The freshwater shrimp (Gammarus pulex) is not a shrimp,
although it may look like one. It is a member of a group called amphipods that
includes marine sandhoppers.
Freshwater shrimps are found in well-oxygenated water that
contains plenty of plant material, which is why they are often found on
watercress farms.
Freshwater shrimps are small creatures, with males being up
to one inch (25 cms) long and females slightly smaller. They have narrow,
curved, translucent light brown bodies, their shape allowing them to crawl into
small crevices.
They have seven pairs of limbs – the foremost two pairs are
used for grasping and the rest for swimming. Three pairs of feathery structures
on the abdomen waft oxygenated water towards the plate-like gills that are
attached to the inner surfaces of the swimming legs. Freshwater shrimps swim on
their sides.
Two pairs of long antennae help in locating the shrimp’s
food, which is decomposing plant and animal material.
At breeding time the male grips the female from behind and
the pair stay locked together for several days. After this time the female
moults her shell, allowing the male to deposit his sperm before breaking away.
The fertilised eggs are retained in a brood pouch between the female’s legs,
eventually being released as small replicas of adult shrimps without going
through a larval stage.
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