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Life in Freshwater

Fish Louse (Argulus foliaceus)

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Argulus the Fish Louse

This is an unmistakable crustacean not often found free-living but as a parasite on the outside of fish. The oval body is semi-transparent and there are two large suckers, characteristic of the group. Four pairs of legs help swimming and there are several sharp spines. The antennae are very restricted but the compound eyes are just visible. The animal is about 8 mm long.

These are the parasites of freshwater fish, unlike the copepod group, which parasitise estuarine and marine fish. The Fish Louse is very common and easily overlooked because of its transparency and smooth outline. It can be found on just about any freshwater fish including sticklebacks, perch, minnows, pike trout, carp and bream. They are widespread across the continent although the species A.coregoni is most abundant there.

ECOLOGY: This is a beautifully adapted parasite. From the moment the eggs hatch they are searching for a host. The suckers give grip as do the spines which dig in and prevent dislodgement. There is a proboscis between the suckers that scrapes flesh off the fish. This causes blood to flow and it is this that is consumed. The Fish Lice may be so dense on a host that severe damage can be made to the fish's body. If death ensues the lice just swim off in search of a new host. The swimming legs beat even when they are attached to a host. The females lay a mass of eggs on stones wrapped up in a jelly coat.











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