News in Science

Fish-eating spiders cover most of the world

Spider stocktake Spiders that prey on fish often more than twice their size live in every continent except Antarctica, a new study has found.

Researchers from Australia and Switzerland reviewed more than 80 reports of fish predation by semi-aquatic spiders.

They found spiders from up to eight taxonomic families attack fish, they report in the journal PLOS ONE.

Some spiders are known to have a varied diet, with many species supplementing insects with amphibians, reptiles, mice, and even birds and bats. But accounts of spiders catching fish in the wild are often anecdotal, come from very old literature, or originate from just a few locations.

To understand how widespread the behaviour is, the researchers compiled all the available literature on fish predation by spiders together with unpublished information from biologists and naturalists.

"It's important to gather as much information as you can into one place so that you can put this phenomenon into some context," says Bradley Pusey, from the University of Western Australia, in Perth.

With the exception of the well-documented diving bell spider, which lives under water, the majority of fish-eating spiders are surface dwellers that are usually found around the edge of water bodies, which are typically rich in insect life.

"The spiders that live there are probably hunting for insects most of the time and occasionally, or quite frequently in the case of some families, they are hunting fish," Pusey says. "This requires a lot of flexibility in hunting strategy."

Twice the size

A spider's insect prey is usually much smaller than the spider itself, typically a fifth of its size. But, when it comes to fish, the victim is on average 2.2 times the size of the spider.

"You would think that there was a much higher risk of a spider being damaged in that ensuing conflict, but there's a really significant payoff … it's a big-ticket food item with lots of protein and flesh to consume, whereas an invertebrate has lots of hard material that the spider discards," Pusey explains.

The typical fish-eating spider parks itself on a rock or a piece of vegetation with three pairs of front legs on the water surface and the rear pair attached to a rock or some other anchor point. They appear to receive information from vibrations on the water surface. However, spiders require a fish to actually touch one of their legs to elicit an attack response.

"They just throw themselves down into the water to latch onto them. From the records that we've reviewed they then usually kill their prey with a bite at the base of the neck," says Pusey.

"There's a rich cocktail of neurotoxins in these spiders, some of them specific to vertebrate nervous systems, which is really interesting I think."

The spiders then haul their victim onto dry land and inject powerful digestive enzymes before eating dinner.

Under-reported

Pusey suggests that on the whole it is an under-reported phenomenon. There are few reported incidents from Asia, for example, despite plenty of Asian spiders that are capable of hunting fish.

As for Australia, there are several reports of spiders eating fish on the fringes of slow moving streams in New South Wales and Queensland. The research also found incidents where spiders were observed catching fish in garden ponds in Adelaide, Brisbane, Lismore and Sydney.

"There are lots of other things that would be riskier for your goldfish," says Pusey, "but you never know."