Have you ever looked closely at the head of a flatfish? Probably not, unless you are a fisherman or fishmonger. In Ireland, we generally like our fish beheaded; for many people, a fish is just a white fillet, or, in the words of the late, inimitable Keith Floyd, ‘an unidentified frying object’. In other parts of the world, people are not so squeamish – think of pilchard heads poking out of a stargazy pie, or a conger head lurking in a pot of Breton fish soup. In China there are restaurants that serve almost nothing but fish heads. I once had a Japanese girlfriend who loved to eat fish eyes.
Many kinds of fish are flat. Rays are dorso-ventrally flattened for life on the sea bed. The John Dory, ugly but delicious, is laterally flattened, all the better for sneaking up on its prey unnoticed. The freshwater angelfish is similarly compressed so it can hide among the stems and leaves of water plants. But only flatfish undergo a radical metamorphosis which totally changes the shape of the head, making them the most asymmetrical of all vertebrates.
Flatfish belong to the order Pleuronectiformes, which contains 16 families and about 800 species. They are found worldwide, from the poles to the tropics. Most are marine, but a few species, such as our own flounder, can live in brackish waters and rivers. Flatfish range in size from the tiny Tarphops oligolepis of the western Pacific, which grows to no more than 4.5 centimetres, to the enormous Atlantic halibut which can reach 2.5 metres and weigh over 300 kilograms.
Flatfish lay their eggs – over a million in the larger species – on the sea bed, but they float up into the plankton so the young fish can develop away from their parents, to avoid competition and possible cannibalism. Some species, for example turbot, live their first few months in the surface plankton and can be observed from a boat on very calm days, with other baby fish such as rockling and garfish, wriggling along in the drift lines of floating weed and other debris that gathers at the boundaries of currents.
Baby flatfish look like any other fish at first, with an eye on either side of the head. But as they grow, hormones produced by the thyroid gland cause a remarkable transformation – while the body becomes laterally flattened, one eye moves from one side of the fish, over the top of the head. By the time the little fish is ready to swim back to the bottom – six to ten weeks in the case of the plaice – both eyes are on the same side of the body. As the eye migrates, the respective nostril does too, and the mouth becomes distorted, giving flatfish their peculiarly mournful expressions. The pectoral fin, scales and lateral line on the blind side are all less developed.
Pigmentation changes too – the side with eyes is coloured, usually in shades of brown or mottled patterns, while the blind side is white. Flatfish colouration, as well as their habit of burying themselves in the bottom sediments, renders them well camouflaged. Most can change their pigmentation to match their background. Some have special mechanisms for avoiding predation – the white-blotched sole from Australia can, when young, mimic poisonous species of flatworms in both their black and orange colouration and their undulating movements. The Moses sole, from the western Indian Ocean, secretes a toxin which paralyses the mouths of predators such as sharks.
Halibut, plaice and European flounders have their eyes on the right side of the head. Turbot, brill and megrim have theirs on the left, as do smaller, lesser-known flatfish such as topknots and scaldfish. The true soles are right-eyed, but are easily distinguished from plaice and flounders by their tiny mouths, rounded heads and longer, narrower bodies.
As happens occasionally in garden snails, whose shells are usually coiled to the right, there can be reversals in symmetry, for example flounders whose eyes migrate the wrong way, so they are left-eyed instead of right.
Why did flatfish develop in such a way? Some might say it was just how God made them (look at a wide-eyed flounder and you would think he was having a joke – it is like something Picasso might have painted in his weirdest cuboid phase). But there is a more logical explanation. Flatfish are related to the perciform fish (perch, bass, wrasse etc.); several species of wrasse are known to lie on their sides on the sea bed, half-buried in sand, to escape predators or to sleep. For such fish, it would be an advantage to have both eyes on the side that is looking up, so presumably that was something natural selection favoured. We will never know exactly how or when the first flatfish evolved, but two fossil fish – Heteronectes and Amphistium, from Eocene rocks in Italy (about 45-million-years-old) – both had an eye half-way over the top of their heads. Similarly, the most primitive of the living flatfish, the Indian halibut or adalah – found from the Indian Ocean to the western Pacific – has one eye situated on the dorsal edge of the head, rather than completely on the same side as the other eye, and it sometimes swims in an upright position, like a normal fish.
Flatfish are important commercially. Plaice is probably the most popular in the British Isles, while turbot and common sole (also called black or Dover sole) are the most highly prized. As with many animals, the English names of flatfish can be confusing – the word ‘flounder’ is used for a variety of unrelated species, and several ‘soles’ are not soles at all, e.g. the lemon sole belongs to the right-eyed flounder family. Some less familiar flatfish have been rebranded as soles by the fishing industry to make them sound more appealing – the witch, another right-eyed flounder, is also called the Torbay sole, while the left-eyed megrim has become the Cornish sole. This fish, Lepidorhombus whiffiagonis, must be one of the most miserable-looking creatures of all; even its English name is depressing – megrim is an old word for migraine, and in plural form, relates to being in low spirits (I get the megrims every winter).
The mighty halibut, now endangered because of overfishing, is possibly the only flatfish to have inspired a poet. “To The Immortal Memory of the Halibut (on which I dined this day, Monday, April 26, 1784)” was written by William Cowper, better known for his religious works. In the poem, Cowper considered the life and habitat of a halibut, before pondering: “ – Wherever thou hast fed, thou little thought’st, And I not more, that I should feed on thee.”
Most people are not so reflective when faced with a dead animal on their plates. If they were, more would become vegetarians. Flatfish and other benthic species are caught by dragging a great big net, with chains and otter boards, across the sea bed, killing everything in its path – small unwanted fish, worms, brittle stars, sea cucumbers and tiny creatures few would even know existed. It is rather like bull-dozing a great swathe through a forest, destroying every plant and invertebrate in it, just to catch the edible mammals.
Some of my best friends are fishermen; they have to earn a living, and people have to eat. I myself spent twenty summers working as an angling boat skipper, and dined on fish every day. But with ever-improving technology and increased demand, commercial fishing has become far too destructive. As well as bottom trawling, there are the ghost nets that haunt the oceans and drown turtles and dolphins; the long lines that have brought some albatross species to near extinction; the super trawlers with nets a mile long that can catch hundreds of tons of fish a day, and the foreign fishing vessels that ruin the livelihoods of inshore fishermen in poor countries. Anyone with a conscience should worry about the provenance of their battered cod, sole bonne femme or tuna sashimi; anyone with any compassion must think of the fish’s suffering. I remember the last time I went fishing, over twenty years ago, and watched mackerel gasping for life in the fish box. I don’t know why it didn’t bother me before, but I have rarely eaten a fish since (and I don’t eat meat, but that is another story).
Cowper ended his poem with the whimsical notion that the halibut’s “brethren of the slimy fin” are envious because it was doomed to be eaten by a bard. But look at those doleful eyes and downcast mouth, even knowing they are the result of evolution, and it is not hard to imagine the flatfish saying “We are all just doomed”.
