Are Snails Male or Female? Hermaphrodites and Love Darts

What does it mean that snails are hermaphrodites?

A hermaphrodite is an animal that produces both eggs and sperm. Land snails such as the garden snail are simultaneous hermaphrodites, meaning they have working male and female parts at the same time throughout their adult lives. A single organ, the ovotestis, tucked into the coils of the shell, makes both kinds of sex cells.

The reproductive system opens to the outside through a small genital pore on the right side of the head, just behind the tentacles. It is almost invisible unless the snail is mating. During mating, each snail turns out its penis through this opening and passes sperm to its partner, while also receiving sperm in return.

Being a hermaphrodite has a clear advantage for a slow animal. Any adult of the same species that a snail meets is a potential mate, so it never has to find a partner of the opposite sex.

Are all snails hermaphrodites?

No. Air-breathing land snails and land slugs are hermaphrodites, as are some freshwater snails such as pond snails. But many sea snails, including periwinkles and whelks, and some freshwater snails, such as apple snails, have separate males and females.

A few snails change sex during their lives. The slipper limpet, a sea snail of the North Atlantic, lives in stacks of shells. Young animals start as males, and they become females as they grow older and end up lower down the stack.

How do garden snails court and mate?

Garden snails mate mostly in late spring and summer, on warm, damp nights. Courtship can last for hours. The two snails circle each other, touch with their tentacles and nibble at each other’s bodies, especially around the genital pore. Gradually the openings on their right sides swell as the reproductive organs prepare.

When they finally mate, the two snails press the right sides of their heads together and each inserts its penis into the other. Sperm is passed in a package called a spermatophore. The exchange can take several hours. Most of the sperm a snail receives is later digested in a special sac, and only a tiny fraction is stored and used to fertilize eggs. That is where the love dart comes in.

What is a love dart?

A love dart is a sharp, slender spike of calcium carbonate, several millimeters long, made and stored in a muscular pouch called the dart sac. Some land snail families have them, including the helicid snails to which the garden snail belongs. Just before mating, when the snails’ bodies touch, each tries to fire its dart into the other’s body at close range. The shots are inaccurate: in garden snails, about a third miss or fail to stick.

For centuries, people guessed that love darts were a calcium gift or a way of exciting the partner. Experiments on garden snails since the late 1990s have shown otherwise. The dart is coated with mucus containing a hormone-like substance. Once injected, it changes the shape of the partner’s reproductive tract, so more incoming sperm is sent to storage instead of to the digestion sac. In one study, snails hit by a dart stored about twice as many sperm from that partner. A 2006 study confirmed that the mucus, not the stab itself, does the work.

The dart is used up when fired. A garden snail needs about a week to grow a new one, so it often mates without firing a dart at all.

Can snails fertilize themselves?

Garden snails reproduce almost entirely by mating with a partner, even though they make both eggs and sperm. Some other species can self-fertilize. The great pond snail, common in ponds and ditches across Europe, can fertilize its own eggs if kept alone, although it prefers to mate. The decollate snail, a Mediterranean land snail with a shell that breaks off at the tip, often reproduces by self-fertilization.

Self-fertilization is a useful backup when mates are scarce, but offspring from a single parent have less genetic variety, which can make them less healthy. This is one reason most hermaphrodite snails still mate when they can.

How do snails lay eggs?

After mating, both garden snails use the stored sperm to fertilize their own eggs. A week or two later, each digs a small hole in loose, damp soil with its foot and lays a clutch of roughly 50–100 round, pearly-white eggs, each about 3–4.5 mm across. The snail covers the nest and leaves; there is no parental care. A single snail may lay several clutches a year. The young hatch after about two to four weeks as miniature snails with thin, almost see-through shells.

Slugs are hermaphrodites too

Land slugs, which evolved from snails, share the same hermaphrodite system. Their courtships can be even stranger. The leopard slug climbs a wall or branch with its partner, and the pair lower themselves on a thick rope of mucus before exchanging sperm in mid-air, as described in how do leopard slugs mate? Both slugs then go on to lay eggs, just like garden snails.