By Sarah Ward
Living Seas Officer
The term ‘flatfish’ is the common name used to describe fishes which are members of the order Pleuronectiformes. As common names tend to be fairly descriptive, you don’t need to be a fisherman or a marine biologist to understand that fish in this group have a flattened body type.
Although there are over 600 species of flatfish found worldwide, we have just a handful of them here in the sea around Sussex, including Dover sole, plaice, dab, brill, turbot and topknot.
Their flattened bodies mean that flatfish are adapted to live on the sea floor. They are masters of disguise, not only with their ability to quickly bury themselves under the sediment, but also by being able to change their appearance to better match their surroundings. Flatfish are commonly overlooked by divers and snorkelers due to being so well concealed – often the only clue to their presence is a tiny pair of eyes just poking up out of the sediment.
Interestingly, flatfish do not begin life flat. Flatfish larvae are actually ‘upright’, with one eye on either side – for the first few weeks of life they look and act just like ‘normal’ fish. However, they will soon start to develop into juveniles and so begins the transformation into one of the most asymmetrical vertebrate species to walk (or swim!) the Earth. The fish will undergo a number of physical changes but perhaps the most surprising is the migration of one eye to the other side of the body.
Flatfish species tend to be known as right- or left-eyed (or dextral or sinistral) depending on which side the eyes are on. Curiously, a similar trait is seen in snails, whereby the shell will coil in one direction or the other. Occasionally, an individual may develop the wrong way round, whereby the wrong eye migrates and the fish ends up ‘back to front’ – this would actually be difficult to notice unless compared to a normal individual of the same species!