Some Spiders Grow Bigger in Urban Areas

Bigger, beefier spiders are emerging because, as a new study has found, some spiders love city and suburban living. Continue reading →

Artificial lighting, lawns, urban "heat islands," and other features of city and suburban life are leading to bigger spiders with an increased ability to reproduce, a new study finds.

The study, published in the latest issue of PLoS ONE, could help to explain why some homeowners are finding particularly big spiders in their gardens. Researchers say the effect is noticeable among common orb weaving spiders. (Orb weavers are spiders that build spiral wheel-shaped webs.)

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Like an unintended Frankenstein experiment, we are facilitating spider growth with light posts.

"Artificial night lighting has many implications for spider fitness as it leads to local increases in insect abundance, and increased prey capture for spiders in lit habitats," wrote lead author Elizabeth Lowe from the University of Sydney, Australia, and colleagues.

The researchers focused on a particular common orb weaver, Nephila plumipes, because it lives in both urban and rural settings. They measured and then compared the body size, fat reserves and ovary weight of the spiders in each habitat. Ovary weight is an indicator of reproductive ability.

Lowe and her team found that spiders had smaller bodies in areas with more vegetation cover, and larger, fatter bodies in areas associated with urban development. One would think that clean, country living would be optimal for the spiders, but not so. To paraphrase the old Alicia Bridges song, they "love the nightlife" and thrive under the bright lights. The spiders also seem to like living on and around hard surfaces.

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The researchers theorize that the spiders also do well with increased heat. Cityscapes create "heat islands" where the temperature is significantly hotter than other regions. According to the EPA, the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4 degrees Fahrenheit warmer than its surroundings.

Heat islands occur where buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry. This is obviously a problem for most, but apparently not for these spiders.

The spiders create a base for themselves in and around structures and often near a prey-filled lawn. When given the choice between being near leaf litter or a lawn, the researchers found the orb weavers selected the latter.

Another important difference between these city and country spiders is that country spiders seem to suffer from more parasites. The heat, hard surfaces and more discourage the growth of the tiny parasites that prey on spiders. City spiders therefore don't have to deal with this problem as much as their country cousins do.

As for ovary weight, it was greater in the city/suburban spiders. This measurement suggests that they will reproduce more often, according to the researchers.

News stories often highlight animals that suffer as a result of human-changed landscapes, but we would also do well to watch out for "urban exploiters" such as these spiders.

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Lowe and her team concluded, "By identifying the elements of cities that influence the success of urban exploiters we gain a better understanding of what drives changes in the biodiversity of urban systems."

Photo: the spider Nephila plumipes. Credit: Elizabeth Lowe

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.