Fish-eating spiders catch prey five times their size

It isn’t easy being a little fish. Predators dart at them underwater. Humans try to snare them with hooks. And other species – more than we’d thought, it turns out can pounce on them from above.

According to a new study, spiders in eight of the world’s 109 arachnid families can catch and consume small fish. Some of them can even subdue fish five times heavier than they are.

These arachnids are nearly everywhere. The study, published on June 18 in the journal PLOS ONE, says fish-eating spiders can be found on every continent except Antarctica. They’re especially prevalent in warm, oxygen-depleted bodies of water like the wetlands of Florida, where fish are more likely to come to the surface in search of oxygen-rich water.

At least 18 species have now been observed catching fish, including six-spotted fishing spiders (Dolomedes triton) in the United States, pond wolf spiders (Pardosa peudoannulata) in India and great raft spiders (Dolomedes plantarius) in the United Kingdom.

These findings were pieced together by Martin Nyffeler at the University of Basel, Switzerland, and Bradley Pusey at the University of Western Australia in Albany.

The two biologists first searched for published reports and internet posts documenting spiders eating fish. What they found – 89 records in total, half of which hadn’t been published in the scientific literature previously allowed them to paint a more complete picture of this unusual behaviour.

“Fish predation by spiders has always been seen as a bit of an oddity,” said Marie Herberstein, an expert on spider behaviour at Macquarie University in Sydney, who was not involved in the study. “But the review makes a compelling argument that it is widespread, both taxonomically as well as geographically. This was certainly a surprise.”

How it’s done

Fish-eating spiders live in freshwater environments like ponds and wetlands, where they hunt for meals on foot instead of using a web.

Some can even swim, dive and walk on the water’s surface.

These semi-aquatic spiders “anchor their hind legs to a stone or a plant, with their front legs resting on the surface of the water,” the authors write. Then the arachnids wait to ambush their prey. The slightest ripple in the water, or anything that touches the spiders’ outstretched legs, can trigger an attack.

That’s because fish-eating spiders are generalist predators – they’ll go for nearly anything that moves. Most of the time, that means their meals are insects that have fallen into the water. But occasionally they purposely attack larger animals like fish.

And they’re well equipped to eat them, with mouths that can pierce flesh. They use those mighty maws to inject a lethal venom packed with powerful neurotoxins – chemicals that attack the nervous system into their fish prey.

When the fish is dead, the spiders haul it to dry ground and administer chemicals that liquefy its body tissues, making the meal easier to eat.

In the animal world, the average predator is 42 times larger than the prey it’s trying to subdue. Some fish-eating spiders, however, are actually smaller than their prey. The authors speculate, for instance, that a giant fishing spider – weighing in at 7 gm (0.4 ounces) would be capable of catching a 30 gm (1 ounce) fish.

Such supersize food sources could be critically important for females in the process of producing eggs, or for spiders that don’t have access to enough insect meals. One thing is for sure: It’s more bad news for little fish.