Are We Floundering, Using the Name Halibut?
When I was contemplating the wonders of Flounder and Halibut,
I was going to write about their evolutionary relationships,
But got mired down with just what was their relationship presently!
Well, we know they are both some of the tastiest fish from the sea.
Halibut is actually really a Flounder, the general name for a whole Flatfish family.
'Flounder' is loosely used ; and for example California Halibut aren't actually Halibut at all.
Halibut grow to be very much bigger (10 to 20 times in size) than Flounder.
Halibut, because of their fin shape are diamond-shaped. Flounder are shorter and round.
The tails of Halibut are slightly forked. Flounder tails are rounded.
Uniquely, Flounder's eyes 'migrate' across their face,
Resulting in both eyes being on the same side!
Depending on which side of the head the eyes obtain,
Determine that they are 'right-facing' or 'left-facing'.
Halibut are almost always 'right-facing'.
Other Flounder species may end up right or left, depending on that species.
Generally, Halibut, both Atlantic and Pacific, live farther north than other Flounder species.
Lastly, back to the taste factor--Halibut is more firm and meaty,
While Flounder is more delicate and flaky.
Halibut, being the least fatty, with firm flesh, is perfect for frying or grilling
Flounder is slightly fattier, fillets much thinner, so is perfect to fry or bake, not to grill.
Sadly the Atlantic Halibut are overfished and thus endangered, but smaller Flounder there are still okay.
Consider the marvelous evolutionary changes of this group through natural selection :
The pressures to exist flat on the sea bed, yield new habitat possibilities with new combined food sources.
It was adaptive to have both eyes on one side, but fins, including the pectoral fins,
Remain almost the same--even when next to the sea bed.
The research shows that with one group there was a recent speciation after the invasion
Of a common ancestor into the Atlantic from Southern Africa during the Pleistocene.
But more to the evolutionary point--
The group of fish called Flatfish are the only vertebrates totally asymmetrical.
Attempts since ancient times have been made to explain the origin of this characteristic.
Flatfish when hatched are symmetrical, with bilateral eyes and fins.
The larvae live pelagically high above the sea bed.
But, with life's advances, they lose the swim bladder and acquire a strange appearance :
Totally flattened, a pigmented upper side, an asymmetrical skull--
Two eyes together and inclined in relation to the twisted mouth.
They are flattened sideways, with a less developed whitish lower side.
They have morphed to become benthic, yielding their strange shape
Which allows them to move, feed, and defend themselves very well in this environment!
These fish have lores with the Jewish people, calling soles 'Moses fish',
And with St. Peter who was burned by the Flatfish,
And the Romans who called them 'The Sole of Jupiter' because of the shape.
Lamarck in 1809, explained through 'gradualism' that with their 'need' to be attentive above,
They experienced the displacement of an eye and their flattening.
Later, Darwin partly accepted this idea, but added his thoughts on the process of natural selection.
In 1933, Richard Goldschmidt argued that Flatfish were a good example of 'hopeful monsters'--
Due to mutations, expressed in beginning stages of development
And finally in adults which give rise to very different morphology--
Asymmetrical fish from symmetrical fish!
Today, recent paleontology studies have found that fossils from 45 million years ago had eyes,
And their morphology obtained positions between symmetrical and asymmetrical arrangement.
Thus we see that this process went through intermediate stages and did not occur abruptly.
As stated before, there is a great variety of 'right' or 'left' species,
But the Thornback Turbot has 'right' and 'left' occurring in equal frequency.
This group is closest to the beginning of the Flatfish evolutionary tree, with the least asymmetry,
So thought concludes that asymmetry first occurred randomly,
Or in relation to environmental variations in this group,
And that later was controlled by genes in other related species.
In general there were no abrupt changes in any single genes during development,
But that in their development there are changes in the expression of certain genes,
And these changes explain the various characteristics of Flatfish : asymmetry, coloration and all !
Note : It is interesting that in 10,000 year old cave paintings,
One can determine to which species the paintings depicting Flatfish belong.
A depiction of one fish with its right-side twist in the cave of La Pileta, Spain,
May just be a Flounder!
Frank Maurer 7 July 2023
Frank Maurer 7 July 2023 1700 Hours.
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