Though you might not be faced with parasites attacking you from all sides, it’s a rather different story if you’re a fish. Life in a three-dimensional aquatic environment is tough and cercariae (larval flukes) drift and swim through the water, burrowing into flesh when they find it.
Fish shoal for many reasons, and it now looks as though there’s an anti-parasite benefit to shoaling. This interesting paper from Journal of Animal Ecology, Shoaling as an antiparasite defence in minnows (Pimephales promelas) exposed to trematode cercariae, covers work done by researchers at University of Lethbridge, Canada, and Minnesota State University, USA.
The trematode Ornithodiplostomum ptychocheilus is a natural parasite of the fathead minnow. Snails pick up the early larval stages and release cercariae, which are capable of piercing the fish’s epidermis and cause a whole host of nasty effects. Stumbo et al. looked at whether being surrounded by other fish reduced the number of parasites that colonised each fish; it certainly looks as though shoaling reduced the number of parasites per fish, particularly for fish in the centre of a group.
One point mentioned tantalisingly in the conclusion is that, depending upon the cues used by the parasite to detect the fish, a shoal may be more easily detectable than a single fish. This would even the odds, meaning that, though an individual fish might suffer higher parasite load once detected, a lone fish may not be as attractive as a shoal of fish.
The abstract’s freely available from the journal’s website.