Four species of Periwinkle show clear zonation, in descending order from the top of the shore these are the Small, Rough, Flat and Edible Periwinkles. They were once all considered to be in the same genus (Littorina), but one has now been shifted to the genus Melarhaphe. They all graze on algae and are well adapted for the zone they inhabit.
The Small Periwinkle – Melarhaphe neritoides
This species is almost a land snail when adult. It is very small (9mm max) and found in crevices, the empty ‘shells’ of dead barnacles and tufts of lichen. It is often found quite a long way above the high-tide mark on exposed shores where occasional splashes of sea-water can reach. Spawn is released into the sea at extreme spring high water, and after developing in the plankton the small young return and crawl back up the shore. It is often difficult to distinguish from small Rough Periwinkles, but it usually has a slightly bluish shell colour.
The Rough Periwinkle – Littorina saxatilis
This is another small winkle of the upper shore, although it does not go much above the high-tide mark and can be found quite low on exposed shores with little seaweed. This species has internal fertilisation and gives birth to live young snails (it is ovoviviparous). These two characteristics mean that local populations remain relatively isolated form one-another and there is much debate about the taxonomy of this species – some authors distinguish several species, others consider them sub-species, while yet others think they are just local varieties of the same species. (These niceties do not often bother beginners who are simply looking at zonation.) It is often difficult to tell small saxatilis from neritoides unless both are available (in the hand!) for comparison. They look different but the differences are not easy to describe – it is probably best to call all small winkles saxatilis unless they are found very far up the shore and look a bit different. (This does not sound very scientific – but it works!)
See below to distinguish large saxatilis from small littorea.
The Flat Periwinkle – Littorina obtusata
As the name suggests this species does not have a pointed shell and it is easy to distinguish from the other three. Eggs are laid on seaweed, and obtusata is only found on shores with Fucoid seaweeds. This means that – like the seaweeds – it is not present on the more exposed rocky shores. It is most abundant on the middle shore.
The Edible (or Common) Periwinkle – Littorina littorea
When an animal is called ‘edible’ it often means no more than ‘large enough to eat’. The other three species probably taste as good, but littorea is the only one big enough to be worth collecting as food – it can be up to 5cm. Like neritoides, this one releases its spawn into the sea, but because it inhabits the middle and lower shore it is not restricted to extremely high tides – any high tide in spring will do. It is often difficult to distinguish small littorea from saxatilis, but an infallible guide (if you have the patience) is to put them in seawater and wait until they show their tentacles. The Edible Winkle has annular dark rings around the tentacles whereas the Rough Winkle has dark stripes running from tip to base. (There are also differences in the way the shell aperture looks, but these are difficult to describe and for beginners to see!)
Ref: British Marine Life Study Society
References for species identification from ‘Marine Life Information Network for Britain and Ireland:
Other articles by John Blatchford
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