Seashells on the Seashore II

Blue-rayed Limpet Patella pellucida, on kelp fronds
Blue-rayed Limpet Patella pellucida, on kelp fronds

The area we call the seashore lies between the high-water mark, the shoreline, and line which marks the lowest level to which the water retreats on a falling tide. This is a transitional habitat shaped by the action of the sea, the geology of the land and geographical location. The composition of the coastal communities of plants and animals depends on the type of habitat which can vary from exposed rocky coasts to pebble, shingle and sandy beaches, and to coastal lagoons, mudflats and salt marshes.

The marine molluscs found on the shore will change with the type of seashore and, if you are collecting empty shells, the nature of the offshore habitats. For example if there are offshore kelp forests or reefs, you may find the shells of molluscs which live in these environments washed up on the beach or in the seaweed in the tide wrack.

The shells found on beaches are mainly from two groups of molluscs. The bivalves, where the shell is in two parts joined by a hinge, and the gastropods which produce a single, conical or coiled, shell. The bivalves are mainly sedentary, filter feeders and either live buried in the sand e.g. cockles, or are attached to hard surfaces e.g. mussels. In contrast gastropods are more like snails and move around either grazing on the rocks or on seaweed or predating other molluscs. Marine molluscs are a very diverse group of animals both in terms of their morphology and ecology. However, as a general rule, bivalves are more common on sandy beaches and gastropods on rocky shores.

Bivalve Molluscs

Seasshells bivalve molluscs
Norway Cockle Laevicardium crassum; Faroe Sunset Shell Gari fervensis; Banded Wedge Shell Donax vittatus;
Great Scallop Pecten maximus; Otter Shell Lutraria lutraria; Baltic Tellin Macoma balthica

The shells of bivalve molluscs exhibit a wide range of forms, from large and robust oyster shells to small and delicate tellins. However, the methods used to identify the species are the same and you will need nothing more than a ruler and a hand lens or magnifying glass. If you are lucky, you can find some shells where the two halves still joined, if not try to collect a selection of shells to show both halves. You will need to look at the outside shape and surface structure of the shell and the inside, particularly the hinge and the muscle attachment scars.

Internal view of a bivalve shell
Internal view of a bivalve shell drawn by Muriel Gottrop. Image:Valva-VistaInterna.png.
Photograph: internal shell of Venerupis corrugata

There is an excellent guide on the Marine Bivalve Shells of the British Isles website, which illustrates the range of morphological features to look at and provides a pictorial identification key.

Gastropod Molluscs

Seashells gastropod molluscs
Common Pelican’s Foot Aporrhais pespelecani; Rough Periwinkle Littorina saxatilis; Auger Turritella communis;
Painted Top Shell Calliostoma zizyphinum; Netted Dog Whelk Tritia reticulata

This group includes animals with conical shells e.g. limpets, or coiled shells e.g. whelks and periwinkles. The shell morphology may be very different from the bivalves , but the approach to identification using an empty shell is similar. However, for an accurate identification of some species, it is necessary to look at the anatomy of the live animal e.g. some limpets and periwinkles.

To begin you need to separate the conical shells from the spiral shells. Molluscs with spiral shells show a range of different forms from the long and slender to the short and squat, but the features to examine are similar. These include the size, the number of whorls, whether the apex is sharp or depressed, the shape, size, position and orientation of the aperture, and whether the exterior is smooth ridge or ornamented.

Shell structure of coiled gastropod marine mollusc.
Shell structure of coiled gastropod marine mollusc.
Photograph: Netted Dog Whelk Tritia reticulata

Unfortunately, there is not an equivalent website for gastropod molluscs, but there are a number of good identification guides and keys.

Beginning to learn now to identify a group of species is always daunting, but marine molluscs are not too difficult, especially when you can recognise the main features. Start with the common and easy to identify species, and become familiar with the important morphological features and their names. If you wish to ask for an identification to be confirmed or need help, please remember to give the size of the shell, describe the surface of the shell, and to photograph the main features.

Identification Guides

  • The Essential Guide to Rockpooling
  • The Essential Guide to Beachcombing and the Strandline
  • RSPB Handbook of the Seashore
  • Naturalist Handbook: Rock Pools
  • Snails on Rocky Sea Shores
  • Animals of Sandy Shores
  • A Student’s Guide to the Seashore
  • Handbook of the Marine Fauna of North-west Europe

General Reading

  • British Wildlife Collection: Rocky Shores
  • A Natural History of The Seashore, No. 94 (The New Naturalist). Out-of-print, but secondhand copies are often available.

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