The Dover sole is probably one of the best known of the flatfishes living in British seas. Its long oval shape is characteristic – a fact its name acknowledges since sole is the Latin for sandal or slipper.
Not so lonely
There are, of course, other members of the sole family in British seas but none is as large (average length 30cm/12in) or as numerous as the Dover sole.
Of the others, the tiny solenette, less than 13cm (5in) long, has a dark bar along every fifth or sixth ray in its dorsal and anal fins; the thickback sole has five regular broad brown bands across its back; and the rare sand sole, 36cm (Win) long, has a hugely swollen, rosette-like front nostril on the underside of the head.
Any sole lacking these features must be a Dover sole. (The lemon sole isn’t a sole at all — it’s a relative of the dab.)
Sea bed life-style
Strongly associated with sandy coastlines and offshore to a depth of 100m (330ft), the sole also occurs on muddy bottoms or mud mixed with sand or stones. It burrows in the surface of the sand, lying hidden with just its eyes showing. Most soles live like this throughout the day, perfectly concealed, the colour of their upper side matching the sea bed. At twilight and during the night they become active, scouring the bottom for food, and even swimming in mid water or near the surface.
The fish’s food consists almost entirely of bottom-living invertebrates – small crustaceans, worms and molluscs and occasionally fish such as sand gobies and sandeels. These are almost all burrowing animals which the sole locates by using complex sensory organs on the underside of its head. These show up as a mat of short protrusions literally packed with sensory cells including taste buds.
The sole spawns in spring or early summer • as late as June in deep northern waters.
The eggs float near the surface, as do the young fish which are about 3-4mm long on hatching. At a length of about 18mm (%in) the young become bottom-living and look like miniature soles.
However, between hatching and becoming bottom-living, enormous changes take place in these minute fish. At hatching, the sole is similar to other fish with an eye on each side of its head and a symmetrical mouth. By the time it is living on the sea bed, the left eye has migrated through the head tissue to lie close beside the right eye, the mouth has changed shape and the sense organs around the mouth on the eyeless side have developed.
It is a nicely timed sequence of events which is co-ordinated with a total change in life-style • from a tiny translucent fish swimming in mid water to a bottom-living fish coloured on one side only to match the sea bed.
While these changes are taking place, post-larval sole migrate inshore to shallow water. This movement in the first weeks of life brings them into the breakers on sandy shores. From here they spread out into deeper water until they are sexually mature and then migrate to the spawning grounds by coming up near the surface and ‘hitching a lift’ on the surface currents flowing in the right direction.