Flatfish: which side up?

Flatfish which side up

Most people are familiar with the peculiar appearance of flatfish such as plaice and flounders – species with both eyes on the same side of the head and a side-ways-on mouth. But what comes as a surprise to nearly everyone is that flatfish are not born this way.

A newly hatched flatfish looks just like any other juvenile fish. As the flatfish larva develops, however, a strange transformation begins: it starts to take up a life-style in which it lies on the sea bed on one side (either its right or its left). For this to happen the eye on the side next to the sea bed must migrate to the opposite side; the mouth twists slightly round at the same time and the dorsal fin also moves towards the snout.

No other group of fishes does this. Even flattened, bottom-dwelling fish like rays and skates are flattened from top to bottom – not from side to side. A ray or skate has a definite top (the back) and an underside (the belly) – just imagine yourself lying down on the sea bed on your stomach!

Right or left?

Once a flatfish has undergone its change, its eyes are located either on the right side of its head (dextral), or on the left (sinistral).

A dextral flatfish ends up lying on its left side, with its right side facing up. With a sinistral flatfish, just the opposite is true.

In a dextral flatfish, the left eye moves over to the right side of the head and body. In a sinistral flatfish, the right eye migrates to the left side.

Different families of flatfish are either right-sided (soles and plaice) or left-sided (turbot). Occasionally, however, you come across one which is the ‘wrong’ way round (just as most people are right-handed, but some are left-handed).

Usually the side of the body with the eyes on it (facing up) is coloured, while the blind side (facing the sea bed) is white.

All change

The transformation in its appearance -called metamorphosis — starts when the fish has accumulated a large food reserve. During the change it doesn’t feed at all.

The eye that is to migrate gradually becomes raised in a tube of soft, transparent skin which stands proud of the head. It moves slowly across the head until it comes to rest next to the other eye. When it has reached its final position, the tube of soft tissue disappears.

In some families of flatfish — the plaice and soles, for example – the foremost rays of the dorsal fin begin to move forward towards the snout once the migrating eye has passed the middle of the head.

In the other flatfish families (such as the turbot) the dorsal fin rays start shifting forwards before the eye has started to move. A temporary gap opens up between snout and dorsal fin through which the migrating eye can pass.

Before the flatfish can lie fully on either its right or its left, its mouth too has to change position. The head and jaw bones of the tiny larval fish remain soft and pliable long enough for metamorphosis to take place. As the transformation occurs, the jaws twist slightly so they lie more or less on the same side of the head as the eyes. Once the migrating eye has passed the middle of the head, the soft cartilaginous tissue hardens into bone.

When this has happened, the body flattens and the fish, now looking like a miniature adult, is ready to take up life on the sea bed.