Cuttlebone
The strong easterly winds of the past few weeks have brought in all sorts of things to the cove at the end of our lane. There was a fleet of By-the-wind-sailors (Velella velella), those strange hydrozoans that look like little blue boats with transparent sails. Some have sails that go from the port bow, as it were, to the starboard quarter, and some go the other way, but none can sail against the wind, so they get washed ashore and die. One of their dangerous cousins, a Portuguese man o’war, turned up in the same week. Several goose barnacles arrived too, poor things; some had hitched a ride on an abandoned fish box, others were stuck to a piece of timber, but all were doomed once they ran aground.
Mermaids’ purses (the empty egg cases of dogfish, various rays and flapper skate) have come ashore in large numbers too, but the most abundant pieces of biological flotsam this spring have been cuttlebones. Many people asked me exactly what they were, so here is an explanation.
Cuttlebones are the internal skeletons of cuttlefish, relatives of squid, which belong to the Phylum Mollusca, one of the major groups of the Animal Kingdom. Molluscs share a common ancestor with the annelids (segmented worms), probably something like a flatworm that lived in Pre-Cambrian times, more than 500 million years ago. The diagnostic features of a mollusc are a muscular foot, a shell and a special device for feeding called a radula, which is like a conveyor belt of teeth used to chomp through your lettuces or scrape algae off rocks – look at the patterns made by snails on greenhouse glass or by limpets on a rock.
The Phylum Mollusca consists of seven classes. Three of these you are unlikely ever to see: the solenogasters, worm-shaped creatures living in deep seas; the monoplacophorans, limpet-like animals thought to be extinct for 375 million years until one was found in 1952; and the scaphopods, whose shells are shaped like miniature elephant tusks. You might, if you look hard enough, find a chiton on the seashore; they don’t have a shell, but instead, their flat bodies are covered by a series of transverse plates.
The other three groups should be familiar to everybody: the bivalves, such as cockles, mussels and oysters, which have two shells; the gastropods, which have one coiled shell (though some have lost theirs) – snails, slugs and periwinkles; and the cephalopods, the most advanced of the molluscs.
The cephalopods have a very extensive fossil record, but there are fewer species alive today. They descended from something resembling a limpet. After millions of years, its shell became taller, the muscular foot turned into arms, and the animal changed from crawling to swimming, with the shell horizontal and the arms outstretched.
There are three sub-divisions within the order Cephalopoda. The nautiloids developed a simple, coiled shell. They were extremely abundant in Palaeozoic seas, but today only nine species remain, including the attractive pearly nautilus. Then there were the ammonites, which had planispiral shells (flat, with many coils). They were one of the dominant marine animal groups until their extinction, around the same time as the dinosaurs. Thirdly, there are the coleoids, the living cuttlefish, squids and octopuses. Cuttlefish and squid have eight arms and two longer tentacles, while octopuses have just eight arms.
In the coleoids, the shell became reduced as the animals evolved different lifestyles. The ram’s horn squid, Spirula spirula, a small, strange deep-water creature, still has a coiled shell like its ancient ancestors, but instead of the squid living in the shell, the shell has shrunk and is actually inside the squid, where it functions as a buoyancy device. Ram’s horn squids are very rarely seen alive, but their shells wash up frequently on tropical beaches.
In the cuttlefish, which live on the sea bottom, the shell has evolved into that elliptical chalky structure, the cuttlebone. As in Nautilus and Spirula, the cuttlefish uses its skeleton to control buoyancy, by changing the ratio of liquid and gas in the pores of the cuttlebone. These bones implode at depths greater than 600 metres, so cuttlefish live in shallower waters than squids.
There are over 120 species of cuttlefish worldwide; the largest is the giant cuttlefish from Australia, which can grow up to 100cm in total length. In Ireland, it is the common cuttlefish Sepia officinalis, that most often leaves behind its bones, but there are other smaller species in the NE Atlantic, for example, the elegant cuttlefish and the pink cuttlefish, as well as the delightfully cute little cuttle (which is not really a cuttlefish, but actually a bobtail squid). Cuttlefish are caught commercially in the Mediterranean, but rarely seen on menus here. Cuttlebones are given to caged birds as a source of calcium.
Cuttlefish are extremely intelligent animals – they have the largest brain to body ratio of all invertebrates. They are masters of camouflage and can change the colour and texture of their skin very rapidly. In the breeding season, males become striped like zebras, to attract females. They live for no more than two years, and die after spawning. That is when their cuttlebones wash up on the shore.
Like most cephalopods, cuttlefish use ink as a method of escaping enemies – they squirt out a cloud of the dark liquid, which confuses predators. The ink mostly consists of melanin, the same pigment that gives us a sun tan. The brown colour sepia is named after the cuttlefish, because cuttlefish ink was once used for writing and drawing. The ink of cephalopods is also used in cooking, for example, in calamares en su tinta, or the black fish stew from Livorno called cacciucco.
Sea hares (a type of sea slug) also use ink to avoid becoming lunch, but their ink is a red or purple colour. In the mammal world, the pygmy and dwarf sperm whales eject a red liquid when disturbed, possibly for the same reason.
Cuttlefish feed on small fish, crabs and shrimps, various molluscs, even smaller cuttlefish or octopuses. They catch the food in their large tentacles, and crunch it up with their parrot-like beak. They in turn are eaten by larger fish and dolphins. The latter are expert at eating only the good parts of a cuttlefish; if a dolphin catches one, it will bash it about so that all the ink is expelled, and then squeeze it until the cuttlebone pops out.
In the other cephalopods, the shell is reduced or absent. True squids are mostly mid-water shoaling creatures who rely on speed and numbers for defence, and their internal skeleton is just a thin, transparent structure called a gladius or pen. Anyone who has gutted a squid will have seen one. Having both pen and ink, the squid was once known as ‘the sea clerk’.
Octopuses, who live solitary lives on the sea bed, have no skeleton at all, so they can crawl into the tightest of crevices to hide from danger or, in captivity, cause mischief. There is, however, one strange species of octopus that has a shell: the female paper nautilus, Argonauta argo, secretes a beautifully delicate shell in which she lays her eggs and where she and her miniature husband reside, floating through the seas, grabbing any passing sea slug or jellyfish.
