Ostracod
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| Class: | Ostracoda Latreille, 1802
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Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species (13,000 of which are extant taxa) have been identified[1], grouped into several orders. This group may not be monophyletic[2]. Ostracod taxa are grouped into a Class based on gross morphology. Their molecular phylogeny remains ambiguous[3].
Ostracods are small crustaceans, typically around 1 millimetre (0.04 in) in size, but varying from 0.2 millimetres (0.0079 in) to 30 mm (1.2 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.
Ecologically, marine ostracods can be part of the zooplankton or (most commonly) they 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, New Zealand and Tasmania.[4] They have a wide range of diets, and the group includes carnivores, herbivores, scavengers, and filter feeders.
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.[5]
Fossils
Ostracods have a long and well-documented fossil record from the Cambrian to the present day. An outline microfaunal zonal scheme based on both foraminifera and ostracoda was compiled by M. B. Hart.[6]
Ostracods have been particularly useful for the biozonation of marine strata on a local or regional scale, and they are invaluable indicators of paleo-environments because of their widespread occurrence, small size, easily-preservable generally-moulted calcified bivalve carapaces, the valves are a commonly found microfossil.
Description
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).
Soft 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. There are two pairs of well-developed antennae, which the animal uses 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. There are two "rami", or projections, from the tip of the tail, that point downwards and slightly forward from the rear of the shell.[7]
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.[7]
The primary sense of ostracods is likely touch, as they have several sensitive hairs on the body and appendages. However, they do possess a single nauplius eye, and, in some cases, a pair of true compound eyes as well.[7]
Life cycle
Male ostracods have two penes, corresponding to two genital openings, or "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 the join the males. Some species are partially or wholly parthenogenetic.[7]
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.[7]
The larva moults eight times before reaching adulthood.
Ontogeny
 | This section may be too technical for most readers to understand. (June 2009) |
During the ontogeny the epidermis (containing mesodermal tissue) invaginates ventrolaterally near the cephalon/thorax area. This invagination proceeds upwards and tailwards, until the whole animal is enveloped by a double tissue layer on both sides: this forms the duplicature. The dorsal region never becomes invaginated, and is called the isthmus. The mesodermal tissue in the duplicature develops into the vestibulum. The vestibulum makes contact with the body near the isthmus. It plays a role in oxygenation. In paleo-ecology, the size of the vestibulum can be cautiously interpreted as an environmental indicator. The two double tissue layers surrounding the animal each have an inner and an outer lamella, which surrounds the vestibulum. These lamellae are surrounded by a chitinous cuticle, that is secreted by the epidermal cells.
Hard parts
The epidermal cells may also secrete calcium carbonate after the chitinous layer is formed, resulting in a chalk layer enveloped by chitin. This calcification is not equally pronounced in all orders. During every instar transition, the old carapace (chitinous and calcified) is rejected and a new, larger is formed and calcified. The outer lamella calcifies completely, while the inner lamella calcifies partially, with the rest remaining chitinous. The partial inner lamella calcification occurs when the ostracod becomes adult. The partial inner lamella calcification is most strongly developed frontally (see electron micrograph). The marginal zone is the area where inner and outer lamella meet, and includes part of the vestibulum. The edge of the marginal zone is called the fused zone, and in this area inner and outer lamella join. The fused zone can contain marginal pore canals. These, along with non-marginal pore canals (that are dispersed evenly along the ostracod's valve) connect the vestibulum to the outer world. The line of concrescence is the visible line between the vestibulum and the fused zone. In many cases, this line is wavering and follows the marginal pore canals. On the inner lamella, a selvage may be present.
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 Cuspidariidae in detecting ostracods with cilia protruding from inhalant structures, thence drawing the ostracod prey in by a violent suction action.[8] Predation from higher animals also occurs; for example, amphibians such as the Rough-skinned Newt prey upon certain ostracods.[9]
Use of non-marine ostracods in palaeoclimatic reconstruction
A new method called Mutual Ostracod Temperature Range (MOTR), similar to the Mutual Climate Range (MCR) used for beetles, is in development which can be used to infer palaeo temperatures.[10]
Bioluminescence
Some ostracods have a light organ in which they produce luminescent chemicals. Most use the light as predation defense, while some use the light for mating (only in the Caribbean). In Malaysia, these ostracods are called "blue sand" and glow blue in the dark at night.
References
- ^ Brusca, R.C. and Brusca, G.J. "Invertebrates" (2002)
- ^ Richard A. Fortey & Richard H. Thomas (1998). Arthropod Relationships. Chapman & Hall. ISBN 9780412754203.
- ^ S. Yamaguchi & K. Endo (2003). "Molecular phylogeny of Ostracoda (Crustacea) inferred from 18S ribosomal DNA sequences: implication for its origin and diversification". Marine Biology. 143 (1). doi:10.1007/s00227-003-1062-3.
- ^ J. D. Stout (1963). "The Terrestrial Plankton". Tuatara. 11 (2): 57–65.
- ^ Definition of ostracod at dictionary.com.
- ^ Malcolm B. Hart (1972). "A correlation of the macrofaunal and microfaunal zonations of the Gault Clay in southeast England; The Boreal Lower Cretaceous". Geological Journal. Special Issue (no. 5).
{{cite journal}}:|issue=has extra text (help) - ^ a b c d e Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 680–683. ISBN 0-03-056747-5.
- ^ John D. Gage and Paul A. Tyler, Deep-Sea Biology: A Natural History of Organisms at the Deep-Sea Floor, University of Southampton ISBN 9780521336659
- ^ C. Michael Hogan (2008). "Rough-skinned Newt ("Taricha granulosa")". Globaltwitcher, ed. N. Stromberg.
- ^ Horne, D. J. "A Mutual Temperature Range Method for European Quaternary Nonmarine Ostracoda". Geophysical Research Abstracts, Vol. 9, 00093, 2007. European Geosciences Union. PDF format.