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Are Snails' ''Love Darts'' Source of Cupid Lore?

Ian Popple
McGill Tribune
February 13, 2002
 
Valentine's Day is upon us, and once again love is in the air. In a
quiet third floor laboratory of Montreal's McGill University's Stewart
Biology building, garden snails, Helix aspersa, are
courting—in slow-motion.

The snails belong to Ron Chase,
who has spent the past 30 years using these organisms as subjects for
his study of neurobiology, behavior and evolution.




Snail reproduction is a curious tale. Snails are hermaphrodites, but although individuals contain both male and female sex organs, they do not self-fertilize. The two- to six-hour marathon session that is snail copulation is actually an exchange of sperm between two individuals, combined with plenty of rubbing, biting and "eye-stalk" waving. Individuals use the received sperm to fertilize their own eggs—a process necessary to maintain genetic diversity in the population.

What makes some snail species particularly interesting to Chase is their use of "love darts" during copulation. About one third of snail species manufacture hard, sharp darts which they "fire" at the objects of their affections.

"The love dart phenomenon has been documented in the literature as far back as the mid-17th century," Chase said. "Love dart snails were known to the ancient Greeks, and it wouldn't be surprising to find that they influenced the creation of the cupid myth."

Chase became intrigued with snails' "love darts."

"It was incorrectly believed that these darts were a nuptial gift of calcium—a major constituent of snail shells—from one snail to another. Like a bunch of flowers or a box of chocolates in humans," Chase said. "Another belief was that the dart was intended to arouse the receiver and indicate the shooter's readiness to mate."

Research conducted by Chase has uncovered the real reason for dart-shooting in snails, and the truth is much more sinister than previously thought.

The answer lies in Darwin's theory of sexual selection. Basically, snails want to reproduce as much as possible. Snails that have a way of ensuring their sperm, rather than another's, is used to fertilize eggs therefore will sire more offspring. This is known as sperm competition.

The love dart is a tool of male manipulation. Received sperm is moved to a storage area within the female reproductive system where it is used to fertilize eggs over a period of months or years; however, many sperm fail to reach the safety of the storage area and instead are digested in great numbers en route.

Research conducted by a graduate student of Chase's revealed of the millions of sperm received by a snail, only 0.025 percent actually survive. Love darts contain mucus that temporarily contracts a part of the female reproductive system in a way that allows a greater number of sperm to reach the storage area and survive; in short, he shoots, she stores.

According to Chase, being hit with a love dart may increase the survival of sperm, but fortunately for some snails it is not essential for copulation.

"Poor shooting is commonplace—one-third of all love darts either fail to penetrate the skin or they miss the target completely," Chase said.

Being hit with a love dart may sound cute and comical, but for the recipient there may be costs. Love darts are the equivalent of being stabbed with a hypodermic needle. Evidence from mating trials conducted in the Chase lab indicates snails try to avoid being hit with love darts. Copulating snails commonly are seen jostling, in an attempt to hit but not be hit.

Copyright 2002 McGill Tribune via U-WIRE
 

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