The left-hand of benthos

When I was young, I was a fussy eater, but could be relied upon to eat enormous amounts of the few foods that passed muster (and pass the mustard). When the family ate out, which was rare enough in the austere 1960s, plaice and chips was an item on most menus and I could be relied upon to choose the familiar rather than trying something a) new b) more challenging; but, hey, I was only fourteen.  So I must have eaten a lot of plaice Pleuronectes platessa in my day.  I knew they were flatfish: hard to escape that knowledge as they were eaten skin-and-all, so were clearly not a fillet or if they were filleted it was clear that there was a black side and a white side.  Yum yum, aNNyway!  When I grew up and left home it was treat to buy a bagful of dabs Limanda limanda for half-nothing because Irish people at that time wanted their fish in big slabs - preferably salmon Salmo salar.  But they're wrong as well as boring in the choice.

One of the peculiarities of dabs and plaice  . . . and halibut Hippoglossus hippoglossus, sole (common or Dover Solea solea; Lemon Microstomus kitt) and turbot Scophthalmus maximus, apart from being moreishly tasty, is that they are flatfish which spend their adult lives on the sea-floor but start off as planktonic hatchlings being woofled around the ocean by the currents.  As fry (juveniles) they look like regular fish with eyes on both sides of their head (so they can see predators and take evasive action) but as they grow up a remarkable developmental transition occurs as one eye moves across the top of the head so that they both finish up looking more or less in the same direction.  Simultaneously, the now adult fish drops out of the water column and settles to the bottom, where it spends the rest of its life scarfing up crustacea worms and molluscs and growing to enormous size over many years.  The record for halibut is a fish 2.5m long weighing 230kg: that's a lot of fish-fingers. Clearly that's a double transition that needs to be quite well coordinated - a free-swimming fish with a blind side will soon be dinner for something else but there's not much to see if one eye is permanently embedded in the sand. The other coordinated change is developing a camouflage pattern topside and depigmenting the underside. 

Footnote: if you image.google.com for "halibut fry"
all you get is pictures of batter.

All the species eaten above are right-eye flounders of the family Pleuronectidae which comprehends about 100 distinct species in total. They lie on their left side and have two right eyes. I say distinct species because the biological species concept of Ernst Mayr defines species as "species are groups of interbreeding natural populations that are reproductively isolated from other such groups". Species are thus the only biologically meaningful item in our classification schemes for making sense of the living world. Fish breed by shedding sperm and egg into the water and hoping that they make contact.  The developmental genetic instructions are likely to be fatally scrambled if sperm from one species meets an egg from a different species. But it turns out that, in the Baltic, viable offspring are found whose parents are plaice (Pleuronectes platessa) and flounder (Platichthys flesus).  Not only different species, but different genera (the classification level up) as well!  That calls out to be investigated.

It turns out there is also another distinct group within the same order Pleuronectiformes which are left-eye flounders Bothidae.  These lads lie on their right side and have two left eyes. There doesn't seem to be any genetic, developmental or evolutionary advantage to being lefty or righty and occasionally you'll find an individual of one species that has flipped the 'wrong' way. The completeness with which the roaming eye migrates varies among the species. Some flatfish can also change colour to match the background like a chameleon. And I should add that rays and skates of the super-order Batoidea related to the shark family are also bottom-feeders that hug the sea bottom, but become flat by spreading out rather than turning on their sides.  Evolution has many ways of cracking essentially the same nut.
More on Southpaws.