Fish-Eating Spiders Are Everywhere

Well, this is terrifying. According to new research published today, dozens of spider species from around the world can capture and eat fish as much as twice as big as they are.

Fish provide some 7% of the protein eaten by humans around the globe. Fish are also a favorite snack for a variety of birds, as well as fish-eating bats, otters, bears, snakes, wolves, turtles, and more. It doesn't exactly boggle the mind to realize that tiny fish hatchlings are good eating for invertebrates like caddisflies, and it's even reasonable to consider that there are some arthropods, like water scorpions, who manage to kill and eat small fish. But spiders?


Writing in PLoS ONE, Martin Nyffeler and Bradley J. Pusey acknowledge that "the notion of fish-catching spiders is rather peculiar if we consider that spiders, as a whole, are traditionally viewed as the classic example of a predator that feeds on insects." However, they point out that some spiders spend their lives at water's edge, or even on the water's surface.

When you stop and think about it, fish eating isn't all that surprising, actually, since quite a few spiders are known to eat vertebrates like frogs, toads, salamanders, lizards, snakes, mice, rats, bats, and small birds. Most of the knowledge of fish-eating spiders has been anecdotal. Nyffeler and Pusey wanted to change that, so they conducted the first worldwide assessment of piscivory (fish eating) in spiders.

It turns out that fish-eating spiders can be found on every continent except for Antarctica, and occurs in dozens of species.


The researchers explain that this is true predation, not just opportunistic scavenging:

Predation requires that a prey item must have been killed and eaten by the predator. Both behavioral traits – killing and consumption – have been witnessed many times by a large number of researchers in the wild and in captivity. These spiders possess large strong chelicerae capable of piercing the skin of vertebrates and are equipped with powerful venoms containing hundreds of different neurotoxins, some of which are specific to vertebrate nervous systems. The vast majority of fish (~85%) are bitten by the spiders at the base of the head.


A fish prey must always first be dragged by the spider to a dry place before the feeding process can begin. Such a dry feeding site can be a rock, tree trunk, halfway immersed log, or an aquatic plant emerging from the water. The behavior of always first moving a fish prey to a dry site prior to feeding can be explained by the spiders' extraintestinal digestion – first pumping digestive enzimes into the prey and thereafter sucking in the dissolved tissue through the mouth opening; otherwise the digestive enzimes would be diluted in the water and, thus, become ineffective. This type of feeding behavior has been witnessed in spiders from all families engaged in fish-catching. A second reason for this behavior may be that on land the spider has physical superiority over its aquatic prey and its potential for escape is greatly reduced.


Behold: spiders feasting on fish, from around the world.


Read the whole paper at PLoS ONE.


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