A Snail’s Dart of Love (A Guest Post)

By Jenna Miskowic

Snails that shoot darts. Who would have thought? Turns out, snails have a lot of competition for mates. Females of some snail species have evolved ways to select which males they want to be the father of their eggs. One of these strategies is a female can mate with multiple males and store their sperm. The female can then “choose” which sperm she wants to fertilize her eggs. This affects how males compete for mates. Males want to make sure they are the father to the offspring because they want their genes to be passed on. So male snails have developed ways to increase their chances of paternity.

Euhadra quaesita gliding through foliage. Image by Angus Davison
and Satoshi Chiba posted at Wikimedia Commons.

Enter the dart-bearing land snail, Euhadra quaesita. Snails of this species are simultaneous hermaphrodites that use cross-fertilization. Simultaneous hermaphrodites are animals that have both female and male reproductive tissues and systems. Cross-fertilization means that the snails require a mate. So, when two dart-bearing land snails cross paths and decide they want to mate, they will take their love-dart and pierce it into their mating partner. Because the snails are simultaneous hermaphrodites, they both perform this behavior before exchanging their sperm.

Love darts are composed of a crystalline form of calcium carbonite, which is what sea shells are made of, called aragonite. They are very sharp and pointed so that they are able to pierce the other snail. The dart is covered with a secretion from its mucous glands. When the dart pierces into the other snail, mucus is transported from the dart’s glands into the pierced snail’s blood. This mucus helps increase the amount of sperm being stored in the recipient snail and increases the likelihood of the donor snail being the father to the offspring of the recipient snail. Researchers Kazuki Kimura, Kaito Shibuya, and Satoshi Chiba from Tohoku University in Japan hypothesized that the dart’s mucus would also reduce future matings and promote laying eggs, also called oviposition.

Drawing of Euhadra quaesita’s love-dart. Cross-section on the left and lateral view on the right.
Image by Joris M. Koene and Hinrich Schulenburg posted at Wikimedia Commons.

To test these hypotheses, the researchers conducted two separate experiments. The first experiment focused on the effects of dart shooting and future matings of the recipient snail. Individually, non-virgin adult snails were presented with a non-virgin or virgin adult for their initial mating. In this species, non-virgin adults shoot their darts and virgin snails do not shoot their darts while performing the mating behavior. Thus, the subjects paired with a non-virgin adult were pierced with their partner’s love-dart, and the subjects paired with a virgin adult were not pierced with their partner’s love-dart. Then the subjects were offered to mate again with an unfamiliar non-virgin snail with a high mating motivation caused by individual rearing. They recorded how long the snail subject went, in days, before mating again with another individual of the same species. The researchers found that the amount of time between matings was longer in pierced snails than in ones not pierced.

The second experiment focused on the effect of injected artificial mucus on future matings and promotion of oviposition behavior. Researchers dissected an extract of the mucous glands out of adult snails and combined it with saline solution to create the artificial mucus. There were two groups used in this experiment: (1) adult snails injected with the artificial mucus, also known as the treatment group and (2) adult snails injected with only the saline solution, also known as the control group. They recorded the number of hatched eggs and their parentage. They found that artificial mucus-injected snail pairs mated less often than the control pairs. Additionally, they found that the amount of the snails that laid eggs was larger in the snails injected with artificial mucus. These findings support the researchers’ hypotheses that dart mucus can subdue future matings in its recipients.

So what are the benefits to stabbing your partner with a love dart? Well, if an animal has multiple partners, then it is quite advantageous for the partner to make sure that they are the parent. Mating suppression after being injected with the love dart is one way to fight off the competition. So, beware to all who search for Cupid’s arrow this Valentine’s Day. There may be more to an arrow of love than you realize.


References

Kimura, Shibuya, & Chiba. (2013). The mucus of a land snail love-dart suppresses subsequent matings in darted individuals. Animal Behaviour, 85(3), 631-635.