Common periwinkle

A few microscopic eggs at a time are enclosed in small egg capsules that drift around in the water. The eggs hatch after about a week and small larvae appear and drift around in the water and eat plankton. After about 3 weeks they transform to a bottom living form and fall to the bottom in shallow water.

Snail larva

Many are sterile

Many of the periwinkles are sterile because of parasitic flatworms have injured the periwinkles intestines and genitals. The parasites reproduce asexually in great numbers whilst eating the periwinkles, but without directly killing them. The periwinkle is an intermittant host for the parasites larvae. When the infected periwinkle is eaten by a gull, the parasitic larvae have arrived at the end host, where adult parasites develope and reproduce. The new larvae are then distributed along the beach in bird droppings and thus infect more periwinkles along the shoreline.


Large areas of the Baltic are far too brackish to support a vigorous population that reproduces on a regular basis. To the contrary, on the Swedish west coast, four specie of periwinkle are common and reproduce annually. Three of these specie have no pelagic larvae and are therefore relative stationary and reproduce generation after generation within the same coastal area. Natural selection forces adaption of these populations to the special circumstances that pervail in just that local environment where they live - they are specialists. The common periwinkle, because of its pelagic larvae, is distributed over a much larger area, generation for generation. This can result in the young periwinkles growing up in an environment that is alien to that which their parents grew up in. Presumably, they are able to survive the different and varying environments because they have generalized characteristics that enable them to function in most environments - they are versitile. Furthermore, they probably react to varying signals in the environment in which they have ended up and grow and therefore develope charcteristics that are compatible with the new environment in which they live.

Bo Johannesson

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Returns home?

Glue their shell

Can survive in ice

Eats brittle algae

The shell protects

Worms in their shell

Pelagic larvae

Many are


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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson