A new research conducted by Prof Martin Nyffeler from the University of Basel and Prof Bradley Pusey from the University of Western Australia provides evidence that fish predation by spiders is geographically widespread, occurring on all continents except Antarctica.

The scientists analyzed more than 80 incidences of fish predation by five families of semi-aquatic spiders – Pisauridae (nursery web spiders), Ctenidae (wandering spiders), Trechaleidae (longlegged water spiders), Lycosidae (wolf spiders) and Liocranidae (spinylegged sac spiders).

These spiders, some of which are capable of swimming, diving and walking on the water surface, have powerful neurotoxins and enzymes that enable them to kill and digest fish. In most cases, they eat fish in combination with other prey, mostly insects.

“The finding of such a large diversity of spiders engaging in fish predation is novel,” said Prof Nyffeler, who is the first author of the paper published in the open-access journal PLoS ONE.

“Our evidence suggests that fish might be an occasional prey item of substantial nutritional importance.”

Most incidences have been documented in North America, especially in the wetlands of Florida, where semi-aquatic spiders have often been seen catching and eating small freshwater fish such as mosquitofish.

Vision plays a relatively minor role in prey detection and while some spiders rely on feeling ripples in the water made by their prey, most rely on the dorsal fin of a fish touching their outstretched legs.

Some spiders anchor their hind legs to a stone or plant with their front legs resting on the surface of the water, ready to sense and ambush their fishy dinner.

While spiders usually feed on prey items that are smaller than themselves, some semi-aquatic spiders capture fish with bodies on average 2.2 times as long as the spiders. And some spiders catch fish which are 4.5 times their weight.

Fishing spiders possess large strong mouthparts capable of piercing their preys’ skin. Most fish are bitten by the spiders at the base of the head. Once a spider catches its fish, it always drags it to a dry place before eating it. This is because the spider has to pump enzymes into its prey before it can be digested, and also because there is less chance of the fish getting away.

It takes a spider many hours to consume a fish and the extra nutrients and calories in a fish – compared to an insect – may give the spider an advantage during the mating period.

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Nyffeler M, Pusey BJ. 2014. Fish Predation by Semi-Aquatic Spiders: A Global Pattern. PLoS ONE 9 (6): e99459; doi: 10.1371/journal.pone.0099459