Ostracod

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Ostracod
Temporal range: Ordovician to Recent, 450–0  Ma
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Ostracod.JPG
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Euarthropoda
Subphylum: Crustacea
Superclass: Oligostraca
Class:Ostracoda
Latreille, 1802
Subclasses and Orders

Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, [1] grouped into several orders. They are small crustaceans, typically around 1 mm (0.039 in) in size, but varying from 0.2 to 30 mm (0.008 to 1.181 in) in the case of Gigantocypris . Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell". The hinge of the two valves is in the upper (dorsal) region of the body. Ostracods are grouped together based on gross morphology. While early work indicated the group may not be monophyletic; [2] and early molecular phylogeny was ambiguous on this front, [3] recent combined analyses of molecular and morphological data found support for monophyly in analyses with broadest taxon sampling [4]

In biological classification, class is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. Other well-known ranks in descending order of size are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the prefix sub-: subclass. For example, dogs are in the class Mammalia.

Crustacean subphylum of arthropods

Crustaceans form a large, diverse arthropod taxon which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill, woodlice, and barnacles. The crustacean group is usually treated as a subphylum, and because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, and comprises all animals in the Pancrustacea clade other than hexapods. Some crustaceans are more closely related to insects and other hexapods than they are to certain other crustaceans.

<i>Gigantocypris</i> genus of crustaceans

Gigantocypris is a genus of ostracod crustaceans in family Cypridinidae, and probably the most famous ostracod in the world. Its members are extremely large for ostracods, measuring up to 3.2 cm (1.3 in) across, have a globular shape, are typically semi-transparent orange or reddish, and have relatively large mirror-like eyes that are used to locate their small animal prey. They are found worldwide in dark, deep and cold oceans.

Contents

Ecologically, marine ostracods can be part of the zooplankton or (most commonly) are part of the benthos, living on or inside the upper layer of the sea floor. Many ostracods, especially the Podocopida, are also found in fresh water, and terrestrial species of Mesocypris are known from humid forest soils of South Africa, Australia and New Zealand. [5] They have a wide range of diets, and the group includes carnivores, herbivores, scavengers and filter feeders.

Zooplankton Heterotrophic protistan or metazoan members of the plankton ecosystem

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word zooplankton is derived from the Greek zoon (ζῴον), meaning "animal", and planktos (πλαγκτός), meaning "wanderer" or "drifter". Individual zooplankton are usually microscopic, but some are larger and visible to the naked eye.

Benthos community of organisms which live on, in, or near the seabed

Benthos is the community of organisms that live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.

The Podocopida are an order of ostracods in the subclass Podocopa. It is divided into five suborders – Bairdiocopina, Cypridocopina, Cytherocopina, Darwinulocopina, and Sigilliocopina. It is the most diverse of the four orders of ostracods, and also has a rich fossil record.

As of 2008, around 2000 species and 200 genera of nonmarine ostracods are found. [6] However, a large portion of diversity is still undescribed, indicated by undocumented diversity hotspots of temporary habitats in Africa and Australia. [7] Of the known specific and generic diversity of nonmarine ostracods, half (1000 species, 100 genera) belongs to one family (of 13 families), Cyprididae. [7] Many Cyprididae occur in temporary water bodies and have drought-resistant eggs, mixed/parthenogenetic reproduction, and the ability to swim. These biological attributes preadapt them to form successful radiations in these habitats. [8]

Cyprididae is "the most diverse group of freshwater ostracods". It contains 1000 species, which represents 50% of the known species of freshwater ostracods. Around 60% of genera in the family are endemic to a single zoogeographic region. The family contains 25 subfamilies, and is most diverse in the Afrotropic ecozone, with over 300 species in 45 genera. Many Cyprididae occur in temporary water bodies and have drought-resistant eggs, mixed/parthenogenetic reproduction and ability to swim. These biological attributes pre-adapt them to form successful radiations in these habitats. Bennelongia is an interesting genus of the family Cyprididae. It may be the last true descendent genus of the Mesozoic lineage of Cypridea, which was a dominant lineage of ostracod in non-marine waters in the Cretaceous.

Etymology

Ostracod comes from the Greek óstrakon meaning shell or tile. The word "ostracize" comes from the same root due to the practice of voting with shells or potsherds. [9]

Fossils

The large ostracod Herrmannina from the Silurian (Ludlow) Soeginina Beds (Paadla Formation) on eastern Saaremaa Island, Estonia HerrmanninaSilurianEstonia.jpg
The large ostracod Herrmannina from the Silurian (Ludlow) Soeginina Beds (Paadla Formation) on eastern Saaremaa Island, Estonia

Ostracods are "by far the most common arthropods in the fossil record" [10] with fossils being found from the early Ordovician to the present. An outline microfaunal zonal scheme based on both Foraminifera and Ostracoda was compiled by M. B. Hart. [11] Freshwater ostracods have even been found in Baltic amber of Eocene age, having presumably been washed onto trees during floods. [12]

The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.

Microfauna

Microfauna refers to microscopic organisms that exhibit animal-like qualities. Microfauna are represented in the animal kingdom and the protist kingdom. This is in contrast to microflora which, together with microfauna, make up the microzoa.

Foraminifera phylum of amoeboid protists

Foraminifera are members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an external shell of diverse forms and materials. Tests of chitin are believed to be the most primitive type. Most foraminifera are marine, the majority of which live on or within the seafloor sediment, while a smaller variety float in the water column at various depths. Fewer are known from freshwater or brackish conditions, and some very few (nonaquatic) soil species have been identified through molecular analysis of small subunit ribosomal DNA.

Ostracods have been particularly useful for the biozonation of marine strata on a local or regional scale, and they are invaluable indicators of paleoenvironments because of their widespread occurrence, small size, easily preservable, generally moulted, calcified bivalve carapaces; the valves are a commonly found microfossil.

Biostratigraphy

Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at some other section. The fossils are useful because sediments of the same age can look completely different because of local variations in the sedimentary environment. For example, one section might have been made up of clays and marls while another has more chalky limestones, but if the fossil species recorded are similar, the two sediments are likely to have been laid down at the same time.

A find in Queensland, Australia in 2013, announced in May 2014, at the Bicentennary Site in the Riversleigh World Heritage area, revealed both male and female specimens with very well preserved soft tissue. This set the Guinness World Record for the oldest penis. [13] Males had observable sperm that is the oldest yet seen and, when analysed, showed internal structures and has been assessed as being the largest sperm (per body size) of any animal recorded. It was assessed that the fossilisation was achieved within several days, due to phosphorus in the bat droppings of the cave where the ostracods were living. [14]

Description

Anatomy of Cypridina mediterranea CypridinaMediterranea.png
Anatomy of Cypridina mediterranea

The body of an ostracod is encased by two valves, superficially resembling the shell of a clam. A distinction is made between the valve (hard parts) and the body with its appendages (soft parts).

Body parts

The body consists of a head and thorax, separated by a slight constriction. Unlike many other crustaceans, the body is not clearly divided into segments. The abdomen is regressed or absent, whereas the adult gonads are relatively large.

The head is the largest part of the body, and bears most of the appendages. Two pairs of well-developed antennae are used to swim through the water. In addition, there is a pair of mandibles and two pairs of maxillae. The thorax typically has two pairs of appendages, but these are reduced to a single pair, or entirely absent, in many species. The two "rami", or projections, from the tip of the tail, point downwards and slightly forward from the rear of the shell. [15]

Ostracods typically have no gills, instead taking in oxygen through branchial plates on the body surface. Most ostracods have no heart or circulatory system, and blood simply circulates between the valves of the shell. Nitrogenous waste is excreted through glands on the maxillae, antennae, or both. [15]

The primary sense of ostracods is likely touch, as they have several sensitive hairs on their bodies and appendages. However, they do possess a single naupliar eye, and, in some cases, a pair of compound eyes, as well. [15]

Palaeoclimatic reconstruction

Articulated ostracod valves in cross-section from the Permian of central Texas; typical thin section view of an ostracod fossil PermianOstracod.jpg
Articulated ostracod valves in cross-section from the Permian of central Texas; typical thin section view of an ostracod fossil

A new method is in development called mutual ostracod temperature range (MOTR), similar to the mutual climatic range (MCR) used for beetles, which can be used to infer palaeotemperatures. [16] The ratio of oxygen-18 to oxygen-16 (δ18O) and the ratio of magnesium to calcium (Mg/Ca) in the calcite of ostracod valves can be used to infer information about past hydrological regimes, global ice volume and water temperatures.

Ecology

Lifecycle

Male ostracods have two penii, corresponding to two genital openings (gonopores) on the female. The individual sperm are often large, and are coiled up within the testis prior to mating; in some cases, the uncoiled sperm can be up to six times the length of the male ostracod itself. Mating typically occurs during swarming, with large numbers of females swimming to join the males. Some species are partially or wholly parthenogenetic. [15]

In most ostracods, eggs are either laid directly into the water as plankton, or are attached to vegetation or the substratum. However, in some species, the eggs are brooded inside the shell, giving them a greater degree of protection. The eggs hatch into nauplius larvae, which already have a hard shell. [15]

Predators

A variety of fauna prey upon ostracods in both aquatic and terrestrial environments. An example of predation in the marine environment is the action of certain cuspidariid clams in detecting ostracods with cilia protruding from inhalant structures, thence drawing the ostracod prey in by a violent suction action. [17] Predation from higher animals also occurs; for example, amphibians such as the rough-skinned newt prey upon certain ostracods. [18]

Bioluminescence

Some ostracods, such as Vargula hilgendorfii , have a light organ in which they produce luminescent chemicals. [19] Most use the light as predation defense, while some use the light for mating (only in the Caribbean). These ostracods are called "blue sand" or "blue tears" and glow blue in the dark at night. Their bioluminescent properties made them valuable to the Japanese during World War II, when the Japanese army collected large amounts from the ocean to use as a convenient light for reading maps and other papers at night. The light from these ostracods, called umihotaru in Japanese, was sufficient to read by but not bright enough to give away troops' position to enemies. [20]

Related Research Articles

Malacostraca Largest class of crustaceans

Malacostraca is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, woodlice, amphipods, mantis shrimp and many other, less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments, and divided into a head, thorax, and abdomen.

Isopoda order of arthropods

Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax. Isopods have various feeding methods: some eat dead or decaying plant and animal matter, others are grazers, or filter feeders, a few are predators, and some are internal or external parasites, mostly of fishes. Aquatic species mostly live on the seabed or bottom of freshwater bodies of water, but some taxa can swim for a short distance. Terrestrial forms move around by crawling and tend to be found in cool, moist places. Some species are able to roll themselves into a ball as a defence mechanism or to conserve moisture. There are over 10,000 species of isopod worldwide, with around 4,500 species found in marine environments, mostly on the seabed, 500 species in fresh water, and another 5,000 species on land. The order is divided into eleven suborders. The fossil record of isopods dates back to the Carboniferous period, at least 300 million years ago, when isopods lived in shallow seas. The name Isopoda is derived from the Greek roots iso- and -pod.

Traditionally, the Myodocopa and Podocopa have been classified as subclasses within the class Ostracoda, although there is some question about how closely related the two groups actually are. The Myodocopa are defined by possession of a poorly calcified carapace, and 8–9 articles in the exopod of the second antenna. The ventral margin of the carapace is not concave, and the valves do not overlap to a great extent.

Gonopod

Gonopods are specialized appendages of various arthropods used in reproduction or egg-laying. In males, they facilitate the transfer of sperm from male to female during mating, and thus are a type of intromittent organ. In crustaceans and millipedes, gonopods are modified walking or swimming legs. Gonopods may be highly decorated with elaborate structures which may play roles in sperm competition, and can be used to differentiate and identify closely related species. Gonopods generally occur in one or more pairs, as opposed to the single (un-paired) reproductive organs such as the aedeagus of insects or the penis of harvestmen.

Kapcypridopsis is a genus of ostracod crustaceans in the family Cyprididae, subfamily Cypridopsinae. It includes the critically endangered species Kapcypridopsis barnardi.

Liocypris grandis is a species of ostracod which was long presumed extinct. It was rediscovered in the Western Cape of South Africa in 2003, having not been seen since its original description by Georg Ossian Sars in 1924. It was assessed as extinct for the IUCN Red List in 1996, and that assessment has not been updated.

Crustacean larva crustacean larval and immature stages between hatching and adult form

Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow. The larvae of crustaceans often bear little resemblance to the adult, and there are still cases where it is not known what larvae will grow into what adults. This is especially true of crustaceans which live as benthic adults, more so than where the larvae are planktonic and therefore more easily caught.

Candonidae is a family of ostracods, containing around 25% of all known species of freshwater ostracods. Around 75% of genera in the family are endemic to a single zoogeographic region. It contains more than 500 species, of which more than 300 are endemic to the Palaearctic ecozone.

Bennelongia is a genus of ostracod crustaceans in the family Cyprididae. It is probably endemic to Australia and New Zealand, and is predicted to be highly diverse. The genus was described in 1981 and named after Woollarawarre Bennelong, the first aboriginal to have a long association with the early European settlers of Australia. Prior to 2012, six species were described in Australia. There are currently 15 species of Bennelongia. Bennelongia may be the last true descendent genus of the Mesozoic lineage of Cypridea, which was a dominant lineage of ostracod in non-marine waters in the Cretaceous.

Potamocypris is a genus of ostracod crustaceans in the family Cyprididae. There are currently 42 extant species of Potamocypris. The majority of the species occur in freshwater habitats; only a few species of the genus colonize marine brackish coastal waters.

<i>Potamocypris mastigophora</i> species of crustacean

Potamocypris mastigophora is a species of ostracod crustacean in the family Cyprididae, subfamily Cypridopsinae. It is known from Africa and the southern areas of the Palaearctic.

Potamocypris variegata is a species of ostracod crustaceans in the family Cyprididae, subfamily Cypridopsinae. It is mainly found in ponds with rich aquatic vegetation, more rarely in slowly flowing streams. The species is distributed throughout Europe, but is also known from North America.

Potamocypris unicaudata is a species of ostracod crustacean in the family Cyprididae, subfamily Cypridopsinae. It is abundantly found in ditches and ponds near the sea shore, where freshwater slightly mingles with sea water. It is known from both Europe and North America.

Potamocypris smaragdina is a species of ostracod crustacean in the family Cyprididae, subfamily Cypridopsinae. It is known from both Europe and North America.

<i>Potamocypris arcuata</i> species of crustacean

Potamocypris arcuata is a species of ostracod crustacean in the family Cyprididae, subfamily Cypridopsinae. It is mainly known from the southern areas of the Palaearctic.

Chlamydotheca is a genus of freshwater ostracods in the family Cyprididae. About 36 species are known to occur throughout continental waters. Four species are found in Argentina.

References

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  9. Definition of ostracod at dictionary.com.
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  12. Noriyuki Ikeya, Akira Tsukagoshi & David J. Horne (2005). Noriyuki Ikeya; Akira Tsukagoshi & David J. Horne, eds. "Evolution and Diversity of Ostracoda. Theme 2 of the 14th International Symposium on Ostracoda (ISO 2001), Shizuoka, Japan". Hydrobiologia . 538 (1–3): vii–xiii. doi:10.1007/s10750-004-4914-z.|chapter= ignored (help)
  13. Oldest penis:
    The oldest fossilised penis discovered to date dates back around 100 million years. It belongs to a crustacean called an ostracod, discovered in Brazil and measuring just 1mm across.
  14. World's oldest sperm 'preserved in bat poo', Anna Salleh, ABC Online Science, 14 May 2014, accessed 15 May 2014
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  20. Jabr, Ferris. "The Secret History of Bioluminescence". Hakai Magazine. Retrieved 6 July 2016.