, with Notes on Its Biology." Zoologica Scripta 15, no. 3 (1986): 221–232.

Olesen, J., J. W. Martin, and E. W. Roessler. "External Morphology of the Male of Cyclestheria hislopi (Baird, 1859) (Crustacea, Branchiopoda, Spinicaudata) with a Comparison of Male Claspers Among the Conchostraca and Cladocera and Its Bearing on Phylogeny of the Bivalved Branchiopoda." Zoologica Scripta 25, no. 4 (1996): 291–316.

Boyce, Sarah Lyn. "The Conchostraca." February 12, 2003 [July 24, 2003].

Eder, Erich. "Large Branchiopods—Living Fossils!" June 24, 2003 [July 24, 2003].

"Introduction to the Branchiopoda." University of California Museum of Paleontology. July 1, 2003 [July 24, 2003].

[Article by: Johana Rincones, PhD; Alberto Arab, PhD]