One of the defining characteristics of ostracods is their carapace or shell, which when closed covers the non-calcified body-parts and appendages.

The carapace originates from the head region, and consists of two valves that are hinged along the dorsal margin.

The valves are closed by the adductor muscles, which are attached directly to each valve usually just anterior of the mid-length of the animal.

These attachment points, called the adductor muscle scars, can often be seen through the exterior of the carapace.

The pattern of adductor muscle scars is a useful feature for higher taxonomic levels.

The two valves of the carapace close with high precision, with one valve slightly larger and hence overlapping the other, and this creates a tight seal against environmental conditions.

The carapace is opened by relaxing the adductor muscles and pushing the valves apart with the appendages.

The shell is composed of low-magnesium calcite, and in some groups is translucent so that the internal parts of the animal can be partly seen. With this Tanycypris species, various features of the internal body are visible through the carapace.

The carapace is covered by a thin membrane or epicuticle consisting of pseudochitin. In some species of the marine family Sarsiellidae, the carapace is covered with a gelatinous substance (Kornicker 1976).

The carapace is shed and regrown during each moult, and hence does not have growth lines like the carapaces of some other crustacean groups, such as the Cyclestherida and Spinicaudata.

Ostracods are typically 0.3 to 5mm in length, although the predatory, planktonic, deep-sea Gigantocypris can reach lengths of more than 30 mm (Kornicker 1975).

Ostracods living interstitially (i.e. in the spaces between sediment grains) tend to be the smallest, while some of the largest live in hyper-saline lakes or temporary water bodies devoid of fish predators.

The carapace provides protection from a wide variety of life threatening situations. Although very small fish can encapsulate an ostracod with their mouths, they can have trouble crushing and eating it due to the ostracod's hard carapace. Ostracods that are taken by such small fish are often ejected seconds later, apparently without ill-effect to the ostracod.

With larger fish, ostracods don't fair so well as larger and more developed teeth can crush the carapace. If an ostracod does manage to by-pass the teeth and make it whole into the fishes gut, they can pass through intact and appear at the other end alive. Experiments on the ostracod Cypridopsis vidua fed to blue-gill sunfish demonstrated that 26% can survive passage through the fish's gut (Vinyard 1979).

The carapace can also protect ostracods against smaller predators. Juveniles of the freshwater ostracod Darwinula stevensoni have been recorded as surviving flatworm predation (Smith & Kamiya 2008). More about the ostracod predators...

Many species of ostracods live in shallow waters, such as rice fields, puddles, shallow ponds, rock pools, intertidal zones along coasts etc, and ultra-violet radiation is a health risk for life in such habitats. However, in experiments, the carapace has been shown to block up to 80% of UV radiation (Van den Broecke et al. 2012).

The carapace also provides protection from short-term environmental stress. Ostracods can survive in damp conditions without standing water by closing their carapaces and waiting.

Similarly, low oxygen levels or changes in salinity can be tolerated by ostracods in this way.

In addition to the adductor muscles, which pull the carapace shut, the carapace provides attachment points for various other muscles and ligaments. The mandibular coxa are attached to and braced against the internal surface of the valves to allow the coxa's endites to crush food in the mouth cavity.

Other muscles and ligaments connected to the antennules, antennae, and mandibles are anchored to the carapace near its dorsal margin (Kesling 1965).

The valves also provide support for the testes in the males and the ovaries in the females of some groups, both of which can leave marks on the internal surface of the valves.

There are two main parts to each valve: the outer lamella and the inner lamella. The outer lamella constitutes the largest proportion of the valves and forms the outer-most surface.

The inner lamella is the layer of the valve inside of the carapace.

The outer lamella has numerous small pores, which are holes in the carapace wall through which sensory setae protrude.

There are two types of pores, normal pores consisting of a simple hole, sometimes with a rim or lip (left), and sieve pores, which have a sieve-like covering over the hole.

Some species have over 2000 pores on each valve, such as Neonesidea oligodentata, a marine species found in the rocky intertidal zone of Japan (Smith & Kamiya 2002).

In addition to the pores on the outer lamella, there are also marginal pore canals (sometimes called radial pore canals) located on the edges of the valves, except along the hinge.

Marginal pore canals can be straight or branched, and are usually more numerous along the anterior margins of the carapace.

They run through the area where the outer and inner lamella meet, called the fused zone.

The surface of the outer lamella can be smooth, or have ornamentation in the form of bumps (tubercles or nodes), depressions (sucli), spines, pits, ridges, striations, and reticulation.

It has been shown that in one species ornamentation on the outer lamella can be influenced by environmental conditions. Cyprideis torosa is a brackish water ostracod found in European coastal waters. Typically specimens found in high salinity environments are smooth, while those found in low salinity environments (1.5 to 0.2%) have nodes (= tubercles). This is caused when the animals moult; when moulting, the ostracod fails to regulate increasing osmotic pressure in low salinity environments, resulting in epidermal cells to rupture and nodes to form (Keyser 2005).

A series of regularly spaced small bumps along the edge of the valves are typically called denticles or crenulations.

The denticles can be seen with transmitted light along the edge of the valves. Usually, the denticles are on the edge of the smaller of the two valves.

The inner lamella consists of calcified and un-calcified parts. The un-calcified inner lamella is a membrane that joins the edge of the calcified inner lamella with the inner part of the body.

A selvage is a ridge on the inner lamella, usually running close to the edge of the valve margin. In some groups the selvage has been displaced internally to form a ridge further away from the edge of the valves. The selvage helps to tightly seal the carapace when closed.

Inner lists are also found on the calcified inner lamella, but are less distinct than a selvage, usually forming nothing more than a slight ridge.

Lists can also be seen in transmitted light.

Septa (singular = septum, Latin meaning partition) are small support structures found along the margins of the carapace of some genera. They consist of small, wedge-shaped structures connected to the outer lamella and calcified inner lamella.

The two valves of the carapace are joined at the hinge, which runs along the dorsal margin. When the valves are open, the hinge is the only contact between the two valves. The structure of the hinge varies between groups. In some groups it is simply a chitinous connection between the two valves, while others it consists of teeth and corresponding sockets. There are eight main types of hinge.

Adont - the most simple type, without teeth and sockets.

Lophodont - with a pair of teeth and sockets at each end of the hinge, and a groove and corresponding bar between them. The teeth and sockets separate when the carapace is open.

Merodont - similar to lophodont, but the teeth and sockets are crenulated.

Entomodont - similar to merodont, but the groove and bar are also partially crenulated.

Schizodont - similar to lophodont but each tooth and corresponding socket are bifid (divided by a deep notch).

Ampidont - similar to schizodont, but the anterior tooth and socket are not bifid, only the posterior one.

Gongylodont - has teeth on both valves, and corresponding sockets on both valves.

Visordont - consists of two teeth at each end of the hinge on the right valve, and two corresponding sockets on the left valve. When the valve open, the teeth and sockets act like pivot points, and the right valve overrides the left along the dorsal margin, like a visor. Only known in the Terrestricytheroidea.

The male and female carapaces are often different in shape, especially in the posterior region, although in some groups the males and females are very similar shapes.

Males can be larger, smaller or of similar size to females.

With the small number of groups that brood eggs, the females have a large brood chamber in the rear of the carapace, giving them a much more inflated back end compared with males.

While the carapace is made from calcite and is hence translucent, pigments just below the outer lamella can give it a variety of colours. Vibrant blues, purples, greens, blacks, browns and oranges are common for ostracods, while others are whitish.

Typically, the carapace is not uniformly coloured, but has patches or stripes of colour presumably providing camouflage. The upper half of the carapace is usually the most intensely coloured, while the underside is lighter, although in the Notodromadinae, a group that feeds and swims up-side down at the water surface, the opposite is true (top right).

Additional colouration is provided by the internal parts of the body seen through the carapace, such as the hepatopancreas, ovaries, and gut.

Information concerning the type of pigments present in ostracods is sparse, but one study reported the presence of carotenoids (astaxanthin and beta-carotene), a pteridine found throughout the body, and a bilin in the gut wall of the freshwater ostracod Heterocypris incongruens (Green 1959).

Pigmented species from temporary habitats (which are typically very shallow) have been shown to be the best protected from UV radiation (Van den Broecke et al. 2012).

In specimens preserved with formalin, and to a lesser extent alcohol, colours can be faded or non-existent.

After death, the internal body and un-calcified parts of the carapace rapidly decay, leaving only the calcified part. In sediments with neutral or higher pH, these calcified parts can become preserved to form fossils.

The photo on the left side is a Cretaceous limestone sample (about 100 million years old), containing numerous ostracod fossils. These ostracods were living in a shallow water body (maybe a puddle or pond) that dried up, resulting in the mass death of the ostracods.