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This article has appeared in:

National Geographic News
The Montreal Gazette
The National Post
The McGill Reporter

Image courtesy of Dr. Ron Chase

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Love Hurts

by Ian Popple

In a quiet third floor biology laboratory of McGill University, garden snails (Helix aspersa) are courting….in slow-motion. The snails belong to Dr. Ron Chase, who has spent the past 30 years using these organisms as subjects for his study of neurobiology, behaviour 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 2-6 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 that is 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 object of their affections (i.e. other snails). Snails manufacture love darts from calcium carbonate, the same material that they use to build their shell. The dart apparatus itself is located in a multifunctional genital pore which also serves as home for the snails' penis, a receptacle for received sperm and a release point for fertilized eggs.

"The love dart phenomenon is documented in the literature as far back as the mid-17th century" recalled Chase. "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 the question of why snails shoot 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", explained Chase. "Another belief was that the dart was intended to arouse the receiver and indicate the shooters 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 that their sperm, rather than another's, is used to fertilize eggs will therefore 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 are instead digested in great numbers en route. Research conducted by David Rogers, a graduate student of Chase, revealed that of the millions of sperm received by a snail, only 0.025% 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". 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 stabled with a hypodermic needle. Evidence from mating trials conducted in the Chase lab indicates that snails try to avoid being hit with love darts. Since snails are hermaphroditic, each snail possesses the love dart apparatus and copulating pairs are commonly seen jostling, in an attempt to hit but not be hit. All is fair in love and war, so they say!