Pea shrimps: the charm of the Ostracoda

Amongst the scurrying, skipping and gliding specks at the bottom of a tray of pond life there are certain animals notorious for being tricky to identify and generally misunderstood. The jelly-bean like threesome above are all members of just such a group, the Ostracods, sometimes called pea shrimps. Most of what you can see are two valves, largely enclosing a scrunched up body with a handful of antennae, legs and other sticky out bits. They are little crustaceans, their jointed limbs at least giving away their kinship with more familiar water fleas and shrimps. Identification is tricky, largely needing a good view of the legs and antennae, “extremely difficult and can only be undertaken by a specialist ” as it says in Wolgang Engelhardt’s classic The Young Specialist looks at Pond-Life. Whilst ecologists may be wary of this awkward group geologists have a wealth of knowledge of their historic distributions, using fossils of the tough valves and the various shapes, spines and surface sculpture. Size, colour and shape are handy for the living adults and these three are fairly distinct: Herpetocypris reptans, Heterocypris incongruens and Eucypris virens. The “cypris” bit of the names is essentially saying “shrimp”. All three are a millimetre or two long, stuttering uncertainly across the sediment in search of food which can be any old detritus but they can gang up on enfeebled prey too. Although individually small their populations are robust, sustained through the ups and downs of drought and flood by desiccation resistant eggs, so they often appear in puddles and flashy pools. Heterocypris incongruens in particular seems to prefer these conditions and is lost from ponds as permanent, dense emergent plants such grasses and spike rush colonise. Under a microscope they are objects of great beauty, their valves sculpted or fringed with bristles and serrations. I am fond of these enigmatic beauties.