Littorina littorea (Linnaeus, 1758)

Periwinkles are the pebbles of the sea. You find them along the entire coast, on breakwaters, dikes, piers, etc. When a periwinkle ventures high onto the dike on a hot summer day, it lets itself roll down the slope when it gets too hot, hoping to land in water. As it crawls over a hard surface, it leaves behind a trail of slime. All kinds of algae stick to this slime. Other snails, such as laver spire shells, graze upon these slimy trails. If you pick up a periwinkle, the snail will hide in its shell. Pick up several snails and shake them in your hands, then the snails often crawl out of their shell. They confuse the shaking with wave movement. They think that the tide has risen and it is time to look for food.

Alikruiken kun je langs de hele kust vooral op de dijken vinden. Wanneer een alikruik over een hard oppervlak kruipt laat hij slijmsporen achter. In dit slijm blijven algen plakken. Andere slakjes, bijvoorbeeld wadslakjes, maken hier gebruik van en begrazen de slijmsporen van de alikruik. Als je een alikruik oppakt, verstopt hij zich in zijn huisje. Als je er een paar tegelijk pakt en ze in je hand tegen elkaar schudt, komen ze tevoorschijn. Ze denken dan dat het hoogwater is en ze in het water terecht zijn gekomen.

The flat periwinkle grazes on detritus and microorganisms. It also feeds on green seaweeds such as sea lettuce (Ulva lactua) and species of Enteromorpha (2). Breeding may take place throughout the year, but tends to vary depending on temperature (4). The sexes are separate (individuals are either male or female), and fertilisation occurs internally after copulation (2). Egg laying is timed to coincide with the spring tide; the eggs are laid in the sea in gelatinous capsules that usually contain around three eggs (although up to 9 eggs per capsule has been known) (4). The egg capsules float in the sea and free-swimming 'veliger' larvae hatch after a few days. After 6 weeks spent in the ocean, the larvae settle on the shore. The young periwinkles attain sexual maturity at two or three years of age and may live for up to five years (2).

Periwinkles are a large family of gastropod molluscs found on the shore. The edible periwinkle is perhaps the best known species of this family, as it has been collected and eaten for centuries. After boiling, the soft body is 'winkled out' from the shell with a pin or a special 'winkle-picker' (3). This species is the largest periwinkle found in Britain. It has a dark grey or black conical shell that develops a smooth surface with age. The flattened tentacles bear obvious black bands, a feature which allows young specimens to be identified easily. Males can be distinguished during the breeding season by the presence of a penis on the right side of the body (4).