Sea Bed Flatfishes of great importance to the fishing industry are found in nearly every sea in the world, and sometimes in freshwater. There are about 500 species of flatfish world-wide-including such familiar species as plaice, halibut, turbot, and flounder. They are found in every ocean except for the Antarctic.
Their distribution ranges from the edge of the Arctic ice to the boundary of the Antarctic Circle, and throughout this vast area they live mostly on the continental shelf. Here they are found from the shallow water, among the breakers, to water several hundred feet deep. Many also live in freshwater.
Sea Bed Flatfish Development
Flatfishes actually lie on their sides on the sea bed, not on their undersides, as do other flattened fishes such as skates and rays. Since they lie on their sides, the flatfishes have both eyes on one side of their heads, yet the newly hatched larvae of flatfishes have an eye on each side of the head, exactly like all other fishes.
The metamorphosis from larva to adult flatfish is quite remarkable and is probably unique among animals with backbones. The eggs of marine flatfishes are laid out at sea, where they are buoyant and float on the surface, or just beneath it. Flatfishes have spherical eggs that are relatively large for fish eggs (about 1.6-2.2 mm in diameter), with bright canary yellow pigmentation covering the developing embryo and its yolk.
When the larva hatches out of the egg, it has dark eyes and there are fine, dark pigment spots on the underside of its body. The egg’s yolk is soon absorbed and the little fish (or post-well-developed mouth, larva), which has begun to feed on pelagic diatoms, copepods, and their larvae, and larval molluscs. The post larva grows, it deepens in the body, relatively in length.
For example, 12 days after hatching it is 7 mm long and less than one mm deep, but at 49 days it is about 10 mm long and 4 mm deep. Throughout this time, it still has an eye on each side of its head and looks like any other young fish. When the eye starts to move rounds the head, (as shown in the illustration below), the fish swims to a lower level in the water. By the time it has acquired the familiar flatfish shape, it has not only come to rest on the sea bed, but it has also drifted into shallower water, about 4 m (13ft) deep.
Classification
Approximately there are 24 species of flatfish have been recorded from the British seas. They are divided into four families, which are most easily characterized by whether their eyes move to the left side of the head, (known as left-eyed flatfishes), or the right, (known as right-eyed flatfishes). Several of the species of small fishes found the edge of the continental shelf and were not often caught except by research vessels using special nets.
They include the three species of scald fish, which only exceptionally grow longer than 7in (18 cm) and which are so delicate that contact with a fishing net tends to rub off their scales and skin so that they more rapidly than it increases look as if they have been scalded. Several others are rarely reported, such as Greenland or black halibut, found only once off the coast of Ireland, although it is common in deep water off the coast of Iceland and in the Arctic seas, where it lives in temperatures very close to the freezing point.
Right-eyed Flatfish
The halibut has the distinction of being the largest flatfish in the world, with huge specimens on record, measuring 2.5 m (8ft) long and weighing in excess of 318 kg (700lb). However, because of intensive fishing, such big fish are rarely caught today. The halibut is common off the coast of Britain, living on sandy, gravelly or rocky bottoms at a depth of 100-1500 m (328-4920ft).
It is most abundant off the western coasts of Ireland and Scotland and around the Orkney Islands. Unlike most flatfishes, which feed on the bottom, it actively hunts for prey in mid-water. Being a large fish with strong teeth in its jaws, it can capture a wide range of squid and fish. Other flatfishes with eyes on the right side of their heads are the plaice, flounder and, dab.
All three are strictly bottom-living and feed on invertebrates such as marine worms, small crustaceans and, molluscs. They all prefer much the same kind of habitat-sandy or gravelly bottoms in about 10-50 m (33-165ft) of water. When young, they are all abundant in shallow water close inshore and can be caught in shrimp nets among the breakers. Plaice and flounder are also common on muddy bottoms, while the dab is less so.
The flounder is perhaps found most often in river mouths, which are usually muddy bottomed, but it also swims up-river until it is in freshwater, This is particularly true of the young fish, for rivers form an important nursery ground for the flounder, In the River Thames, for example, in late spring and early summer thousands of young flounders, mostly about the size of a postage stamp, can be seen passing up-river on the tide, and fish as big as a man’s hand is common between Richmond and Teddington.
In appearance, the plaice is distinguished by its orange-spotted top side, clear white underside, and smooth skin. The flounder sometimes has pale orange spots on its ‘back’, but it is usually blotched and dull brown, while the underside is opaque and dull white. The skin is smooth, apart from a line of prickles around the bases of the dorsal and anal fins and behind its head. In contrast, the rough-edged scales of the dab mean it is always rough to the touch, and it has a lateral line that curves sharply behind the head.
Left-eyed Flatfishes
Two other large flatfishes are the turbot and brill, which belong to the group of flatfishes with eyes on their left sides. Both live on sand or gravel at depths of around 9-80 m (29-262ft), but the young are found in shallower water. They are near the northern limit of their range in British seas and are most common off the southern coasts, where they are active hunters of bottom-living fishes.
Abundant Food Fish
The same is true of the sole, which is much more common in the Channel, the Irish Sea and, the southern North Sea than it is in the north. The most abundant member of the sole family found in British waters, it is a popular food fish commercially. The sole has both eyes on the right side of its head, and its dorsal fin running round to the front of the snout with the mouth set on one side.
It lives mainly on sandy bottoms in 10-100 m (33-328ft) of water, feeding on worms and small crustaceans. Although it is often marketed as Dover sole, it is actually the same species that biologists known simply as the sole, and very few of them come from Dover.
The plaice (Pleuronectes platessa) is a right-eyed flatfish-that is, both its eyes are on the right side of its head. The nostrils are also on this site and as the fish develops its characteristic flattened shape, its skull becomes asymmetrical. As a result, the mouth becomes twisted. Its teeth are larger on the underside of the jaws than on the upper-colored side of the fish. Feeding on bottom-living invertebrates, it attacks burrowing molluscs such as cockles by nipping off and eating their breathing siphons as they are pushed up through the sea bed. Worms.
The plaice is a right-eyed flatfish that is both its eyes are on the right side of its head.
Masters of Camouflage
Flatfishes that live on the sea bed, like the brill experts at matching the coloring of their topsides to the color of the sea bed. The pigment in a fish’s skin is composed of three types of colored cells (red, yellow and black) and silvery iridocytes (reflecting cells). Each cell can expand or contract to vary the amount of pigment visible and so change the tone of its skin.
Interpretation of the background color is made through the eyes and the tone of the skin is automatically varied to match it. Experiments with captive fish show that changes to match backgrounds of sand or dark gravel are accomplished within a minute. A fish placed on a chessboard takes much longer to change but produces a creditable imitation of the black and white background colors.
Plaice: from egg to adult. When the post larva of the plaice measures about 13 mm (Bin) the left eye begins to move rounds, the upper edge of the head until it almost touches the eye on the right side. While the left eye is moving, the young plaice swims at an angle to keep the eyes level. When the change is complete-within three days-and the eye is in position, the dorsal fin begins to grow forwards along the edge of the head. The plaice now swims with its eyeless left side downwards.
The topknot, like all flatfish, swims with an undulating movement. Topknots can be found offshore, but are more common in rock pools along the shoreline where they have the habit of clinging to the underside of boulders.
