By Will Dunham
WASHINGTON (Reuters) - "Will you walk into my parlor?" said the spider to the fly; "'Tis the prettiest little parlor that ever you did spy."
English poet Mary Howitt's "The Spider and the Fly" doesn't tell the half of it. Spiders of course are happy to devour flies, but their appetites sometimes go beyond mere insects.
Researchers documented this wide-ranging palate in a study published on Wednesday detailing how at least two dozen species of spiders on every continent but Antarctica eat small freshwater fish that often exceed them in size.
Scientists have long known that some spiders consume fish, but the study -- considered the first systematic review of the topic -- showed that the practice is far more common and geographically widespread than previously understood.
"Fish may represent a 'big-ticket item' in the nutritional budget of semi-aquatic spiders," said zoologist Martin Nyffeler, of the University of Basel in Switzerland, who led the study, which was published in the scientific journal PLOS ONE.
"Fish meat is high-quality prey regarding protein content and caloric value," Nyffeler said. "Feeding on fish may be particularly advantageous during the mating period when the elevated energy and protein requirements of gravid (pregnant) female spiders require increased food intake, or at times of limited availability of invertebrate prey."
The spiders employ potent neurotoxins and enzymes to kill and digest fish. Semi-aquatic spiders -- able to swim, dive and even walk on the water surface -- lurk at the edges of shallow streams, lakes, ponds or swamps. They often anchor their hind legs to a stone or plant, with their front legs ready for the ambush on the water's surface.
After the spider snares a fish, it drags it to a dry place like a rock or tree trunk to begin the feeding process: pumping digestive enzymes into the fish and sucking out the dissolved tissue like a milkshake.
"It takes a spider usually many hours to devour a fish until nothing is left but bones and scales," Nyffeler said.
Fish-catching is largely limited to warmer climates. The Florida wetlands and neighboring regions are a particular hot spot, with mosquitofish in large numbers.
The researchers said at least 18 spider species from five families have been observed catching fish in the wild. And six more species, including some from three other families of spiders, have been documented doing it under laboratory conditions.
About 30 species of fish ranging from just under an inch to 2-1/2 inches long (2 to 6 cm) have been observed being captured by spiders in the wild.
In North America, the species Dolomedes triton frequently catches fish. In Australia, the species Dolomedes facetus steals goldfish from ponds in suburban gardens.
Fish are not the only small vertebrates that spiders include on the menu. Some have been known to eat frogs, toads, salamanders, lizards, snakes, mice, rats, bats and birds, Nyffeler said.
Brad Pusey of the University of Western Australia, who also worked on the study, noted: "Spiders are more adept predators than most give credit for."
(Reporting by Will Dunham; Editing by Leslie Adler)