Evolution and systematics
The fossil report of the Conchostraca is likely one of the oldest among the many Branchiopoda, extending from the Lower Devonian. This report consists largely of fossils of carapaces, and for the reason that main diagnostic options of the clam shrimps relate to this carapace, these fossils are simply labeled as conchostracans. Nevertheless, these traits don’t present particulars of the evolution throughout the group. The Conchostraca and Cladocera belong to the subclass Diplostraca, which, along with the subclasses Calamanostraca and Sarsostraca, type the three dwelling teams of the category Branchiopoda; two fossil teams, Kazacharthra and Lipostraca, additionally belong to this class. The time period Branchiopoda actually means “gilled feet,” and branchiopods are consequently stated to breathe by means of their ft, that are leaflike and divided into lobes, every containing a gill plate. The presence of gills on the ft of the Branchiopoda is nearly the one frequent attribute among the many numerous members of this group, though minor similarities may be discovered within the group of the trunk segments and trunk limbs. In 1986 the conchostracans have been divided into 5 households of bivalve crustaceans: Lynceidae, Limnadiidae, Cyclestheriidae, Cyzicidae, and Leptestheriidae. However, the relationships amongst these households usually are not nicely understood and plenty of authors consider that these relationships might not even exist. In reality, the validity of the time period Conchostraca as a taxonomic title is debatable, since no proof has been discovered that helps a monophyletic origin for clam shrimps. Nevertheless, up to the current, no writer has offered a phylogenetically supported or typically accepted reclassification for this group.
Conchostracans are generally often 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 2 halves linked and managed by a powerful adductor muscle. This carapace covers the complete physique and limbs (or virtually), and, usually, is marked by concentric strains of development. The trunk is split into 10–32 segments, every with a pair of appendages. The second pair of antennae is nicely developed and biramous, and the compound eyes are sessile. Conchostracans may be distinguished from different teams of the Branchiopoda as a result of their our bodies are fully enclosed throughout the carapace and since they possess a relatively lowered stomach. They vary in measurement from just a few millimeters to up to 0.7 in (1.7 cm). Their shade is normally translucent to pale, though some species would possibly current a pink, reddish, or orange coloration due to the presence of hemoglobin within the haemolymph; the shell shade varies from translucent to darkish brown.
Modern conchostracans happen completely in freshwater our bodies on all continents, besides Antarctica and northern polar areas. Some extinct species apparently inhabited marine habitats throughout the Devonian and Carboniferous.
Clam shrimps are generally present in non permanent water our bodies akin to ephemeral ponds and troughs. Nevertheless, there have been stories of family members Lynceidae in prairie streams, whereas the household Cyclestheriidae primarily inhabits everlasting our bodies of water which are all the time related to a thick algal mat. Clam shrimps spend most of their time on the underside of these water our bodies filtering nutritious particles, they usually can regularly be discovered burrowed within the mud.
Clam shrimps use their second antennae as well as to their legs for swimming, generally in an upside-down place, performing spiral or staggered actions. They can reproduce ####ually, a####ually (by way of parthenogenesis), or by each means. The females carry a number of hundred eggs connected to a specialised appendix; these eggs are normally shed when the feminine molts, though in some species, the eggs hatch in a brood pouch connected to the carapace. Some species produce drought-resistant eggs that may be dispersed by water or wind and are broadly distributed.
Feeding ecology and eating regimen
Conchostracans are typically acknowledged as filter feeders, however they’ll additionally scrape and tear at their food, and can scavenge virtually any organism of their atmosphere. Some species additionally scrape supplies from rocks or different substrates. Conchostracans use their forefeet to gather food, whereas the hind appendages are modified as mandibles for grinding massive food particles.
During copulation, the male clam shrimp grasps the ventral fringe of the feminine shell and deposits a spermatophore inside the feminine’s carapace by extending his stomach. The eggs are brooded inside the feminine’s shell and are shed on the time of the feminine molt. Breeding is fixed all through the grownup lifetime of clam shrimps, and the feminine sheds eggs with each molt. Conchostracans normally produce two sorts of eggs: vegetative and a####ual eggs, often known as summer season eggs, and extra resistant, dry-season or winter eggs which are normally ####ually produced. The summer season eggs possess a skinny shell they usually would possibly develop parthenogenetically or be fertilized; their improvement is speedy they usually might hatch whereas nonetheless connected to the feminine. The winter eggs possess a thick shell and, in some species, they’ll stay dormant for lengthy durations of time. Winter eggs are produced in lesser numbers and their manufacturing could be stimulated by exterior components akin to inhabitants density, temperature, and photoperiod. With the exception of the household Cyclestheriidae, the eggs of clam shrimps hatch as a nauplius larva that develops rapidly into an grownup with out present process metamorphosis, though the acquisition of the carapace offers a misunderstanding of metamorphosis. The household Cyclestheriidae reproduces largely by parthenogenesis, with the eggs hatching in a brood pouch connected to the feminine and creating into replicas of the grownup earlier than being launched with no naupliar stage.
No species of clam shrimp are listed by the IUCN. Some endemic species have restricted distributions, however none have been thought of endangered up to now, in all probability as a result of this group has been poorly studied.