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The Seashore

Flat Periwinkles

Flat Periwinkles

 

If you have read the section on Rough Winkles you will know the difficulty of telling different species apart. Here is another one. Hence there is no scientific name given at the top. Read on:

They are very abundant on rocky shores in the middle and lower shores where they are associated with the Fucus seaweeds. The shell is around 1 cm long. There is no distinct spiral tip and is very rounded and smooth. There are many different colour forms from white through yellow, orange, brown to black.

Flat perwinkle feeding on Pepper Dulse

Flat perwinkle feeding on Pepper Dulse

This herbivore feeds (well, nibbles) on Fucus seaweeds and Pepper Dulse. It has gills and breathes dissolved oxygen from the water. Hence, it is found further down the seashore than the Rough winkles. It is the periwinkle with the least tolerance of changes in the temperature and humidity.

The smooth, rounded shell blends in with the bladders of the bladderwrack. Numerous colour variations give camouflage at different seasons as algae change. This is another good example of genetic polymorphism. The eggs are laid in gelatinous masses which helps to prevent drying. These can be found on Fucus seaweeds where the eggs hatch into small periwinkles (no larval form). In this way they can start feeding as they arrive directly on to the food.

The story of which species is which changes but one of the latest accounts can be seen in Seashore by Peter Hayward, New Naturalist series, HarperCollins 2004. There are two species which cannot easily be told apart in the field. Littorina obtusata lives up to three years and feeds on adult Ascophyllum and Fucus vesiculosus and hence favours the middle shore. This species delays reproduction until its second year, reproductive output is low and the adult population is maintained at close to the maximum the habitat can support (referred to as K selection). L. fabalis lives only a year and reproduces within six months of hatching. This species lives on F.serratus, grazing algae and epiphytes off its surface and hence favours the lower shore. Intensive crab predation has favoured a strategy where this species grows rapidly and breeds early (referred to as r selection).

Widespread in Atlantic regions.

Other periwinkle species include:

Melaraphe (small periwinkle), Rough Periwinkles and Edible Periwinkle

 


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