WHAT SPIDERS EAT IS A BIG SURPRISE

by Whit Gibbons

April 17, 2016

What predator can set a trap to catch bats and snakes, grab fish from lakes and supplement this carnivorous diet by eating plant material? Considering this wide-ranging assortment of prey, plus the fact that this voracious group of hunters have been documented to make their predatory attacks on every continent except Antarctica, should we be worried that humans might also be on the menu? Not likely, because the animals are spiders, and the largest ones in the world get no bigger than our hands.

I was once again appreciative of what a diverse world we live in when I received an unusual request from Martin Nyffeler, senior lecturer in zoology at the University of Basel in Switzerland. He sent me some photos of snakes to identify.

Each year I see hundreds of cell phone photos people send in wanting identification of snakes and other animals. But these snake photos were different.

First, most were snakes from regions from which I do not typically get requests: Taiwan, India, Malaysia and Australia. But the truly bizarre feature was that each was a small snake that had been entangled in a spider web and then consumed by the spider. I did the best I could in helping him with the snake identifications, turning to colleagues in other countries for confirmation of a couple of them.

In my communications with Nyffeler, I found out that he has published several scientific papers on spider diets. I was intrigued. Among the more fascinating of his publications about what spiders eat were ones on bats, fish and plants.

I knew of a fish-eating spider in the Southeast and once saw one that had caught a small mosquitofish. But I had no idea that fish predation by spiders is a global phenomenon.

More than a dozen species of spiders have been documented to engage in capturing fish, with a total of 89 observed incidents.

One common strategy is for the spider to sit alongside shore, stretch out its front legs, and when a fish swims by, to ambush it.

Whereas most spiders eat insects that are smaller than they are, the average size of fish eaten is more than twice the length of the spider itself. The documented size record for a fish caught by a spider is a goldfish 3.5 inches long.

The giant fishing spiders of the Amazon are suspected of catching even bigger fish, but with little likelihood of anyone observing the operation.

Equally high on the surprise meter for me, as well as other ecologists, was Nyffeler's scientific review paper on bat-catching spiders. Incidents of spiders preying on bats have been primarily ones where a bat was caught in a web before the spider proceeded to kill and eat it.

Most spiders have fangs and venom, so they come well-equipped for the job. As is often true in science, some biological phenomena are not recognized as being widespread, even common, until someone does a scientific review of the literature, much of which may be anecdotal.

Many biological observations that are mentioned incidentally in books or scientific papers are often not recognized as being the norm rather than the exception until someone accumulates the records. Such is the case with bats in which more than 50 accounts of spiders eating bats were confirmed.

Zoologists have traditionally considered spiders to be active carnivores or scavengers. However, ecologically significant, although less dramatic than predator-prey accounts, are 95 compiled records of 60 species of spiders eating plants.

Not nearly as exciting as a spider ensnaring a bat or snake or pouncing on a fish but a clear indication that ecologists still have much to learn about the intricate interactions among plants and animals.

The intriguing documentation of the dietary versatility of spiders is a real eye-opener. Let's be thankful that spiders don't get as big as horses or we would surely find ourselves on their list of preferred prey.

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