Periwinkles

Periwinkles must be the most known of “snail like” molluscs found on Cornish shores. There are four species widely found on rocky shores; flat periwinkle Littorina littoralis, edible periwinkle Littorina littorea, rough periwinkle Littorina saxatilis and the small periwinkle Littorina neritoides.

Periwinkles are a very adaptive group, the four species between them have manage to settle most of the zones. The small periwinkle living in the splash zone on the extreme upper shore; the rough winkle periwinkle the upper shore to the middle shore; while the flat periwinkle and edible periwinkle the middle shore to the lower shore.

The zones where the species live has affected their development. The two species the small and the rough periwinkle that live higher up on the beach and hence are uncovered by the tide for most of the time, have evolved the ability to breath in air. The other two species living further down the shore have not evolved this ability.

        Flat Periwinkle Littorina littoralis

           Small Periwinkle Littorina neritoides

        Edible Periwinkle Littorina littorea

           Rough Periwinkle Littorina saxatilis

Flat Periwinkle Littorina littoralis

A very common shellfish on beaches with brown seaweeds, this is particularly so if bladder wrack dominates and are therefore found the same zone. The shell’s size and shape is similar to the bladders on this seaweed and provides better camouflage.

The colour of the shell widely varies, most commonly being brown or yellow, but can be white, green or even banded.

The prevalence of differing colours is dependent on the habitat. The common bright yellow colour seems an unusal defence mechanism. However when yellow shells on seaweed are seen underwater from below, the shell appears to be a similar colour to bladders on the seaweed.  This is due to brown seaweed with the light from above shining through it, creating a natural light filter.

Flat periwinkles live on the middle to lower shore and are prone to desiccation hence are found in moist conditions under seaweed and rocks. The empty shells are seen more often than live specimens, especially the bright yellow shells which stand out against the beach background.

Flat periwinkles do not go through a planktonic stage. They lay eggs on seaweed, the young hatching as smaller versions of the adults.

Edible Periwinkle Littorina littorea

This is the most well known of all of the periwinkles as they are commonly eaten. They are an adaptable species as they thrive in different salinities and exposure levels; and across most beach zones, making them very abundant.

Edible periwinkles are the largest periwinkle reaching a size of 2 inches.

Edible periwinkles are mostly black and grey, but can be found in a variety of colours and stripes. Shells in the adults tend to be smooth but in the young are ridged.

The sexes are separate. After mating the female releases packets of eggs in floating capsules, at the time of spring tides. Each capsule containing three eggs.

The eggs hatch, producing planktonic larva. These eventually settle on the shore as small shelled individuals.

Rough Periwinkle Littorina saxatilis  

A small shell with colour variations from, black, yellow, green, red to white. The shell is ribbed shell with many growth lines. Shells vary to such a degree that it raises questions as to whether this is only due to the species adapting to the local habitat, or whether they are sub-species themselves.

The rough periwinkle lives on the upper shore, it has evolved to bypass the planktonic stage, it is ovoviviparous. The females retain the fertilised eggs in their body and bear live small, shelled, copies of the adults.

This reproductive strategy bypasses the planktonic stage where the larva face a high predation rate, instead the young are born already with a protective shell. The drawback being that a planktonic stage provides a means for the species to spread large distances and find new areas to colonise.

Small Periwinkle Littorina neritoides

This common small periwinkle lives in the extreme upper shore and splash zone. This zone is only reached with spring tides, as such the small periwinkle requires a high resistance to drying out. Originally a marine mollusc it has even evolved a partial ability to be air breathing. This hardy periwinkle can survive on the stormiest of coasts.

The splash zone is not inhabited by any of the large algae i.e. seaweeds, instead the small periwinkle feeds on the lichens and plants found in this zone. The small periwinkle breeds at spring tides, as these are the only times that they are reached by the sea, a necessity for their eggs to be dispersed.