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Ostracods, or ostracodes, are a class of the crustacean (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon) have been identified,'''
on 2023-09-12. ' grouped into 7 valid orders. They are small crustaceans, typically around in size, but varying from , the latter in the case of the marine Gigantocypris. The largest known freshwater species is Megalocypris princeps'', which reach 8 mm in length. Freshwater Life: A field guide to the plants and animals of southern Africa The Triassic-Jurassic Terrestrial Transition: 37 In most cases, their bodies are flattened from side to side and protected by a bivalve-like valve or "shell" made of chitin, and often calcium carbonate. The family Entocytheridae and many planktonic forms do not have calcium carbonate. (2025). 9780195365764 ISBN 9780195365764 The hinge of the two valves is in the upper (dorsal) region of the body. Ostracods are grouped together based on shell and soft part morphology, and molecular studies have not unequivocally supported the group's monophyly. They have a wide range of diets, and the class includes carnivores, herbivores, scavengers and filter feeders, but most ostracods are Detritivores.
Ostracods have been particularly useful for the biostratigraphy 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.
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. 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. 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. World's oldest sperm 'preserved in bat poo', Anna Salleh, ABC Online Science, 14 May 2014, accessed 15 May 2014
The head is the largest part of the body, and bears four pairs of appendages. Two pairs of well-developed antennae are used to swim through the water. In addition, there are a pair of mandibles and a pair of maxillae. The thorax has three primary pairs of appendages. The first of these has different functions in different groups. It can be used for feeding (Cypridoidea) or for walking (Cytheroidea), and in some species it has evolved into a male clasping organ. The second pair is mainly used for locomotion, and the third is used for walking or cleaning, but can also be reduced or absent. Both the second and third pair are absent in suborder Cladocopina. In the Myodocopina the third pair is a multisegmented cleaning organ that resembles a worm. Their external genitals seem to originate from the fusion of three to five appendages. The two "rami", or projections, from the tip of the tail point downward and slightly forward from the rear of the shell. (2025). 9780123706263 ISBN 9780123706263 Limited preview via Google Books
All ostracods have a pair of "ventilatory appendages" that beat rhythmically, which create a water current between the body and the inner surface of the carapace. Podocopa, the largest subclass, have no gills, heart or circulatory system, so the gas exchange take place all over the surface. The other subclass of ostracods, the Myodocopa, do have a heart, and the family Cylindroleberididae also have 6–8 lamellar gills. Certain other larger members of Myodocopa, even if they don't have gills, have a circulatory system where hemolymph sinuses absorbs oxygen through special areas on the inner wall of the carapace. In addition, the respiratory protein hemocyanin has been found in the two orders Myodocopida and Platycopida. Nitrogenous waste is excreted through glands on the maxillae, antennae, or both.
The primary sense of ostracods is likely touch, as they have several sensitive hairs on their bodies and appendages. Compound eyes are only found in Myodocopina within the Myodocopa. The order Halocyprida in the same subclass is eyeless. The darkness syndrome in subsurface-shallow and deep-sea dwelling Ostracoda (Crustacea) Podocopid ostracods have just a naupliar eye consisting of two lateral ocelli and a single ventral ocellus, but the ventral one is absent in some species. Platycopida was assumed to be completely eyeless, but two species, Keijcyoidea infralittoralis and Cytherella sordida, have been found to both possess a nauplius eye too.
As of 2008, around 2000 species and 200 genera of non-marine ostracods are found. However, a large portion of diversity is still undescribed, indicated by undocumented diversity hotspots of temporary habitats in Africa and Australia. Non-marine species have been found to live in sulfidic cave ecosystems such as the Movile Cave, deep groundwaters, hypersaline waters, acidic waters with pH as low as 3.4, in plants like Bromeliaceae, and in temperatures varying from almost freezing to more than 50 °C in hot springs. Of the known specific and generic diversity of non-marine ostracods, half (1000 species, 100 genera) belongs to one family (of 13 families), Cyprididae. 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.
Class Ostracoda is divided into following living clades: Ostracods - what are they? - Lake Biwa Museum
Palaeoclimatic reconstruction
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. 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.
Distribution
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. Ostracods has been found as deep as 9,307 m (genus Krithe in family Krithidae). Subclass Myodocopa and the two podocop orders Palaeocopida and Platycopida are restricted to marine environments (except for Platycopida which have a few brackish species), Zooplankton and Micronekton of the North Sea 2.0 – Ordo Platycopida but we find non-marine forms in the four superfamilies Terrestricytheroidea, Cypridoidea, Darwinuloidea, and Cytheroidea in the order Podocopida. Terrestricytheroidea is semi-terrestrial and usually found in brackish and marine-influenced environments such as salt marshes, but not in freshwater. Thorp and Covich's Freshwater Invertebrates: Ecology and General Biology The other three superfamilies also live in freshwater (Darwinuloidea is exclusively non-marine). Lost Sex Seed Shrimp, Mussel Shrimp (Freshwater Ostracods) scientific name: (Crustacea: Ostracoda: Podocopa) Of these three, only Cypridoidea have freshwater species able to swim. Thorp and Covich's Freshwater Invertebrates, Volume 5: Keys to Neotropical and Antarctic Fauna Representatives living in terrestrial habitats are also found in all three freshwater groups, such as genus Mesocypris which is known from humid forest soils of South Africa, Australia and New Zealand.
(1998). 9783510470549, Advances in Limnology. ISBN 9783510470549
Ecology
Lifecycle
Male ostracods have two , corresponding to two genital openings () 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 Parthenogenesis. Superfamily Darwinuloidea was assumed to have reproduced asexually for the last 200 million years, but rare males have since been discovered in one of the species.
In the subclass Myodocopa, all members of the order Myodocopida have brood care, releasing their offspring as first instars, allowing a pelagic lifestyle. In the order Halocyprida the eggs are released directly into the sea, except for a single genus with brood care. In the subclass Podocopa, brood care is only found in Darwinulocopina and some Cytherocopina in the order Podocopida. In the remaining Podocopa it is common to glue the eggs to a firm surface, like vegetation or the substratum. These eggs are often resting eggs, and remain dormant during desiccation and extreme temperatures, only hatching when exposed to more favorable conditions. Field Guide to Freshwater Invertebrates of North America Recent Freshwater Ostracods of the World: Crustacea, Ostracoda, Podocopida Species adapted to can reach sexual maturity in just 30 days after hatching. Specieswatch: ancient crustaceans still going strong after 450m years There is no larval stage or metamorphosis (direct development). Instead they hatch from the egg as juveniles with the bivalved carapace and at least three functional limbs. As the juvenile grows through a series of molts they acquire more limbs and develop further the already existing ones. The Light and Smith Manual : Intertidal Invertebrates from Central California to Oregon – Page 419 They reach sexual maturity in the final instar and then never molts again. The number of instars they go through before adulthood varies. In Podocopa it is eight or nine (but family Entocytheridae and suborder Bairdiocopina has only seven), Recent Freshwater Ostracods of the World: Crustacea, Ostracoda, Podocopida the Halocyprida goes through six or seven, and Myodocopida only four to six. They are able to produce several offspring many times as adults (iteroparity). Strategies of crustacean growth - Australian Museum Crustacean Issues 3: Factors in Adult Growth Evolution and Phylogeny of Pancrustacea: A Story of Scientific Method
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 Cytherocopina in the Cuspidariidae in detecting ostracods with cilia protruding from inhalant structures, thence drawing the ostracod prey in by a violent suction action. (1992). 9780521336659, University of Southampton. ISBN 9780521336659 Predation from higher animals also occurs; for example, amphibians such as the rough-skinned newt prey upon certain ostracods. Whale sharks also seem to eat them as part of their filter feeding process.
Bioluminescence
Some ostracods, such as Vargula hilgendorfii, have a light organ in which they produce luminescent chemicals. These ostracods are called "blue sand" or "blue tears" and glow blue in the dark. 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. Bioluminescence has evolved twice in ostracods; once in Cypridinidae, and once in Halocyprididae. In bioluminescent Halocyprididae a green light is produced within carapace glands, and in Cypridinidae a blue light is produced and extruded from the upper lip. Most species use the light as predation defense, but the male of at least 75 known species of the Cypridinidae, restricted to the Caribbean, use pulses of light to attract females. Some species are the opposite where the females use pulses of light to attract males. This is seen in one example such as the glow worm. This bioluminescent courtship display has only evolved once in ostracods, in a cypridinid group named Luxorina that originated at least 151 million years ago. Ostracods with bioluminescent courtship show higher rates of speciation than those who simply use light as protection against predators. Bioluminescence Is Nature's Love Light The male will continue to swim after releasing its small ball of bioluminescent mucus, but the female is able to read the display to pinpoint the male's location. Watch ‘Sea Fireflies’ Make Underwater Fireworks as They Seek Mates In one species hundreds of thousands of males synchronize their light display, and when one male creates a pattern of light, the new pattern will spread out as the neighboring males repeat it. Mating dance of sea fireflies is 'the coolest fireworks show that you've ever seen'
Classification
Early work indicated that Ostracoda may not be monophyly, (1998). 9780412754203, Chapman & Hall. ISBN 9780412754203 and early molecular phylogeny was ambiguous on this front. Recent combined analyses of molecular and morphological data suggested monophyly in analyses with broadest taxon sampling, but this monophyly had no or very little support (Fig. 1 - bootstrap 0, 17 and 46, often values above 95 are considered sufficient for the taxon support).
See also
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