Watch Out for Little Goldie: Fish-eating Spiders Are Everywhere

Grossed out? There are spiders that eat bats, too – and they're also a lot more common than had been thought, scientists say.

Ruth Schuster
Will Dunham
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A fishing spider (Dolomedes facetus) with a captured fish (Xiphophorus) in a garden pond in Brisbane, Australia.
A fishing spider (Dolomedes facetus) with a captured fish (Xiphophorus) in a garden pond in Brisbane, Australia.Credit: Reuters
Ruth Schuster
Will Dunham

And here you thought spiders were creepy, but forgave them because they eat mosquitoes. It turns out, the arachnids eat a lot more than that, and small fish and bats are not the rarity in their diets that had been assumed.

Spiders of course are delighted to devour bugs, but it turns out that there are at least two dozen species of spiders that like to dine on little freshwater fish; and they are on every continent with the exception of Antarctica.

Remarkably, the fish they predate may be even bigger than the spiders themselves, especially in the case of spiders that can swim, say researchers documenting the spidery supper. Typically the fish they catch are between two to six centimeters in length – at the longer end, more than double the length of the spiders in question.

Scientists have long known that some spiders consume fish, but the study -- considered the first systematic review of the topic -- showed that the practice is far more common and geographically widespread than previously understood.

Just as in the case of humans: "Fish meat is high-quality prey regarding protein content and caloric value," said zoologist Martin Nyffeler, of the University of Basel in Switzerland, who led the study published in the scientific journal PLOS ONE.

"Feeding on fish may be particularly advantageous during the mating period when the elevated energy and protein requirements of gravid (pregnant) female spiders require increased food intake, or at times of limited availability of invertebrate prey," he adds.

Hunting by the fen

Where are the spiders indulging in their taste for fish? The arachnids have been observed at the edges of shallow freshwater streams, rivers, lakes, ponds, swamps, and fens, say the scientists, mainly in the warmer areas of the world – between 40° S and 40° N. The Florida wetlands and neighboring regions are a particular hot spot.

The spiders that prey on fish don’t use webs, or build any for that matter – they're all in the category of hunters, which employ potent neurotoxins and enzymes to kill and then to digest fish.

Semi-aquatic spiders -- able to swim, dive and even walk on the water surface - lurk at the edges of shallow water. They may anchor their hind legs to a stone or plant, keeping their front legs ready for the ambush on the water's surface.

After the spider snares a fish, it drags it to a dry place like a rock or tree trunk to begin the feeding process: pumping digestive enzymes into the fish and sucking out the dissolved tissue like a milkshake. (It's the same digestive mechanism other spiders use to eat insects.)

"It takes a spider usually many hours to devour a fish until nothing is left but bones and scales," Nyffeler said.

The researchers said at least 18 spider species from five families have been observed catching fish in the wild. And six more species, including some from three other families of spiders, have been documented doing it under laboratory conditions.

In North America, the species Dolomedes triton frequently catches fish. In Australia, the species Dolomedes facetus steals goldfish from ponds in suburban gardens.

And now for something completely different: Bats

Another study by Nyffeler and his team, also published in PLOS ONE, describes spiders that hunt bats. Yet again, they found the prevalence of this predilection is a lot more common than had been assumed.

The paper, "Bat Predation by Spiders," describes more than 50 incidents of spiders catching and feasting on the flying mammals – and again, it's happening on every continent except Antarctica. Like in the case of fish predation, bat-eating spiders are more common in warmer areas, between 30° N and 30° S.

One big difference is that almost all bat hunters are web-builders (88 percent); the other 12 percent of incidents observed were by hunting spiders.

The bats in question were tiny insectivorous species that got caught in strong spider webs. Unable to get free, some died of exhaustion, dehydration and the like – but others were observed being killed and eaten by spiders.

Brad Pusey of the University of Western Australia, who also worked on the study, noted: "Spiders are more adept predators than most give credit for."

With reporting by Reuters.

A mouse-tailed bat, which is of a type that spiders could catch in big webs.Credit: Zafrir Rinat

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