NATURE The Cuttlefish, AKA The Clew Bay Casanova

The Casanova of Clew Bay


Marine Life
John Paul Tiernan

We were momentarily stumped last weekend. The second most common question asked of a marine biologist (after ‘what’s wrong with my goldfish?’), is ‘I found this on the shore, what is it?’. And we can usually make a confident identification and throw in some interesting information on the species for good measure.
But when an angelically white elliptical shell arrived in our care last weekend, plucked by a friend on Clew Bay’s southern shore, we were at a loss to identify it. Its delicately hollow calcified structure bulged unhelpfully at one end.
This was not the hard mantle of a familiar bivalve such as an oyster or clam, no; this was the softer interior skeleton of something, but what? We were all at sea.
Its chalky weightless centre reminded me of a sand dollar which is an echinoderm (starfish and urchin family) from a different part of the world. It lacked the radial symmetry of an echinoderm however, and anyway, if such a species existed in our waters, wouldn’t I know about it? Completely doubting my marine taxonomic skills, I made a plea for help via Twitter, which, when not aiding revolutions in the Arab world, is useful for identifying seashells in County Mayo.
An artist from Achill was the first to tweet back across the Bay, and then several others confirming that this confounding carcass was the not too uncommon ‘cuttlebone’ from the cuttlefish, an often ignored but certainly intriguing local resident of our waters. The cuttlefish looks like and is related to the octopus and squid, and lives on the sandy seafloor of Clew Bay from shallow areas out to depth.
The cuttlefish is more closely related to other molluscs such as winkles and mussels than they are to true ‘fish’. They are also extremely intelligent, and more social than the solitary octopus. The cuttlefish has the ability to change its colours rapidly, and males have been reported to be able to show a dominant display of colours on one side to other males, while showing a calmer display to potential mates on the other – the Casanova of Clew Bay.
The cuttlefish uses the air-filled porous bone, which bewildered us so much with its intricate structure, to regulate buoyancy. Humans, on the other hand commonly use it as a dietary supplement for caged Budgies, as it provides a good source of calcium for the incarcerated birds. The last puzzle that this mollusc gave us was its Irish name. As Gaeilge, the cuttlefish was known as  An LΡir bhΡn or ‘the white mare’ –  one mystery we couldn’t figure out. Speculations are welcome to irishmarinelife@gmail.com.