Freshwater Ostracods and Shrimps.
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Like the copepods, the ostracods are very numerous in both freshwater and marine environments. There are 2000 living species. The larger marine species are also known as mussel shrimps or seed shrimps, but the freshwater ostracods are usually smaller than a millimetre.
There are 10,000 or so fossil species dating from the Late Cambrian period (about 500 million years ago) to recent times. Their great abundance and widespread distribution have made them useful index fossils for dating marine sediments, notably in oil exploration.
In freshwater ponds they are usually found scuttling around among the submerged plants and debris at the shallow edges, and less commonly in the open waters. They swim smoothly with appendages extended from between the two halves of their carapace. When disturbed, they withdraw their limbs and clamp the halves of their tiny shells tightly together.
They are perhaps less attractive creatures than the other small crustaceans due to the opaque and sometimes strongly patterned shell which makes it difficult to see their internal structure. Young specimens are the most rewarding for microscopical examination, as their shells are generally more transparent than those of the adults.
Ostracods are very similar in appearance, making it less than easy for the non-expert to distinguish one species from another -- or even one genus from another.
The ceaseless activity of ostracods can make them something of a nuisance to the microscopist when, under the microscope, they disturb the observation of some other creature. They are bulls in the chinashop of the sub-millimetre world.
The pictures below are probably of Cypris.
Click for a diagram of Cypris.