Food for thought

Forest and Bird, Issue 285, 1 August 1997, Page 50

Food for thought

Ann Graeme

AMN, TIVE LOST a filling," exclaimed my husband at breakfast this morning. We indulged in a little moaning about the cost of dental work. But we didn’t complain that seriously, for we are glad that modern dentistry keeps our teeth in good repair. And how lucky we are to have teeth, I mused later. The development of jaws and teeth was of extraordinary importance in the evolution of animals with backbones. Without it, vertebrate animals could not have colonised the land, developed limbs and speciated into frogs and lizards, birds and mammals — and us. The earliest vertebrates were called agnathans. These fish, mainly jawless, fed by drawing seawater in through the mouth and pumping it out through the gill slits. Rotting material and tiny organisms

swirled in with the water and were trapped in mucus strands which — slowly moved into the stomach. The left-over water flowed out the

gill slits. This is filter feeding, a method of food collection still widely employed by invertebrates like sea squirts, sponges, shellfish and the larvae of hagfish and lampreys, as well as the largest animals of all — the baleen whales. Five hundred million years ago, jawless fish vacuumed their way over the sea floor. They were heavily armoured against two arthropod groups, now long-extinct — prowling trilobites and huge scorpion-like animals known as eurypterids. Then came a big advance. Fish developed jaws, probably as modifications of the bones and muscles supporting the gill arches, and the jaws became studded with teeth. No longer were fish defenceless and limited to shovelling and straining soft

food. Now fishes could bite. They could eat large plants, and crustaceans, and even each other. It was the end of most of the jawless fish, preyed upon by fierce arthropods and by their own, toothed, relations. The food web became more complex. These earliest jawed fish, the placoderms and their descendants, dominated the oceans and the fresh waters of the earth. This was "the Age of Fishes", the Devonian period, which began some four hundred million years ago. And when, in that period, the first amphibians crawled on to land, the hinged jaw and its teeth became adapted to suit the extraordinary variety of niches filled by land vertebrates. They became the poison fangs of the snake, the grinding molars of cows and sheep, the chisel teeth of the rodents, the cutting teeth of the carnivores, and human teeth, suited to our own , omnivorous diet.

We share a lot more than teeth with the fish. Our limbs evolved from the bony, finger-like fin skeleton of the lobed-finned group of

fish. This ancient group

is long extinct except for the

coelacanth, a fish discovered, to the amazement of scientists, deep off the coast of Madagascar 60 years ago.

We share with fish the sense organs for sight, hearing, taste, smell and touch. But fish have a unique organ for "hearing" in the water. This is the "lateral line", a series of sense organs in the skin linked in a canal along the side of the fish, like the plimsoll line on a ship. This lateral line senses movements and currents and allows fish to avoid obstacles in murky water, and

flash into hiding in dark crevices where their eyes cannot see. It enables vast shoals of fish to swim and wheel in perfect yet leaderless formation. While fish breathe through gills, land vertebrates breathe through lungs. Again we are indebted to fish for this organ. The lung is thought to be derived from the swim bladder which developed in bony fish as a device to maintain effortless buoyancy in the water. Gases from a blood supply similar to our pulmonary vessels inflate or deflate the swim bladder and regulate the fish’s density. It works like a scuba diver’s buoyancy compressor. Sharks and other cartilaginous (or non-bony) fish lack a swim bladder and so have to keep on the move to avoid sinking to the sea floor. But despite our links to fish, humans show little empathy for the creatures of the sea. Most of us are unmoved by the flapping fish in the dinghy, slowly dying of asphyxia. Neither does the exhausted marlin, towed, often to death, by deep sea sport fishers, excite great sympathy. While a hunter astride a bloodied lion would today be reviled in most circles, the proud . fisher beside his shark hung

on the weigh-station is still a "good news" story. And on a larger scale, we endorse the wholesale slaughter of fish in a manner no prudent farmer would ever condone, as well as turning 1

a Dlind eye to the Careless killing — the bycatch — that frequently accompanies undiscriminating fishing methods such as trawling. Why do we show such little compassion for fish? Partly I think it is because they live in the sea, an environment alien to land creatures such as ourselves, and a place strange, frightening and often hostile when we venture to explore it. Perhaps it is because fish have features which peculiarly fit them to life in the

water, and which make it difficult for us to identify with them. One such feature is the lack of eyelids. Land mammals have these to protect their eyes from dust and dirt, and to keep the surface of the eye moist by blinking. Fish eyes are constantly bathed in water, so they have no need for eyelids. A fish will gaze unblinking at a diver, a look that is "alien" to humans accustomed to the modest curtain of eyelid and eyelashes. The front of the fish eye bulges to focus in water, adding to its gog-gle-eyed, strange appearance. Even when fish sleep, their eyes must remain open. Many fish make sounds loud enough for us to hear. They whistle and click, squeak and drum, in order to warn or startle enemies or attract other fish for spawning. The whistling of the Mediterranean maigre fish is so loud it is

thought to be the origin of the sirens of Greek mythology whose singing lured unwary sailors onto the rocks. Many more fish make sounds of very low frequency and inaudible to the human ear. Such sounds amongst schooling fish have been described as "like water pouring" and, along with the lateral line sensory system, help keep the school together. # But fish make no recognisable noise os of distress, and can only gulp for water when they are hauled out on a line. Fish feel pain, but they do not ES cry out when hooked and : dragged into a boat. They need water to transmit their sounds, so they cannot cry out in eo 5

air. Perhaps if they could we would have greater feeling for their suffering. Fish are an important food for humans, and in a protein-hungry world, we are catching fish in quantities never before dreamed of. We are also overfishing — taking them in quantities that even their great capacity for egg-laying cannot sustain. Already some fisheries have become commercially extinct and others are steeply declining. Perhaps if we better understand fish, we may better husband them to both our own advantage and to theirs.