THERE CAN BE FEW FISH AS COMMON, YET SO RARELY SEEN, AS THE TOPKNOT. Widespread around most of the UK, they frequent either rocky areas or wreckage, which distinguishes them from just about every other flatfish. Most diving takes place in such areas, yet ask around and many divers would agree that they have never spotted a topknot.
The two species you might see, common and Norwegian, both have interesting and unusual habits. The only possibility of confusion is with the lemon sole, which is sometimes also found on hard ground but this is more unusual and in my experience significantly larger.
A third type, Eckstroms topknot, is much rarer.
The common topknot is the larger of the two, growing to about 25cm long. It has a very deep, oval shape, becoming almost square towards its rear. Its head is broad and blunt and it has a largish mouth which, because of a series of joints, can be projected forward instantly like a telescopic tube.
This is used to good effect to catch tiny fish and a variety of crustaceans as it inches slowly and deliberately over the rocks, exploring every crack and cranny. It is coloured a nondescript grey to brown, with lots of irregular dark blotches. It appears to prefer fairly drab areas into which it blends perfectly.
Occasionally, the common topknot can be found on colourful sections of rock. Unlike many flatfish, it seems to have only limited ability to modify the colours and patterns on its back. Here, the best it can do is to lighten itself a little and fade out the blotches.

BUT WHAT IT LACKS IN CAMOUFLAGE ABILITY, IT MAKES UP FOR WITH A MOST UNUSUAL ATTRIBUTE. The common topknot can fix itself to any rock surface, from vertical to upside-down, on overhangs! This is not where you expect to see a fish, and as it stays immobile, it is small wonder that it is easily overlooked.
The fish appears to grip the rock by moulding its very broad and flexible fringing fins into every tiny imperfection in the surface. Having tried to move topknots on occasion, I can testify that they get quite a grip, and suspect (though no fish ID book mentions it) that they also arch the centre of their bodies away from the rock to create a vacuum effect. What can give them away is their eyes, which stand well proud of the head on prominent little turrets that constantly swivel independently of each other, giving an all-round view of impending danger or perhaps an approaching snack.
However, the common topknot is so confident of not being detected that if you do see one, no special care with your approach is needed. You should easily be able to move in for a close look. When it does move away, it does so slowly, gliding just millimetres above the rock surface with its surrounding fins, rather like the skirt on a cuttlefish, gently rippling.

OTHER THAN INHABITING SIMILAR AREAS AND BEING FLAT, THE NORWEGIAN TOPKNOT bears little resemblance in either appearance or habits to the common variety. You could be forgiven for wondering why they share a name.
It is a very slender, oval shape with a much more pointed head and is, at around 12cm maximum, considerably smaller. Neither does it appear to have the ability to cling to awkward surfaces, and prefers more level rocks. However, it possesses the most superb camouflage ability of any fish I have encountered, and this is a necessity, as the Norwegian topknot is regularly found in the shallows, down to about 18m.
This is probably the most colourful zone of our inshore waters and rocks are often a warm, pinkish-purple colour, thanks to encrusting algae. Remaining a normal brown here would be fatal, but its no problem for the Norwegian topknot, which can rapidly turn on patches of pink and purple.
And if there are bryzoans and sponges around, this fish can also add small areas of yellow and orange to blend in perfectly.
To find one bedecked like this is not uncommon and the contrasting combination of pink and yellow with an emerald-green eye transforms this little fish into a living jewel.
Although relatively common in many places, with such wonderful camouflage Norwegian topknots can be difficult to find. And unlike their larger relative, they are not inclined to sit still while you blunder around the bottom. So the first indication that they might be about is often a sudden flash of movement and perhaps a cloud of silt hanging over the rocks, showing that something has shot off to find a safer position.
They tend not to move more than a metre or so and if you train your eye always to look quickly beyond where the movement has been, you are likely to see a Norwegian topknot settling into a new position. A stealthy approach with gentle exhalations is then required to get close. Have a go - its well worth it.

POPULAR GUIDEBOOKS SAY THAT LITTLE IS KNOWN ABOUT THE NORWEGIAN TOPKNOT, and most that it is restricted to Atlantic coasts. This is incorrect - the fish is also widespread in rocky inshore areas of the North Sea, and far more likely to be seen than the common variety. Diver observations are, of course, a relatively recent source of information.
For example, the leopard-spotted goby, which is conspicuous and easy to identify, was until about 20 years ago thought to be extremely rare and confined to the South-west, but is now known (thanks to divers) to be common in all rocky areas around the UK. The small size and difficulty of identifying the Norwegian topknot under water means that its presence has largely been overlooked and, when seen, it has probably been mistaken for a stray juvenile of a better-known species.
The big giveaway is that all topknots are left-eyed flatfish - that is, looked at head-on, their eyes are always to the left of the gill slit. Other flatfish you are likely to find, with the exception of turbot and brill, which are both uncommon, are right-eyed.
Short of catching one, the best way to be certain of what you have seen is through a photograph. My own, all taken in the North Sea, prove conclusively that the Norwegian topknot has a much wider distribution than was once thought.

Brilliant camouflage ability distinguishes (or often prevents divers distinguishing) the Norwegian topknot.
How to tell a topknot - the common variety
a Norwegian topknot
and a typical right-eyed flatfish - this one is a plaice