Love Darts

Shot Through the Head, for a Reason
By HENRY FOUNTAIN

Some garden snails have an unusual notion of the niceties of courtship. When one of these snails is wooing another, before mating it shoots a small, sharp dart into the other's head.

These "love darts," which are made of calcium carbonate and are less than half an inch long, were once thought to be a gift of calcium from one snail to another. But in recent years, researchers have determined that dart-shooting has a different purpose: to increase the amount of sperm that survives after mating.

"Really what the dart does is, it enables more of the sperm to be stored," said Ronald Chase, a professor at McGill University, who in years of snail research has helped unlock the secrets of this odd behavior. Snails can have many partners and can store the donors' sperm, mixed together, up to four years before fertilizing eggs. By increasing the amount of its sperm that survives, a snail can outcompete other donors and improve its chances at paternity.
Now Dr. Chase has gone a step further, showing that it's not the dart itself, but mucus coating it, that causes the changes in the receiving snail. By shooting the dart, the snail is injecting its partner with a compound that ensures that more sperm survives, Dr. Chase and Katrina C. Blanchard report in The Proceedings of the Royal Society B.
Dr. Chase studies brown garden snails, Cantareus aspersus, one of a number of land snails that are dart shooters. (Like other snails, these are hermaphroditic, so at the same time they are shooting darts and depositing sperm, they are getting shot and receiving sperm. Talk about a complicated lifestyle.)
The researchers surgically removed darts from some snails, and using a hypodermic needle instead, injected some with mucus and others with saline solution. After the snails mated, they found that those with the mucus had many more sperm.
Dr. Chase said a snail's female reproductive tract has openings to a gland that produces digestive enzymes that, under normal conditions, destroy most of the sperm. He suspects that a compound in the mucus, probably a peptide, results in contractions in the reproductive tract that close off the openings. He is currently conducting more research to determine which of the many peptides in the mucus is the active one. "That will be the magic stuff," he said. "The love potion."


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