Flat like a flounder...and really not too difficult to identify!

The situation is well known: One is diving over sandy or muddy bottom and concentrates on documenting the depth distribution of blue mussel banks for TaMOs when all of a sudden a flat fish takes flight in a cloud of resuspended sediment. Without having been detected in advance. As this not only can happen during TaMOs dives but anytime one is submerged and as this can clearly be related to the perfect camouflage of the fishes dwelling on the sediment, the sole and ultimate dive-log entry “flounder” is all that remains of the submarine encounter. Therefore, a closer look might pay off – and if one is acting cautiously and not moving overly hectic, the flatfish relies on its camouflage to a certain degree and can be observed and identified from small distance.   

Aside from rather uncommon flatfish species like lemon sole (Microstomus kitt) and sole (Solea solea), five flatfish species commonly occur in the western Baltic. These are flounder (Platichthys flesus), plaice (Pleuronectes platessa), dab (Limanda limanda), turbot (Psetta maxima) and brill (Scophthalmus rhombus). All five belong to the extensive order Pleuronectiformes, i.e. flatfish. Flounder, plaice and dab within this order belong to the family Pleuronectidae; these are “flounders” in the narrow sense. Turbot and brill in turn belong to the family Scophthalmidae. The affiliation to the different families is also reflected by appearance: The three “flounder” species are comparatively difficult to discern, just like the “scophthalmid” species are – a differentiation between the two families however is no further problem.

As a start, a superficial look should be given to the body shape – rather round to circular leads to a representative of the scophthalmid family. More discrete characters are a base of the dorsal fin anterior to the eyes and a left-eyed body (i.e. eyes are on the left side of the body). If the body is rather elongate (insecure criterion) and right-eyed (exceptions exist) with the basis of the dorsal fin above the eye, one has encountered a species from the pleuronectidae family.  

Often you can read of orange spots as identification characteristics for plaice (Pleuronectes platessa). The problem is, that often these spots are not easily to recognize. And above all: flounder and dab also often have these spots on their eye side. Therefore, a good criterion for discerning the according species is the lateral line on the “upper side” of the fish. If the lateral line is straight, one has either encountered a plaice or a flounder. If the lateral line is strongly and distinctly arched over the pectoral fin, one has discovered a dab (Limanda limanda). Plaice and flounder again can be discerned by a closer look to the superficial structure of the body: If merely the head is characterized by a bony ridge and the rest of the fish is smooth, it is a plaice (Pleuronectes platessa). Flounder (Platichthys flesus) is characterized by rough skin along the lateral line and especially along the basis of the dorsal and anal fin. A certain fraction of flounders additionally is left-eyed.   

Fishes of the family Scophthalmidae can be discerned as follows: a circular body with numerous bony tubercles is characteristic for turbot (Psetta maxima). A rather oval body with a smooth upper side that is spotted with numerous “open circles" is characteristic for brill (Scophthalmus rhombus).

This table recapitulates the most important identification traits:

1. Characteristic: Body more or less circular and right-eyed?
YES		NO
		2. Characteristic: Lateral line strongly and distinctly arched over pectoral fin?
		YES	NO
Body circular. Left-eyed. Basis of dorsal fin anterior to eye. Mouth large. Eye-side with numerous bony tubercles.	Body rather oval. Left-eyed. Basis of dorsal fin anterior to eye. Mouth large. Eye-side smooth with a pattern of numerous "open circles".	Lateral line strongly and distinctly arched over pectoral fin. Basis of dorsal fin above eye. Right-eyed. Mouth small. Coloration variable.	Lateral line straight. Basis of dorsal fin above eye. Right-eyed. Mouth small. Ridge of bony tubercles on head. Eye-side otherwise smooth.	Lateral line straight. Basis of dorsal fin above eye. Mostly right-eyed. Mouth small. Rough scales along lateral line. Bony tubercles along basis of dorsal and anal fin.
Turbot (Psetta maxima)	Brill                      (Scophthalmus rhombus)	Dab (Limanda limanda)	Plaice (Pleuronectes platessa)	Flounder (Platichthys flesus)
			n.a.
(Picture: T. Kujawski)	(Picture: R. Rathke)	(Picture: R. Rathke)	-	(Picture: H. auf dem Venne)