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Slideshow: Fish-eating spiders more common than thought

Lying in wait at the water's edge, four legs on a leaf and four legs feeling the water for vibrations, the Dolomedes triton spider can catch a fish twice its size in an instant. Holding on tight, it injects its aquatic prey with a deadly dose of venom and drags it to dry land, where it spends hours pumping it full of digestive enzymes and slurping up the fish’s liquefied flesh.

Fish-eating spiders sound like something out of a nightmare, but until recently, scientists were pretty sure only a few kinds of arachnids could do it. Now, a new survey shows that at least 26 species of spiders know how to fish—and they live on every continent except for Antarctica.

Martin Nyffeler, an arachnologist from the University of Basel in Switzerland, began collecting reports of unusual spider prey as a side interest while researching more traditional spider-insect interactions. After publishing reports on spiders that eat snails and bats, he turned his attention to spiders that fish, and discovered a mismatch between what textbooks say about fishing spiders, and what a Google Images search clearly shows.

According to the textbooks, Nyffeler says, fishing spiders are mainly those in the Dolomedes genus and the related genus Nilus. These spiders live in North America, Europe, New Zealand, and Australia, where they earn names like dock spiders and raft spiders because they live near bodies of water, can walk on the water’s surface, and have hairs on their bodies that trap air to help them float and breathe when they go under.

Photographs of Dolomedes eating fish—often in laboratory settings—aren't hard to find, but it turns out these spiders aren’t the only ones that do this. After digging up 89 reports of wild spiders preying on fish, including some known only from photos shared online, Nyffeler and his colleagues discovered that spiders from at least eight different families have been documented preying on fish in the wild,  they report today in PLOS ONE. For example, spiders in the genus Ancylometes, which can be as wide as 20 centimeters from toe tip to toe tip and can dive for up to 20 minutes, prowl the edges of South American ponds at night, catching fish in addition to their known prey of frogs, tadpoles, and lizards—behavior often documented by amateur photographers but never before recognized by scientists.

Although spiders that catch fish were found across the globe, almost all were in tropical latitudes. Nyffeler thinks that may be because warmer water holds less oxygen, forcing fish to spend more time at the surface where spiders wait.

Fish seem to be a "big ticket" prey item for these spiders, which are probably catching aquatic insects for most of their diet but really luck out when they land a fish, Nyffeler says. While insect prey tends to be much smaller than a spider's body, the fish that spiders catch can be much larger—the average fish in this survey was twice the body size (not counting legs) of the spider that caught it. The team found reports of fish as big as 9 grams, but believes the largest spiders could probably catch prey as big as 30 grams—about an ounce.

"This review is a valuable introduction to the phenomenon of fish predation by spiders," says David Wise, an ecologist who studies spiders' role in food webs at the University of Illinois, Chicago. Still, he calls the work "largely confirmatory" because the known fishing spiders like Dolomedes made up about 80% of the observations.

"Given the sheer number of fishes in most water bodies, I doubt spider predation has much of an impact on the fish populations," says Richard Vari, an ichthyologist at the Smithsonian Institution in Washington, D.C. Still, he says, "there is every reason to believe that future observations will both increase the number of species demonstrating this feeding pattern and the areas of the world where it occurs."