Strandline Shells on Swansea Bay

Seashells on the beach at Swansea Bay

Shells drift lines on the beachSeashells in situ on Swansea Bay strandlines or drift lines are mostly tiny immature common cockle shells but there are many other species of bivalve and gastropod mollusc shells too. I noticed mussels, tellins, and oysters, winkles, top shells, netted whelks, sting winkles, slipper and common limpets, and I am sure there were many more types. There were seven drift lines of shells lying parallel to the water line and decreasing in the number of accumulated shells sequentially up the shore. Each line represents the highest reach of the sea on a series of subsequent falling tides that were decreasing in reach each time.

It was interesting to see that wave-worn pieces of black coal and dark clinker from industrial plants across the bay were scattered amongst the light coloured shells together with a fair number of burial-blackened periwinkles. Many of the shells were fragmented and the accumulations included the calcareous tubes of marine worms. It would be lovely next time to take a sample home and sort it through under a binoc. I am sure that it would reveal much more information.

Click on any image below to see the details in a larger version.

Just a Common Whelk Shell (4)

This empty Common Whelk shell, picked up on the beach, is mostly covered with the calcareous tubes of a marine polychaete worm called Pomatoceros triqueter, also known as “German writing”. The tubes are a frequent sight on rocks at the beach and also on objects such as pebbles and driftwood. The tubes attached to the outer surface of the shell may have been made while the whelk was still alive and moving about. Those within the mouth of the empty shell were definitely attached after the gastropod mollusc had died and its flesh had been removed from the shell.

Organisms that live on the outside of other creatures in this way are known as epibionts. Usually epibionts are neither parasites that occupy a rather one-sided relationship with the host where they rely on it for nutrients and frequently damage it; nor symbionts where both organisms depend on each other in a mutually beneficial relationship; but maybe they could be called commensals in that the host is not damaged by the attached organism but merely provides a surface of attachment, and both organisms share the same environment.

The images below show the distinct patterns where calcareous tubes of Pomatoceros triqueter have been attached (but now removed) on a pebble at Chesil Cove in Dorset; also some in situ shots of Pomatoceros tubes attached to low shore rocks, along with the sand-grain tubes of the Honeycomb Worm (Sabellaria alveolata), at Mewslade Bay in Gower.


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Just a Common Whelk Shell (3)

Lots of barnacles on a whelk shell

Whelk shell with an encrustation of mostly acorn barnacles – some complete with all plates and in other areas only the basal plate remains

Acorn Barnacles (Cirripedia) settle on almost anything in the sea or on the seashore. These images show the empty shell of a Common Whelk (Buccinum undatum) that I picked up on the beach at Rhossili on the Gower Peninsula in South Wales – it has proved to be an ideal substrate for them.

The outer surface of the shell is almost entirely covered with barnacles. The majority are intact with the lateral and also the terminal plates. Many specimens are mature but there are juveniles too. In one area, the barnacles have been knocked off but you can still see the basal plates by which they were attached. Some barnacles may have been living on this common British seashell while it was still alive. However, it is equally possible that the shell became colonised by barnacles once it was empty. The few calcareous tubes of marine worms which are stuck on the inner surface of the aperture or mouth of the shell would have settled there once the whelk flesh had disappeared.

The close-up shots reveal the details of the structure of the barnacles, made up generally from six fixed lateral plates overlapping each other to form the shell for the animal, with four articulating terminal plates forming the lid to the chamber. The whole barnacle shell is in this instance securely attached to the whelk shell by a basal plate that often remains in place even when the barnacle becomes detached. Not all species of barnacle have a basal plate.

The macro-photographs also show the intricate pattern and texture of the whelk shell surface with a regular criss-crossing of ridges. This gives an almost lattice-like effect where the growth lines intersect with the natural ornamentation or sculpturing of the shell. In close-up, it is also possible to see small areas of the colonial microscopic animals called Bryozoa or Sea Mats (resembling fragments of lace) which are clinging to the bases of some of the barnacle shells.

Macro-photograph of growth lines and natural sculpturing on a whelk shell

Close-up image of pattern and texture in a barnacle-encrusted whelk shell

Barnacle encrustation on a whelk shell

Whelk shell with mature and juvenile barnacles attached

Macro-photograph of growth lines and natural sculpturing on a whelk shell

Close-up image of growth lines and natural sculpturing in a barnacle-encrusted whelk shell

Apertural view of epibiont encrustation hard parts on a Common Whelk shell

Whelk shell with barnacles attached to the outside and calcareous tubes inside


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Miscellaneous Seashells

Black-lined Periwinkle Seashells

Black-lined Periwinkle seashells (Littorina nigrolineata)

Banded Wedge seashells

Banded Wedge seashells (Donax vittatus)

Turban Top seashells

Top Shells (Gibbula spp.)

Variegated Scallop seashells

Small Scallop  seashells (Chlamys sp.)

Dog Whelk Seashells

Dog Whelk seashells (Nucella lapillus)

Common Cockle seashells

Common Cockle seashells (Cerastoderma edule)

Netted Dog Whelk seashells

Netted Dog Whelk seashells (Hinia reticulata)

Small Scallop seashells and Manila Clam

Small Scallop seashells (Chlamys sp) and Manila Clam

Common Whelk seashells

Common Whelk seashells (Buccinum undatum)


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Common Winkle


Seashore Creatures: Molluscs: Gastropods: 

Common Winkle Littorina littorea (Linnaeus)

  • This has a small, squat and robust spired shell upto 30mm high.
  • The rounded shape and the thick shell protect the animal from damage caused by the force of the waves and rolling around when they are dislodged from rocks.
  • The animal can withdraw into the shell and shut itself inside with a horny lid or operculum to stop it being dried out when it is exposed to the air at low tide. 
  • Seems to have a smooth surface but actually has numerous fine spiral ridges around the whorls which are more obvious in younger shells.
  • The final identifying characteristic for the species is the way the outer lip of the aperture joins the last body whorl at a tangent.
  • The shell is usually a dark grey-brown colour but it can be paler at the pointed spire end and the colour gets paler as the winkle grows older (as you can see in the photograph at the top of this page).
  • In the shells of younger winkles there may be red, orange or white colour in the grooves between the fine ridges (this can be seen in the photograph at the top of the Post for 11th February 2009).
  • The head tentacles have many cross-wise black stripes.
  • Winkles live on the upper (intertidal) shore down to the shallow sublittoral (underwater) zone.
  • They are herbivores that love weedy shores and are found in greater numbers on more sheltered beaches.

For more information about the common winkle see the Marine Life Information Network:

Book references to common winkle include:

  • Barrett, J. and Yonge C. M. (1958 but reprinted many times) Collins Pocket Guide to the Seas Shore, Collins, ISBN 0 0 219321 3, page 135.
  • Gibson, C. (2008) Pocket Nature Seashore, Dorling Kindersley, ISBN 978 1 4053 2862 3, page 116.
  • Graham, A. (1971) British Prosobranch and Other Operculate Gastropod Molluscs, Keys and notes for the identification of the species, Synopses of the British Fauna No. 2, The Linnaean Society of London, Academic Press, ISBN 0-12-294850-5, page 53 & 58.
  • Hayward, P. J.  and Ryland J. S. (1995) Handbook of the Marine Fauna of North-West Europe, , Oxford University Press, ISBN 0 19 854055 8 (Pbk), page 512.
  • Hayward, P., Nelson-Smith, A. and Shields, C. (1996) Sea shore of Britain and Europe, Collins Pocket Guide, , ISBN 0 00 21995, page 188.  



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