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.
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