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.
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