Ostracods are small aquatic crustaceans, with a body entirely covered by a shell consisting of two valves, hinged along the back. Even though they are small, usually ranging from 0.3mm to 5 mm in length (although some species reach 30mm) they have a complex body consisting of typically eight pairs of appendages. These appendages have a range of functions including swimming, sensory, crawling, feeding and mating. When the shell is open the appendages are extended through the gap for locomotion, feeding and mating.

Ostracods are either planktonic (only some marine groups) or, more commonly, live on the bottom on sediments or plants. Many species that live on the bottom have the ability to swim from place to place, whereas other groups can only crawl on or through the sediment.

The ostracod shell is hard and calcareous and can take many different shapes and forms. Many marine species have very ornate shells, with pits, ridges and spines. Many freshwater species have smooth, rounded shells, that look like tiny seeds (hence their common name seed shrimps).

Appendages

Antennules - used for swimming (only some groups), crawling and sensory (both chemical and tactile).
Antennae - used for swimming (only some groups), crawling, sensory (chemical), copulation and feeding (moving food to mouth).
Mandibles - used for feeding (moving food to mouth and mastication).
Maxillulae - used for feeding (moving food to mouth) and respiration.
Fifth limbs - uses vary between groups from feeding (moving food to mouth), copulation or walking.
Sixth limbs - used for walking.
Seventh limbs - uses vary between groups from cleaning, walking or climbing.
Caudal rami - (absent or reduced in some groups) used for locomotion.

Ostracods eat a wide variety of food, including organic detritus, algae, plant material, dead animals and sometimes live animals such as young snails, worms and larvae of other crustaceans. (Right, ostracod mouth parts)

There are approximately 33 000 fossil and living species of ostracods so far described, but there are many more yet to be discovered. It is estimated that the total number of living species is between 10 000 and 15 000.