The fishhook waterflea (Cercopagis pengoi) hails originally from the Caspian Sea and Black Sea regions. Some years ago, it became invasive in waterways of Eastern Europe and in the Baltic Sea. It has also been introduced to the Great Lakes region of North America.
With its elongated tail-end, it can be as long as 1 cm in length.
The waterflea is a predator, eating smaller zoo- and phytoplankton.
Since it thrives in brackish water, the waterflea feels right at home in Finnish coastal waters, and can also cope with life in freshwater lakes.
The incidence of the fishhook waterflea is governed by temperature, or more precisely by cold weather.
The species spends the winter in the mud and sludge on the sea-bed in egg form, and the first waterfleas generally hatch and emerge when the surface water has reached something like 14-15°C.
As the water cools down again, Cercopagis pengoi vanishes. Large populations are generally observed towards the end of summer and into the early autumn.
Helsingin Sanomat / First published in print 13.1.2008
