LITMIR.BIZ

      Several nautilus (Nautilus pompilius) on a coral reef at night, Pacific.

      Cephalopod eyes are very elaborate. In some ways they are even better than our own, for a squid can distinguish polarised light, which we cannot do, and their retinas have a finer structure, which means, almost certainly, that they can distinguish finer detail than we can. To deal with the signals produced by these sense organs they have considerable brains and very quick reactions.

      Some squids grow to an immense size. The aptly named colossal squid lives in the seas around Antarctica. It can reach nearly 100 kilos in weight and measure six metres from the end of its body to the tip of its outstretched tentacles. Its rival for the claim to be the largest species of all is the giant squid. The biggest so far discovered have in fact been slightly smaller and substantially lighter. Although there are records of even larger specimens of this species, it seems that these were not accurate. Nevertheless, we are unlikely to have discovered the biggest individuals of either species, so the record may yet be broken. The eyes of these huge cephalopods are even larger than might be expected. The biggest recorded were 27 centimetres across and are the largest known eyes of any kind of animal, five times bigger, for example, than those of the blue whale. Why the squid should have such gigantic eyes is a mystery.

      It could be, however, that they need extremely sensitive eyes to detect the presence of their great enemy – the sperm whale. Squid beaks are often found in the stomachs of sperm whales, and their heads often carry circular scars with diameters that match a squid’s suckers. So there seems little doubt that squids and whales regularly fight in the dark depths of the ocean. Maybe the squids’ huge eyes help them to detect the presence of the only animal big enough to hunt them.

      The intelligence of all the cephalopods – octopus, squid and cuttlefish – is well known. Octopus have been observed disguising themselves from an approaching enemy by covering themselves with shells or picking up two halves of a coconut and hiding within. Many species in all three groups have an extraordinary ability to change their colour and shape. They can camouflage themselves by matching almost any environment and also signal to one another with patterns and shapes that sweep across their bodies. A female squid has even been filmed signalling to a male lying alongside her that she is not ready to mate, while at the same time displaying a pattern on the other side of her body to summon another male. Octopus and squid, two of the most advanced animals in the ocean which least resemble human beings, are among the few, it seems, that can rival mammals in their intellectual abilities.

      But what of the second great category of animals without backbones, the one represented in ancient rocks by the flower-like crinoids? As these are traced upwards through the rocks, they become more elaborate and their fundamental structure becomes clearer. Each has a central body, the calyx, rising from a stem like the seedhead of a poppy. From this sprout five arms which, in some species, branch repeatedly. The surface of the calyx is made up of closely fitting plates of calcium carbonate, as are the stems and branches. Lying in the rocks, the stems look like broken necklaces, their individual beads sometimes scattered, sometimes still in loose snaking columns, as though their thread had only just snapped. Occasionally gigantic specimens are found with stems 20 metres long. These creatures, like the ammonites, have had their day, but a few species, sometimes called sea lilies, still survive in the ocean depths.

      Bigfin squid (Sepioteuthis lessoniana) hovering in open water above a coral reef at night. Dampier Strait, Raja Ampat, West Papua, Indonesia. Tropical West Pacific Ocean.

      Crinoid (feather star, centre) on a gorgonian (sea fan, red) with a Dendronephthya soft coral in the background, Andaman Sea, Thailand.

      Sea lilies show that the calcium carbonate plates, in life, are embedded just under the skin. This gives their surface a curious prickly feel. In other families, related to the crinoids, the skin has spines and needles attached to it so the creatures are known as echinoderms, ‘spiny-skins’. The basic module on which the echinoderm body is built has a fivefold symmetry. The plates on the calyx are pentagons. Five arms extend from it, and all the internal organs occur in groups of five. Their bodies work by a unique exploitation of hydrostatic principles. Tube feet, each a thin tube ending in a sucker and kept firm by the pressure of water within, wave and curl in rows along the arms. The water for this system circulates quite separately from that in the body cavity. It is drawn through a pore into a channel surrounding the mouth and circulated throughout the body and into the myriads of tube feet. When a drifting particle of food touches an arm, tube feet fasten on to it and pass it on from one to another until it reaches the gutter that runs down the upper surface of the arm to the mouth at the centre.

      Tube feet of a red cushion sea star (Oreaster reticulatus), Singer Island, Florida.

      Though stalked sea lilies were the most abundant crinoid in fossil times, the commonest forms today are the feather stars. Instead of stalks, they have a cluster of curling roots with which they attach themselves to coral or rocks. In places on the Great Barrier Reef, they swarm in huge numbers, covering the floor of the tidal pools with a tufted coarse carpet of brown. When disturbed, however, they can suddenly swim away, writhing their five limbs like Catherine wheels.

      The fivefold symmetry and the hydrostatically operated tube feet are such distinctive characteristics that they make other echinoderms very easy to recognise. The starfish and their more sprightly cousins, the brittle stars, both possess them. These creatures appear to be crinoids that have neither stalk nor rootlets and are lying in an inverted position with their mouths on the ground and their five arms outstretched. Sea urchins too are obviously related. They seem to have curled their arms up from the mouth as five ribs and then connected them by more plates to form a sphere.

      The sausage-like sea cucumbers that sprawl on sandy patches in the reef are also echinoderms, although in most species their shelly internal skeletons are reduced to tiny structures beneath the skin. Most lie neither face up nor face down, but on their sides. At one end there is an opening called the anus, though the term is not completely appropriate for the animal uses it not only for excretion but also for breathing, sucking water gently in and out over tubules just inside the body. The mouth, placed at the other end, is surrounded by tube feet that have become enlarged into short tentacles. These fumble about in the sand or mud. Particles adhere to them and the sea cucumber slowly curls them back into its mouth and sucks them clean with its fleshy lips.

      One highly specialised deep-sea sea cucumber, called a sea-pig, lives in the mud of the deep seabed at depths of up to 5,000 metres. They are rotund little creatures about 15 centimetres long and have large tube-like structures on their underside with which they rootle about in the mud. They have been filmed in the deep sea, assembled in herds, perhaps for reproduction or attracted by the smell of a new source of food drifting down from the surface.

      If you pick up a sea cucumber, do so with care, for they have an extravagant way of defending themselves. They simply extrude their internal organs. A slow but unstoppable flood of sticky tubules pours out of the anus, fastening your fingers together in an adhesive tangle of threads. When an inquisitive fish or crab provokes them to such action, it finds itself

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