Spiders Invented Knees

We have spiders to thank (or not to thank) for knees, with the consolation that spiders have 48 knees while we only have two.

Spiders were the first creatures on Earth to evolve knees, according to a new study that also explains how this happened.

Knees began as a gene in fruit flies and other insects that duplicated and evolved in spiders, resulting in primary leg joints, reports the study, published in the journal Molecular Biology and Evolution.

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"Species constantly adapt and evolve by inventing new body features," lead author Nikola-Michael Prpic of the Göttingen Center for Molecular Biosciences explained in a press release.

Prpic and his colleagues focused their work on a particular gene called dachshund, or dac for short. The name reflects a bit of nerd humor, because the gene was first discovered in fruit flies, and was named for the missing leg segments and shortened legs that result from dac mutant flies.

What clued the scientists in on this gene in the first place is that arachnids, such as spiders, possess a second dac gene, dac2. It results only in the spider's kneecaps (patellas) during the individual's overall growth and development.

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The researchers used a technique called RNA interference to deactivate dac2. The deactivation caused the kneecaps to fuse to the tibias, resulting in single knee-less leg segments.

Spiders therefore likely experienced ancient gene duplication of the original dac gene. Over time, dac2 led to the evolution of an entirely new function and unique way of walking in spiders.

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Prpic said, "Our work shows how a gene can be duplicated and then used during evolution to invent a new morphological feature."

In humans, scientists continue to investigate how genes affect our knees. Many medical experts have theorized, for example, that fragile knees could be inherited. At least we only have two knees to deal with, as opposed to spiders and their multiple leg joints.

Costa Rican zebra tarantula (Aphonopelma seemanni) showing off its knees.

Spiders are known for nabbing insects, but many species frequently go fishing too, and researchers have the photos to prove it. A new study in the journal PLoS ONE documents fish-eating spiders all over the world. Most are semi-aquatic species that usually dwell at the fringes of shallow freshwater streams, ponds or swamps, keeping an eye out for a fish dinner. The spider

Dolomedes tenebrosus

was photographed devouring a creek chub on the banks of Bullskin Creek near Brutus, Kentucky, according to lead author Martin Nyffeler, a zoologist and spider expert from the University of Basel. He told Discovery News that this spider is less than an inch long and is often found in the U.S. south.

This fish-eating spider is huge. Its leg span alone is close to 7 inches. The spider lives near freshwater streams and rivers in Central and South America, but visitors likely won't see it during the day. "Adults are strictly nocturnal," Nyffeler explained. In addition to fish, Trechalea sp. eats a diverse array of other critters, including insects, shrimp and frogs.

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Many fish-eating spiders such as this are capable of swimming, diving and walking on the water surface. They have powerful neurotoxins and enzymes that enable them to kill and digest fish, which often exceed them in size and weight. "The finding of such a large diversity of spiders engaging in fish predation is novel," Nyffeler said. "Our evidence suggests that fish might be an occasional prey item of substantial nutritional importance."

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Australia is home to fish-eating arachnids like

Dolomedes facetus

. It's abundant in Oz, according to Nyffeler. He added that the spider is "known for its habit of occasionally catching goldfish and platies (another type of fish) in garden ponds in suburban Australia."

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An adult

Ancylometes sp.

was photographed preying on fish near Samona Lodge in the Cuyabeno Wildlife Reserve of Ecuador. It is another nocturnal spider that hunts at the edge of bodies of water when the sun goes down. Nyffeler said that the spider can dive and remain underwater for up to 20 minutes. In addition to hunting fish, it seeks out tadpoles, frogs, toads and lizards for supper.

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The photographer in this case snapped the spider as it ate fish on the bank of the Rio Maicuru in Pará State, Brazil. Nyffeler and colleague Bradley Pusey from the University of Western Australia suspect that the spider is a juvenile, since it was out hunting in the daylight even though the species is known to be nocturnal. This fish-eating spider also hunts insects, shrimp and frogs.

A fisherman was surprised to find this spider. He had just mis-cast his fishing line, with the bait landing just off the edge of a dock near Sebago Lake, Me. That's when the spider, according to Nyffeler, "scuttled out very quickly from underneath the dock attempting to attack the live bait fish." He continued, "Such incidences might be considered as predation attempts since the spider is grabbing a living fish with the intention to kill and devour it. The fact that fish are attacked even outside the water shows the high propensity for such spiders to feed on fish."

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This spider typically inhabits moist forests, where they hunt at night at the edge of bodies of water.

Ancylometes sp.

is enormous for a spider, with a leg span of close to 8 inches. Like many other fish-eating spiders, it goes after a variety of prey, such as tadpoles, frogs, toads and lizards.

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Based on the new study, naturally occurring fish predation by spiders has been reported from all continents with the exception of Antarctica. This spider was snapped near Samona Lodge, Cuyabeno Wildlife Reserve in Ecuador. In order to catch prey, this and other fish-eating spiders will typically anchor their hind legs to a stone or a plant, with their front legs resting on the surface of the water, ready to ambush. The fish will then be dragged to a dry place before the feeding process can begin. The spider will usually chow down for several hours on its fish feast.

A mosquitofish bit the dust in this moment, when a

Dolomedes triton

individual caught the fish on the edge of a small, slow-moving stream near Fayetteville, NC. The species is one of the most abundant North American fishing spiders with a strong affinity to water. It is common in the wetlands of Florida and neighboring states. The Florida wetlands are ground zero for fishing spiders, which seem to love the habitat there. Arachnid fans hoping to spot and photograph such spiders would do well to look for them at this location.