Fish-eating spiders more common than you think

By Signe Cane 19 June 2014
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Researchers have determined that spiders prey on fish on every continent except Antarctica

WATER-LOVING SPIDERS that catch and eat fish – often twice their own size – are found on every continent except Antarctica, according to a new study.

Spiders from five different families engage in fishing to complement their insect-based diet, according to s systematic review published today in PLOS ONE.

“I’ve seen a lot of spiders prey on fish in the wet tropics in Cairns,” says co-author Professor Bradley Pusey, a fish ecologist at the University of Western Australia.

His Swiss colleague from University of Basel, zoologist Martin Nyffeler had already found that spiders eat small bats, and do so fairly regularly. “Martin wanted to find out what else spiders might be feeding upon,” says Bradley.

Fishing spiders quite common

The researchers found 89 examples of fish predation across all continents except Antarctica. The predominant fish eaters are spiders from the Dolomedes genus, such as the Australian water spider, which usually hangs out along the edges of shallow freshwater streams, ponds and lakes.

A fishing spider anchors its hind legs onto a plant or rock and extends its front legs on the surface of the water to sense ripples. While this technique is mostly reserved for catching insects that accidentally fall into the water, it also works for snaring a small fish – if it’s unlucky enough to brush its dorsal fin against the outstretched spider legs.

The spider then bites the fish at the base of the head and kills it using the powerful mix of neurotoxins that is spider venom. The last step is to drag the fishy meal somewhere dry so it can be pumped full of digestive enzymes before devouring.

Spiders usually catch prey that is only about 20 per cent of their own size, and insects have a relatively small tissue mass versus the crunchy parts that spiders can’t eat.

However, in the case of struggling with a fish the risk seems worth it – on average the fish caught were 2.2 times larger than the spiders’ body length.

“A fish is a significant meal for a spider,” says Bradley. “It’s a big ticket item.”