OSTRACODS: CHARACTERISTICS, HABITAT, CLASSIFICATION, FEEDING - BIOLOGY - 2025

The ostracods (Ostracoda) are a class of bivalve crustaceans with the body completely enclosed between the valves, and without evident division of the body. Its size is generally small (between 0.1 and 2.0 mm), although there are some species that can exceed 3 cm in length.

They are the crustaceans with the lowest number of body appendages. In addition to four pairs of cephalic appendages, they only have one to three pairs of thoracic appendages. The two pairs of antennas (antennas and antennas) are generally used for locomotion.

Ostracodo Myodocopa. Photo: Carlos Lira.

About 80 thousand species are known, of which about 80% are fossil forms. The earliest records of fossil ostracods date from the Lower Cambrian, with species characterized by having a poorly calcified chitinous shell.

Currently they inhabit both marine and brackish and freshwater waters. Some species are benthic, others are part of plankton.

Characteristics and morphology

The carapace is made up of two valves joined dorsally by a hinge. These leaflets are made up of calcium carbonate and chitin, and can be equal or unequal in size. These shells are laterally compressed and their surface can be smooth or have granules, grooves or other ornamentation.

The valves are composed of two layers, one of chitin and the other of calcium carbonate. The amount of this compound that permeates the exoskeleton varies in different species. This shell is completely shed when the body needs to grow.

The body is completely enclosed between the two valves, contrary to what occurs in cladocerans and conchostracos. There are no external signs of segmentation, which is only indicated by the presence of the paired appendages.

They present four pairs of cephalic appendages, since the second pair of maxillae is absent. The appendages of the thorax can vary between one and three pairs, and there are no abdominal appendages.

The first pair of antennas (anténules) has a single branch, while the second has two branches. Both pairs of antennae can differ in both sexes.

The final portion of the body is represented by a pair of caudal branches that can vary in shape and structure depending on the species.

The larvae also have a bivalve shell.

The size of the ostracods generally does not exceed 2 mm in length. However, Gigantocypris species can measure up to 3.2 cm. These last species are inhabitants of deep waters (below 900 meters of depth).

Ostracoda of the family Cylindroleberididae. Taken and edited from: Anna33 at English Wikipedia.

Habitat

Ostracods are almost exclusively aquatic. Only two species have been reported in terrestrial habitats, associated with mosses and humus.

In fresh water, they can be found in practically any body of water, from rivers and lakes, to temporary ponds and phytotelmatas. Phytotelmatas are plant containers for water, such as tree trunks and leaves.

In marine and estuarine environments they are also ubiquitous species; they can be found from estuaries and marshes, even in oceanic waters. They can inhabit from shallow environments up to 7 thousand meters deep.

Most of the species are benthic, living on the seabed, climbing on sessile plants and animals, or burrowing in the substrate. Some species have been found as commensals of echinoderms or other crustaceans, mainly lobsters and crabs.

Taxonomy and classification

The Ostracoda taxon was erected by the French entomologist Pierre André Latreille, in 1802. Until recently, some authors included ostracods as a subclass within the Maxillopoda class, however, they are currently considered as a separate class.

The taxonomic location of ostracods in higher categories is uncertain, mainly due to the difficulty in making comparisons between fossil and recent species.

Classification in this group is based on both body and leaflet characters. In most of the fossil records only leaflets are available.

Another difficulty is the lack of uniformity in the terminology used by the different authors to describe the species.

The World Register of Marine Species (WORMS) portal offers an updated classification of the group, suggesting the presence of six subclasses, two of which include only fossil species.

However, this portal suffers from several errors. First, it does not point to the source of such a classification. Neither does it indicate the taxonomic authorities of various groups, nor does it have all the synonyms, making it difficult to determine whether some taxa (eg Family Egorovitinidae Gramm, 1977) have been rejected, synonymised or involuntarily omitted.

One of the most widespread classifications considers the presence of three subclasses:

Palaeocopa

Exclusively fossil forms, there are no recent species.

Podocopa

Ostracods lacking a face and rostral incision. They also do not have a heart. The shell, for its part, presents different levels of calcification.

The antennae are used for walking, they are birramos, with the internal branch (endopod) more developed than the external (exopod).

Myodocopa

The members of this subclass have a face and a rostral incision. The circulatory system features a dorsally located heart. The carapace is poorly calcified in representatives of this group.

The antennae are used for swimming, they are barren and its external branch (exopodite) is the most developed, presenting 8-9 gears.

Feeding

Filtration, using the maxillary appendages, is believed to be the primitive basic feeding pattern for ostracods, while the remaining feeding mechanisms are believed to be derived from it.

The diet of current ostracods can be suspensory, that is, they feed on organic matter in suspension. This type of feeding can be observed in both planktonic and benthic forms.

Benthic species can also feed on carrion or detritus. Some species are predators of invertebrates and fish larvae. Some species of cypridinid ostracods can even attack adult fish.

At least four species of ostracods are of parasitic habits. One of the parasitic species is Sheina orri, which lives in sharks in Australian waters. This species has been found parasitizing the gills of fish; it attaches itself to its hosts using the claws of its jaws and maxillae.

Reproduction

The reproduction of ostracods is generally sexual, with the participation of two parents (dioecious). However, asexual reproduction can also occur through parthenogenesis. Males and females are often sexually dimorphic.

The parental care of the eggs varies between the different species. Most species of podocópids lay their eggs freely, or attach them to any substrate and then abandon them.

Some species, however, temporarily incubate their eggs in a cavity between the carapace and the dorsal part of the body.

The egg hatches into an atypical nauplius larva, as it has a bivalve shell. Later it goes through six larval substages until it reaches the adult stage.

Sexual

Some species can use bioluminescence as a mechanism to attract a mate.

The ostracods present copulation, which can occur in different ways: the male can be placed in an inverted way and the copulation occurs belly to belly, or the male can mount the female dorsally or posteriorly.

The male presents a pair of penises. During copulation, the male deposits the sperm in the female's seminal receptacle. Individual sperm are generally coiled while in the testicle and, once uncoiled, can be more than 5 times larger than their parent.

Asexual

Asexual reproduction occurs by parthenogenesis, however, it can occur in various ways among ostracods. There are species in which parthenogenesis is the only known form of reproduction.

Other species exhibit both sexual and parthenogenetic reproduction. When parthenogenesis is present, it can be both geographic and cyclical.

In geographic parthenogenesis, populations of the same species, which reproduce sexually or parthenogenetically, present different geographic distribution.

In cyclic parthenogenesis, the population generally consists of only females that are reproducing by parthenogenesis, and when conditions become adverse, both sexual and parthenogenetic forms appear.

Uses and applications

Ostracods are the most common arthropods in the fossil record. Due to this, they are used as one of the most common tools to determine the age of different geological strata, as well as indicators of environmental conditions in prehistoric times.

Studies of ostracod fossil records have helped to understand climate trends from thousands of years ago, as well as historically important climate events such as the Younger Dryas or the Antarctic Cold Reversal.

On the other hand, researchers have also used recent ostracods to interpret climatic changes, such as the anthropic impacts caused mainly by the Industrial Revolution.

Fossils are also useful as a tool in the search for oil fields. Among the groups most used for these purposes are foraminifera, radiolaria, ostracods and mollusks.

Cretaceous fossil ostracods, from wells in the southeastern US. FoSwain, Frederick M. (Frederick Morrill), 1916-2008; Brown, Philip M. (Philip Monroe), 1922; Geological Survey (US), via Wikimedia Commons.

The ostracods, during their growth, can absorb trace metals present in seawater and incorporated into the shell during their secretion. Up to 26 trace elements, including heavy metals and rare earth elements, have been detected in the shells of some species of ostracods.

Due to this, some authors have proposed the use of the chemical composition of the ostracod shell as an indicator of environmental contamination.

References

1. RC Brusca, W. Moore & SM Shuster (2016). Invertebrates. Third Edition. Oxford University Press.
2. C. Laprida, J. Massaferro, MJR Mercau & G. Cusminsky (2014). Paleobioindicators of the end of the world: ostracods and chironomids of the extreme south of South America in Quaternary lake environments. Latin American Journal of Sedimentology and Basin Analysis.
3. PA McLaughlin (1980). Comparative Morphology of Recente Crustacea. WH Freemab and Company, San Francisco.
4. FR Schram (1986). Crustacea. Oxford University Press.
5. T. Hanai, N. Ikeya & K. Ishizaki (1988). Evolutionary biology of Ostracoda. Its fundamentals and applications. Kondansha, LTD & Elsevier Science Publisher.
6. MB Bennett, MR Heupel, SM Bennett & AR Parker (1997). Sheina orri (Myodocopa: Cypridinidae), an ostracod parasitic on the gills of the epaulette shark, Hemiscyllium ocellatum (Elasmobranchii: Hemiscyllidae). International Journal for Parasitology.
7. MN Gramm (1977). A new family of Palaeozoic ostracods. Palaeontology.
8. Ostracoda. In World Register of Marine Species. Recovered from marinespecies.org.