Barnacle

When: All year round

Scientific name: Cirripedia

Gaelic name: bàirneag-cathan

Size: Individually around 5mm across, but they can grow in large groups, covering entire boulders

Where: rocky shorelines of the Inner Forth, Limekilns beach

Most people wince at the very mention of these rock dwelling beasties - the memories of tip toeing barefoot over jagged, barnacle encrusted shorelines on a rock pooling quest still too sharp to remember fondly. But have you ever taken a close look at these wondrous little creatures? Their existence is far more fascinating than many would imagine...

Often mistaken for a mollusc, Barnacles are in fact crustaceans, making them more closely related to crabs, lobsters and shrimps. Barnacles begin life in a larval stage, hatching out as small organisms that resemble shrimps, floating in large clumps of plankton that are moved around the sea by currents and tides. When the larvae reach maturity, they find an appropriate place to settle and begin life in their adult form. The larvae secrete a glue-like substance that fixes them to their chosen substrate - usually a rock, but occasionally the underside of a boat or even a whale's fin! They then begin to build their hard encasing shell by producing calcium based plates that surround the barnacle in a protective layer. As the barnacle eats more and grows, larger plates are produced, allowing the barnacle to expand.

The truly remarkable features of a barnacle are revealed when you assess its feeding and reproductive habits. The hard, outer casing has four retractable plates at the top; these open when the tide is high and the barnacle is submerged underwater. From this opening, six wispy strands known as cirri protrude, and it is with these appendages that the barnacle catches food! Each of these structures is covered in small hairs that detect movement in the water around the barnacle, enabling it to feed and also defend itself from predators. 

Barnacles are hermaphrodites, meaning that they are equipped with both male and female sex organs. However, they do require the assistance of a neighbouring barnacle in order to fertilise their eggs, and so this group of organisms has evolved a remarkable adaptation to get around the issue of being sedentary but needing to mate; they have the largest penis to body size ratio of any other creature in the animal kingdom. The male reproductive organ, which can be several inches long, reaches out of the confines of the barnacle's shell, feeling around for other barnacles in order to release sperm and complete the reproduction process. A young barnacle egg then develops in the parent's shell, until it is released in its larval form and the process begins once more.

Being creatures of the sea, barnacles have been speculated upon by seafaring Scots for centuries. It was once believed that barnacle geese hatched from the shells of their namesake crustaceans - the feathery cirri were reminiscient of a bird's fluffy plumage, and so sailors thought that that the groups of crustaceans were actually geese developing over the summer, hatching out in the autumn and coming inland over winter. This was such a popular concept that many traditional communities happily ate barnacle geese on a Friday, as they considered it to be a type of fish! In fact, it is now known that barnacle geese migrate north over the summer to breed, returning to territories in the south over winter, and so we can safely say that barnacles are indeed creatures in their own right, and extraordinary ones at that! Find out more by visiting: https://www.whoi.edu/science/B/people/kamaral/Barnacles.html

Image: Kim Hansen, Wikimedia