Species Spotlight Challenge - Day 15 - Common (or Edible) Periwinkle

[EDIT - IMPORTANT NOTE - this blog is not a recommendation to eat periwinkles off your local seashore. It may not be safe (due to sewage or industrial effluent) and also the identification of the species may not be  straightforward (as I found).]

I went with my family to the beach at South Gare today, taking advantage of the sunny weather. After a bit of reading and a bit of paddling I went for a walk along to the rocky breakwater to see what I could see in the way of wildlife. As well as terns, gulls and wading birds (and wading humans), I found some marine creatures and  brown, green and red  marine algae (aka seaweed) on the rocks which were exposed by the low tide. 

The one I am going to focus on today is the Common Periwinkle (Littorina littorea). This is the largest and commonest of the British periwinkle species and can be found on rocky coasts (except the most exposed areas) from below the low tide mark to the upper shore. Periwinkles are marine snails (gastropod molluscs) which use their file-like tongue, called a 'radula' to feed on a broad range of the more delicate seaweeds.

You might think of migration as something that birds or herds of wildebeest or caribou do, over huge distances, but in more northerly locations (including the north of England, where I live) the Common Periwinkle does it too, but on a much smaller scale. They migrate down shore in the autumn as the temperature gets colder, as they wouldn't be able to tolerate the sub-zero temperatures which they might be exposed to above the water. In the spring they head back up shore. They can tolerate being exposed to the air (so long as it is warm enough) but they are usually inactive above water unless the environment is very moist. This is apparently because it is easier (less energy-expensive) to move on mucus (as all gastropods do) underwater.

The alternative name, Edible Periwinkle, is because this is the species which was most commonly eaten by humans and apparently can still be found for sale in coastal areas of Scotland and Ireland, in paper bags with a pin attached. The pin is to stick into the operculum (the little plate that closes the aperture of the shell), so that you can twist it to remove the animal inside in order to eat it. No periwinkles were hurt in the writing of this blog today though.

Thanks are due to my friend, the marine biologist Dr Bob Sluka who confirmed my identification [EDIT - and to Hannah Hereward who corrected both our IDs for at least some of the snails shown in the pictures below😳]. 

Common Periwinkles (Littorina littorea) [EDIT - The one on the right is a Common Periwinkle. The other three are now thought to be Thick Top-shells (Phorcus lineatus)]

Common Periwinkles, with an unidentified limpet (Patella sp) and some small
barnacles on the bottom left hand rock. [EDIT - I think the three in the top right, plus one of the others, are Common Periwinkle. The others are likely to be Thick Top-shells, or possibly another species.]

Common Periwinkles above and below water, with some more limpets (possibly a different
species from the other one). [EDIT - I am no longer sure of which species the snails in this
picture belong to, but have decided to leave the photo in because a) I like the fact that it shows
them above and below water, and b) it is a good lesson in making
sure of your identification before publishing 😊]