Shells of the Cornish Seashore

This article aims to give the reader a brief overview of the common shelled species to be found around the coasts of Cornwall, obviously it is impossible to list every one and the best way for the reader to discover more is to go out there and look!

Limpets, Family Patellidae

Limpets are dominant members of rocky shore communities and are especially adapted to the intertidal zone of rocky coasts, where they are often extremely abundant. Feeding on microscopic algae they “garden” the shore by taming seaweed growth. Limpets feed by rasping at algae with a specially designed structure called a radula. The radula is like a conveyor belt of teeth, strengthened by iron and silica particles, moving over the surface of the rock scraping at the algae. On soft rocks where the limpet scrapes, you can sometimes see clear feeding trails. Limpets feed at high tide, leaving a mucus trail behind them. After feeding the limpet follows chemical cues laid down in the mucus to return to the same starting place, a shallow depression in the rock called a home-scar. Home-scars are created by chemical action and abrasion between the limpet shell and the rock. The shell fits snugly into the depression and helps the limpet to maintain a firm hold on the rock hence the expression “cling like a limpet”. Limpets can live up to 16 years!

Ten species occur in European seas. To differentiate between the various species of limpet it is essential to look at the colour of the limpets main muscle called the foot, which is on the underside of the animal. For example, the common limpet, Patella vulgata has an olive-green foot and the black-footed limpet, Patella depressa lives up to its name. However, removing a limpet from a rock without damaging its shell is extremely difficult and it is strongly recommended not to attempt to do so, as using force may cause permanent, even fatal, damage.

Common limpet (Patella vulgata)
photograph copyright Keith Hiscock from www.marlin.ac.uk

Winkles, Family Littorinidae

These are the most familiar and often most conspicuous and abundant of the intertidal gastropods. All winkles are grazing herbivores.

Identification of periwinkles

(Please note that this is only a guide, the best way to identify any animal is by using a good identification book).

1. Run a fingertip lightly along the shell. Is it smooth or rough to the touch?

• Rough Question 2
• Smooth Question 3

2. Look at the specimen with the opening of the shell towards you. (Please be careful not to damage the animal in any way when doing this). Look at the angle at which the opening of the shell meets with the main spire.

• If the angle is at right angles - Rough Periwinkle, Littorina saxatilis
• If the angle is less than a right angle – Young common periwinkle which often have spiral grooves and can become confused with the rough winkle.

3. What shape is the spire of the animal?

• The top of shell appears flattened – Flat periwinkle, Littorina obtusata
• If the spire is tall and sharply conical – Common periwinkle, Littorina littorea

Common periwinkles, Littorina littorea are collected for food. They feed on soft seaweeds on moderately exposed shores from the middle down to the waters edge. They are typically blackish to greyish-brown.

Common periwinkle (Littorina littorea)
photograph copyright Keith Hiscock from www.marlin.ac.uk

Flat periwinkles, Littorina obtusata exhibit a wide variety of colours, which are directly linked to their habitat. Green shelled winkles dominate sheltered shores whilst patterned brown-shelled winkles dominate exposed shores. Green and brown are not the only shades a flat periwinkle comes in; they can also be yellow, orange and red.

Flat periwinkle (Littorina obtusata) photograph
copyright Peter Barfield from www.marlin.ac.uk

Rough periwinkles, Littorina saxatilis vary in colour from red to black and they live very high up on the shore and in mudflats and estuaries. The rough periwinkle is a hardy species able to tolerate exposure for extended periods of time in the upper shore environment. Here they are usually found in cracks and crevices or among algae such as Pelvetia and Fucus spiralis where they find moisture.

Top Shells, Family Trochidae

These are globular or pyramidal shells with few whorls, often brightly coloured with darker bands. Top shells graze on rocky surfaces, feeding on microalgae and detritus. These animals are often very common on rocky shores. The toothed top shell, Monodonta lineata, the largest of the three species mentioned here, can reach a height of to 30mm. Toothed topshells have a tendency to stay sitting out on the rocks when the tide is out, and hence appear to be extremely abundant. With age the tip of the shell wears away and becomes more rounded and tends to reveal the shiny ‘mother of pearl’ beneath. The grey topshell, Gibbula cineraria is more elusive, hiding beneath stones and in crevices when the tide is out. It is distinguished by very fine patterning on its shell. The flat topshell, Gibbula umbilicalis is somewhat flatter and more rounded than the toothed topshell, however areas at the top of this shell also get worn away to reveal a shiny ‘mother-of-pearl’ surface. The flat topshell tends to be smaller than the toothed topshell only growing to about 15mm in height.

Flat top shell (Gibbula umbilicalis)
photograph copyright Keith Hiscock from www.marlin.ac.uk

Grey top shell (Gibbula cineraria)
photograph copyright Keith Hiscock from www.marlin.ac.uk

Dogwhelks, Family Muricidae

Dogwhelks, Nucella lapillus are common on all English shores and have a strong, heavy greyish-white to dark-brown shell with spirals running down it. They are one of the most voracious predators on the shore, but they are also one of the shores slowest killers. To kill a mussel the whelk locates its prey using a siphon to sniff it out. The whelk then begins a process of drilling into the mussel’s shell using chemical and mechanical means to get to the soft fleshy mussel beneath. The whole act of locating, drilling and finally consuming the mussel takes up to week!

During the 1970’s and early 1980’s vessels were coated with anti-fouling paint containing tributyl-tin (TBT) in order to prevent the settlement of marine fouling organisms on the hull. Marine fouling organisms led to mechanical damage on vessels and caused increased drag leading to higher fuel consumption. However, it has since been discovered that TBT is the most toxic material that humans have ever introduced into the marine environment. The impact was greatest in areas with heavy boat traffic and close to marinas, where boat moorings and maintenance activities were concentrated. Even very low concentrations of TBT can lead to a condition known as imposex in dogwhelks. Imposex is the development of male sexual characteristics by females eventually leading to sterility, and a change in the population structure. The use of TBT paints on small boats was banned in the late 1980s. However, recovery has been impeded because dogwhelks are slow to re-colonise areas which have suffered population decline. Furthermore, it is now recognised that TBT originating from larger vessels has also had detrimental effects (Davies and Bailey, 1991)

Dog whelk (Nucella lapillus)
photograph copyright Keith Hiscock from www.marlin.ac.uk

Mussels - Family Mytilidae

The common or edible mussel, Mytilus edulis is the most familiar and economically the most important mussel species. These mussels are deep bluish to purple in colour and the mantle edge is always light yellowish brown. They range in length from 2-20cm and usually occur in dense beds. Upper shore specimens can be long-lived reaching up to 15 years of age. Mussels attach themselves to rocks and stones by sticky threads called byssus. Recently, a method of extracting the sticky glue has been found and it is now being synthetically reproduced in laboratory conditions. The glue could be used in medicine and dentistry, as it is a water resistant adhesive and so ideal for use within the human body when mending fractures, during eye surgery and organ transplantation.

Common mussel (Mytilus edulis)
photograph copyright Keith Hiscock from www.marlin.ac.uk

This article provides only a very brief overview of the marine shelled molluscs that can be found along the shores of Cornwall. There are many others that have not been mentioned here but are waiting to be identified and recorded by you. Remember to bring a good shore guide (such as Collins Pocket Guide to the Seashore of Britain and Europe) and look under the rocks, as many creatures lurk below.

You can request a seashore recording form by contacting Cornwall Wildlife Trust on 01872 273939 or by email. Please record what you find and send completed seashore recording forms to the Cornwall Wildlife Trust.

Thanks to Caroline Jewell for providing this excellent information and to Keith Hiscock for permission to use his photographs.

Other items that may be of interest:

• Goose barnacles in Cornwall
• Spirula spirula in Cornish waters
• Visit the Marine Life Information Network (MarLIN) website at http://www.marlin.ac.uk/