Further to the flatfish...

The very first image on this blog was a random choice that has now lead me to discover some fascinating information about this enigmatic fish.

Some background:

The flatfish are an order of bony fish called the Pleuronectiformes (meaning "side-swimmers" in Greek: pleuro = side + nektos = to swim). This order includes some important food fish, including the plaice, sole, halibut and flounder.
The young larval fish hatch out form the bouyant eggs and swim like normal fish. But then the amazing! The young fish settle to the bottom, on their sides, and their heads begin to twist around. The orbit (that's techno bone-speak for eye socket), with the eye in it, moves across the skull. The end result of this anatomical change is a rather laid-back fish, lying on the bottom, with two eyes on one side (the top side) of its face. It is easy to imagine the relief sighed at this evolutionary development as the grit no longer accumulated in the corner of the downward facing orb. Wink wink. In fact this is a spectacular adaptation to bottom-feeding, as the fish flounders along, chewing on brittle-stars, tube-worms and other benthic delicacies.

An American researcher (Matt Friedman) has analysed some fossil flatfish and found them to be intermediate in their degree of "eye migration" between upright fish and the modern flatfish, as shown by the diagram below:

(courtesy of the NY Times)

This clearly shows the gradual nature of the evolution of the modern flatfish form. But why? Lying on the side would precede and thus stimulate such an eye migration What pressures could result in early forms taking to the bottom in such an unfishlike way? Was this hoist upon them by an inability to swim? A deflated gas bladder (internal fish-floatie)?

One possible reason is constipation. The clogged intestinal tract of goldfish can press against their gas bladders and interfere with the inflation and deflation of the organ. This can be remedied with a pea and Daphnia puree, apparently. Sounds like a nice side-dish for whole deep-fried flounder to me.