Barnacles on Flat Oyster shells

If you can get to grips with the identification of acorn barnacles, sometimes called sessile barnacles, their presence on objects such as rocks and shells can tell you where these objects came from. Different barnacle species grow in different geographical locations. Different species also tend to live in a variety of depths of seawater and at varying levels of the intertidal zone.

Acorn barnacles or Balanomorphs have a calcareous shell made up of four, six, or eight overlapping plates. These are called laterals at the side, rostrum at the front, and carina posteriorly. Two pairs of opercular plates articulate on top (the smaller being terga and the larger scuta). All the plates together form an approximately conical box, in which the animal lives, with a lid that opens on the top. The whole structure is stuck to a hard substrate which can be natural or man-made. Common substrates include the shells of either empty or living marine molluscs, rocks, pebbles, driftwood, piers, pilings, and boat hulls. The top pictures show barnacles attached to modern European Flat Oyster shells.

The bottom of the barnacle’s shelly cone is sealed by the basis. This can be membranous or calcareous. The calcareous basis can remain glued to the substrate when the barnacle is knocked off. It is sometimes referred to as a barnacle ‘scar’. This roughly circular white plate bears indentations and markings that reflect the structure of the barnacle shell of which it once formed a part. It therefore provides a potential for identification of the missing barnacle. Some of these barnacle scars are illustrated in the photograph above and below in recent oyster shells from the south coast of England. 

In certain circumstances, acorn barnacle shells can actually become incorporated into the shells of Flat Oysters. This usually happens when a young oyster settles on a stone or shell that is already encrusted with barnacles. The shell may be empty or may still be occupied by the living animal. The young oyster grows over the barnacles and, as its shell develops, the barnacles become embedded. This means that the calcareous plates of the acorn barnacle are preserved within the oyster shell.

The photographs below show a range of both modern and archaeological European Flat Oyster shells with emedded barnacles. The barnacle shells  are inverted and the basis absent. It is possible in some specimens to see all the side and opercular plates inside the hollow cone of the barnacle shell. The shape of the barnacle plates is diagnostic for the species. Identification of the particular type of barnacle is theoretically possible. 

Acorn barnacles on ancient shells, stones, or artefacts from archaeological excavations, either marine or land-based, can provide important clues for site interpretation. Even when barnacles have become detached from the object, the presence of an adherent calcareous basis may aid identification. The recognition of the presence of embedded barnacles is vital for the retrieval of diagnostic remains. Knowing which species of barnacle is present on oyster shells may indicate the geographical location being fished (as in solving the riddle of whether oysters were really brought from Rome to Britain two thousand years ago). It may also help to tell us how deep were the waters in which the oysters were growing, and suggest whether oysters were in intertidal relaid beds.

The accurate identification of acorn barnacles is not easy. However, there are some very useful reference books available. The most significant work is:

Southward A. J. (2008) Barnacles, Synopses of the British Fauna (New Series) No. 57. Edited by J. H. Crothers and P. J. Hayward. Published for the Linnaean Society of London and The Estuarine and Coastal Sciences Association by The Field Studies Council. ISBN 978 1 85153 270 4

An excellent general work that provides a key for barnacle identification is:

Handbook of the Marine Fauna of North-West Europe (1998) Editors P. J. Hayward and J. S. Ryland. Oxford University Press. ISBN 0 19 854055 8 (Pbk)

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