Flatfish Metamorphosis

Pity the flounder! Pity, I beg thee…

Adolescence is difficult. Puberty is an awkward transitional time when the winsome cuteness of childhood departs forever but is not fully replaced by the graceful strength and confidence of adulthood. But before you have a PTS flashback to those rough years, spare a moment of pity for the poor flounder. Flounders (and other flatfish like halibuts, soles, and flattest of them all, turbots) are born…err hatched like us with two eyes on either side of their skull. As tiny transparent larvae living among the zooplankton, flounder fry can see a panoramic view of the ocean so as to better evade predators. As they dart through three dimensions, their bilateral symmetry is like that of the rest of the vertebrates.

As searingly depicted in this stunning diagram

Then, as they grow into adult fish, a strange and remarkable metamorphosis occurs. Bones in the flounder’s skull distort and one eye migrates across its head so that both eyes are on one side of its face. Imagine if your left eye traveled over the bridge of your nose to permanently join your right eye on the right side of your face!

An adolescent flatfish–the eyes are just beginning to creep to one side (photo for PBS Nova)

Of course eye migration is but one aspect of the flounder’s change to adulthood. The fish begin to swim at an angle. One side of their body becomes flecked with color while the other becomes white (the better to merge into the two dimensional world of the bottom). Speaking of color, the once transparent fry becomes opaque! Their mouth opens on one side of their head and they must learn to swim like a flying carpet.

But don’t let their remarkable transition and their comic appearance deceive you. The flatfish are extraordinary predators and they are also geniuses at avoiding the many toothy hunters of the ocean. Their close set eyes protrude above the sand and see unwary prey with acuity and laser focus. Fossil finds from Monte Bolca, a beautifully preserved Eocene coral reef, show that the flatfish were evolving into their current form 45 million years ago (as the primates were taking to the trees, the bats were first taking wing, and the little dawn horses were scampering through the endless tropical groves). For at least 45 million years the flatfish (which, I should have mentioned, constitute the order Pleuronectiformes) have been camouflaged at the bottom of the sea feasting on shrimp and minnows while the world blinked and didn’t notice them. They are still out there thriving, even as whole parts of the ocean ecosystem collapse. It is a striking reminder that wrenching changes can work out for the best!

An adult Turbot

Rodhocetus

Rodhocetus (by Pavel Riha)

This endearing beasty is Rodhocetus, a long extinct proto-whale which lived during the mid-Eocene (approximately 40 to 50 million years ago). Rhodhocetus fossils are found in contemporary Pakistan, but the world has changed greatly since the warm Eocene: the creatures did not live on the tops of mountains, but rather in estuaries and shallow seas.

Rodhocetus

The early cetaceans shared ancestors with the artiodactyls (cows, pigs, hippos, goats, and suchlike even-toed ungulates) and indeed the first cetaceans, from the beginning of the Eocene, look somewhat like weird squashed hippos or water cows. By the middle of the epoch, however the familial similarities were beginning to fade.

Rodhocetus specimens have elongated hands and feet–which were almost certainly webbed. Their hipbones were not fused to their backbones, which gave them additional speed and maneuverability in the water, where they hunted for fish and squids. Although the creatures were adapted for an aquatic predatory lifestyle, they could still drag themselves up on land, unlike their descendents the modern whales and dolphins. Additionally they still retained fur, and double-pulley heelbones (the latter of which convinced paleontologists that whales and cows are relatives who share an ancestor).