New Zealand’s fish-eating spider

It seems that New Zealand, along with many other countries, is home to spiders keen on a fishy dinner. In a first of its kind study into fish predation by spiders, researchers from Australia and Switzerland found that New Zealand’s Dolomedes dondalei was capable of catching little fish and indeed did so in laboratory experiments.

Intrigued by anecdotal evidence of fish hunting by spiders, arachnologists Dr Martin Nyffeler, University of Basel, and Dr Bradley Pusey, University of Western Australia, undertook a systematic review of spiders around the world. They compiled evidence from more than 80 reports, published and unpublished, of fish predation and hunted for photos only shared online. They found that spiders from at least five different families prey on fish in the wild, using powerful neurotoxins and enzymes to kill and digest fish even larger than they are (captured fish were on average 2.2 times longer than the spiders hunting them). Fish-grabbing arachnids were observed at the fringes of shallow freshwater streams, rivers, lakes, ponds, swamps and fens – making the occurrence of this type of predation far more widespread than previously imagined and identifying that fish are of substantial nutritional importance to these types of spiders.

In their published paper, the pair writes that, “… fish predation by semi-aquatic spiders is geographically widespread, occurring on all continents except Antarctica”.

Despite the geographical spread, fish predation is more common in warmer areas. Dr Nyffeler suggests that this might be because warmer water carries less oxygen, forcing the fish to live nearer the surface.

The researchers write that fish dinners typically range from 2–6 cm in length, with the species of fish hunted usually among the most commonly occurring in their respective geographic area, such as mosquitofish in the southeastern USA, fish of the order Characiformes in the Neotropics (mostly South and Central America and southern Florida), killifish in Central and West Africa as well as Australian native fish.

“Naturally occurring fish predation has been witnessed in more than a dozen spider species from the superfamily Lycosoidea, in two species of the superfamily Ctenoidea and in one species of the superfamily Corinnoidea.”

However, over three-quarters of the compiled evidence reported on fish predation is by pisaurid spiders of the genera Dolomedes and Nilus. This category includes New Zealand’s Dolomedes dondalei, the largest of the three species of Dolomedes native to New Zealand. Pisaurid spiders don’t spin webs or use silk to catch prey – they hunt down it down or sit and wait to ambush a passing meal.

“There is laboratory evidence that spiders from several more families (e.g. the water spider Argyroneta aquatica, the intertidal spider Desis marina and the ‘swimming’ huntsman spider Heteropoda natans) predate fish as well. Our finding of such a large diversity of spider families being engaged in fish predation is novel

While they can’t breathe underwater, one thing all these fearless semi-aquatic spiders have in common is their ability to walk on water, swim and/or dive. Typically, a fish-hunting spider will wait poised on a rock or on vegetation overhanging a body of water with three or four legs sitting on the water surface feeling for vibrations from below. When they feel a fish nearby, they leap into the water to wrestle and subdue their prey with venom before dragging the unlucky fish back to shore. And although the catching might be quick and ruthless, the researchers write that it can take many hours for a spider to consume a fish, injecting the body with digestive enzymes before eating the liquefied flesh. This all suggests “that spiders can extract a substantial amount of energy while feeding on such large prey”.

The researchers write that another noteworthy feature is the size of the prey in relation to the spider. They write that seldom in nature do lone predators go up against prey substantially bigger than they are – it is usually the other way around.

Although fish predation is obviously more common than previously thought, the researchers believe that a spider living solely on a fish diet is “probably rare and restricted to those occasions when semi-aquatic spiders gain easy access to small fish kept at high density in artificial rearing ponds or aquaria or in small shallow waterbodies. Additional research will be needed to reveal the extent and nutritional importance of fish in the diet of these spiders.”

The research was published on 18 June 2014 in the journal PLoS ONE.