Spiders Hunt Fish!

Since I'm a biologist, friends and relatives sometimes assume I know a lot about the living world. In fact, like most people trained in the sciences, I know quite a lot about a relatively specific subject and progressively less the further away you get from my focus. I can tell you a good deal about plants and evolution and even ants, but I'm not nearly as well informed about the human body, for example. I'm well-equipped to quickly learn about humans, but it's not a subject I've studied and internalized. The same goes for the countless other life forms on Earth and the multifarious interactions that keep the whole show going, minute to minute and month to month. That means there's a whole world of excitement for me to discover, even in seemingly simple facets of life on Earth! For example, I was happily surprised by the findings of a recent paper in PLoS ONE which reviewed published evidence of fish-hunting by spiders. In some innocent combination of naïveté and ignorance, I had always pictured spiders hunting insects and other arthropods (and maybe the occasional vertebrate), but it turns out that many species (from at least eight families!) prey on fish, too.


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The researchers surveyed published literature for evidence of fish predation by spiders, as well as searching online and following up with the original authors and photographers. They found 89 documented cases of free-living spiders attacking fish, including a few cases where semi-aquatic spiders attacked fish in an aquarium after "wandering into buildings", and nearly 70 "staged" observations in which spiders killed fish in aquaria, tanks or pools. Fish are a big meal for spiders; on average, they were twice as long as the spiders, but there were also instances of spiders preying on fish up to four times their size! These were probably incidents of genuine hunting and not scavenging, since spiders in captivity only eat dead fish when they're quite hungry.

So how do spiders catch fish? Semi-aquatic spiders hunt by anchoring their hind legs to a solid surface and stretching out their front legs on the water's surface. Waiting in that position, they use the water similarly to a web, sensing vibrations in the surface tension caused by insects. It's an amazing way of hunting, but it doesn't work as well for fish; spiders don't seem to respond to the surface waves from underwater fish. In most cases, spiders attacked after an accidental touch by the fish -- its dorsal fin brushing against the spider's outstretched leg, for example. The spider lashes out and bites the fish, usually just behind its head, and injects it with a potent cocktail of neurotoxins. The toxins can kill the fish within seconds or minutes, though larger fish may take as long as 50 minutes to die. After attacking, the spider drags its meal onto dry land, where it can more easily overpower it and eat in (relative) safety. In fact, spiders would have a hard time eating in the water; they digest their food externally by pumping them full of enzymes and then sucking out the innards, and their digestive enzymes would be diluted beyond use in water.

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Although spiders were found hunting fish on every continent except Antarctica, over half of the observations came from North America. This bias probably results from a combination of factors. Semi-aquatic spiders are more common in some parts of the world, including the freshwater wetlands of the southeastern USA and south-east Asia, and relatively rare in Europe. North America is also home to more potential observers than south-east Asia; although the population is smaller, there are probably more people likely to observe and report spider predation.

Observed incidents of fish predation by free-living spiders. Larger dots
indicate several observations in the same area.

The availability of suitably-sized fish also seems to have influenced the development of fish predation in spiders. According to the researchers, there are fewer small-bodied fish species in the northern latitudes, setting a boundary of roughly 40°N for fishing spiders. Of course, young fish — even those of large species — are still small in the northern reaches, but their seasonal availability probably may make spiders less likely to rely on them as a food source, reducing the selective pressures to evolve fish predation. A similar gradient in fish size also explains the higher prevalence of fish predation in the eastern US, where there are more small-bodied fish species than in the western states.

Finally, semi-aquatic spiders can only catch fish that come near the water's surface. The evolution of this behaviour therefore depends to a certain extent on the oxygen profile of the water column. In poorly-oxygenated water, fish will swim closer to the surface to take advantage of the oxygen-rich surface water, but at the cost of becoming exposed to arachnid attacks. Water can retain more oxygen at the colder temperatures of northern latitudes, including Canada and much of Europe, so fish in those areas can swim at safer depths. Of course, fish might still end up as spider-food if they come to the surface for other reasons, such as hunting insects.

It's not clear how large a role fish actually play in the spiders' diet. The researchers venture that fish probably constitute "a minor proportion of the diet of semi-aquatic [spiders]", but they also note that fish are likely much more rewarding prey than the spiders' usual arthropod fare. Not only are fish bigger than arthropods, but they're mostly made of good, nutritious muscle, while arthropods come with a bulk of indigestible exoskeleton. The importance of fish as a food source for spiders probably varies as their energy needs wax and wane (e.g., gravid females) and depending on the relative abundance of fish and other prey. In the researchers' words, "semi-aquatic spiders are an important component of freshwater and terrestrial food webs with multiple linkages within and between both", and a better understanding of the importance and ecology of this interaction — which is more common than we had realized — will only emerge with further research.

Ref
Nyffeler, M and Pusey, BJ. Fish Predation by Semi-Aquatic Spiders: A Global Pattern. PLoS ONE 9(6): e99459. (2014) doi:10.1371/journal.pone.0099459

Image credits
All the images are figures from the paper and are distributed under a CC-BY license.