Fish-eating spiders are the stuff of nightmares

Ancylometes fish-eating spider

This adult Ancylometes spider feasts on a fish in Ecuador. Fishing spiders often consume prey much larger than themselves.

M. Nyffeler and B.J. Pusey/PLOS ONE 2014

The little spiders I find hiding in the corners of a room or hanging out among the plants outside my front door are usually fascinating little creatures — and nothing to be frightened of. Those spiders are usually insectivores and not interested in anything with a backbone, let alone a full-grown human.

But insect eaters aren’t the only kinds of spiders out in the wild. There’s a whole category of spiders that hunt for their meals rather than sit and wait for something to get caught in a web. And I will admit that a new review of hunting spiders that eat fish creeped me out. (Weird, I know.)

Scientists have come across fish-eating spiders from time to time. The review, published June 18 in PLOS ONE, tallies up these arachnids for the first time. For their study, Martin Nyffeler of the University of Basel in Switzerland and Bradley Pusey the University of Western Australia in Albany searched through the scientific literature, perused the Internet and asked biologists about any unpublished findings to get as full a census as possible.

They found that fish-eating spiders come from several different families and genuses and can be found on every continent but Antarctica. But they’re more common between 40 degrees north and south latitude. Most are semi-aquatic and live on the edge of water bodies, like streams, lakes and swamps. There was one exception: the world’s only known completely aquatic water spider, Argyroneta aquatica.

Exactly how the spiders catch the fish isn’t quite clear. But some of the fish may simply be unlucky — they get close enough to brush by a spider’s leg and trigger an attack. Death comes through venom, injected through large, piercing chelicerae, usually at the base of the head. Most die within a few seconds to minutes. Larger fish can linger for nearly an hour. To consume its meal, the spider has to find a dry spot (the aquatic arachnids find a pocket of air among underwater vegetation) where it can pump their prey full of enzymes that will digest it.

These spiders are usually going after the most common fish found in their neighborhood, and they aren’t just attacking the small fry. On average, a spider’s prey is 2.2 times as long as the arachnid. (And this is where my nightmares started — some of these spiders are huge: The South American Ancylometes rufus, for example, has a leg span of 20 centimeters and is bigger than my hand.) And spiders in the genera Dolomedes and Nilus have been spotted catching fish that are 4.5 times heavier than themselves.

But none of these spiders would seem to be a threat to vertebrates as large as us. Even so, and even though I consider myself to be pretty low on the arachnophobia spectrum, there’s something about fish-eating spiders that I fear might make its way into my dreams.

This Dolomedes triton was spotted feasting on what’s probably a mosquitofish near a garden pond in Florida. M. Nyffeler and B.J. Pusey/PLOS ONE 2014
Fish-eating spiders, such as this Ancylometes sp. in Peru, have to haul their catch out of the water to eat. M. Nyffeler and B.J. Pusey/PLOS ONE 2014
Sometimes catching a meal is easy. Spiders have been photographed eating bait fish on boats. M. Nyffeler and B.J. Pusey/PLOS ONE 2014

Sarah Zielinski is the Editor, Print at Science News Explores. She has a B.A. in biology from Cornell University and an M.A. in journalism from New York University. She writes about ecology, plants and animals.

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