Dive Dry with Dr. Bill

#519: Fascinating "Flounder"

I've started working on a new episode of my cable TV show "Munching & Mating in the Macrocystis." This one focuses on the flatfish that frequent Catalina waters. I grabbed my trusty video camera, donned my dive gear and descended into King Neptune's realm in the waters off the Casino building. Flatfish live on the ocean floor so I pointed my camera (and eyes) down towards the bottom to look for and film them. Curses! The dastardly "devil weed" from Asia, Sargassum horneri, totally obscured my view of the sand where these interesting fish primarily reside.

Now most of my readers know that the majority of fish swim through the water with one eye on each side of their body. Flatfish have adopted an entirely different lifestyle... lying on the bottom preferably over soft substrate such as sand. It is obvious that having one eye on the side facing the ocean floor would greatly limit their vision! Somehow, the eyes on these bottom dwellers ended up on the top surface of the body so they could locate prey and detect potential predators. How this happened was a mystery until recently.

Even Charles Darwin, considered by many to be the father of evolution, pondered the question of how both eyes came to be on the same side in flounders, halibut, sole, turbot, plaice, "dabs" and the rest of the some 400-500 species of "flatties." Based on his theories, we should observe fossil species that are intermediate between fish with eyes on both sides of the head and the flatfish. Creationists pointed to the absence of such transitional stages as evidence that evolution was not responsible. They believe that all species were created as they currently exist, and are in denial regarding all the scientific evidence to the contrary.

However, recently Dr. Matt Friedman formerly of the University of Chicago and the Field Museum of Natural History (a childhood favorite of mine) made an important discovery that appears to have solved this debate. Friedman looked at fossils from the Eocene, 50 million years ago, in northern Italy. These fossils, from the genera Heteronectes and Amphistium, have eyes which have not fully made the journey from one side of the head to the other. With one eye stuck about half way, these fossils form the missing "missing link" between "normal" eyed fish and the flatfish.

There is also an existing genus (Psettodes) of fish from the Indian and eastern Pacific Oceans that has one eye near the top of its head instead of on one side or the other. These fish divide their time between lying on the bottom and swimming free above it. The position of their eyes thus allows them to see under either posture. Amazing how evolution works.

So tell us, Dr. B., how does all this come about? OK, I will do just that. Flatfish mate and cast their eggs into the plankton where they drift and hatch out as larval forms. Their larvae (see image) are similar to other fish in that they have one eye on each side of the head. This is good since they are swimming midwater and need to see to both sides.

As the larvae develop, they begin to prepare for their adult life on the bottom in the briny depths. The eye on one side of the head begins to migrate across the top of the skull through a notch to the other side. The bones in the tiny fish's skull do not completely harden until after the eye migrates. Some species have both eyes ending up on the right side of the head (referred to as dextral) and others have both on the left side (sinistral). In some species the majority of individuals have both eyes on the same side of the head, but others may have a mix of left- and right-eyed individuals just like we have left- and right-handed human beings.

There are two other modifications from standard fish anatomy that occur during this transition from the free-swimming larval stage to the bottom dwelling habit of the adults. The swim bladder, which adjusts buoyancy and allows a fish to remain in the water column at a given depth, is no longer necessary so it disappears. Pigment creation in an animal's skin involves some expenditure of energy, and there is no reason to have the underside of a flatfish pigmented, so most are a white to cream in color there. Only the upper surface receives the cells that create pigment for blending in with the bottom. This camouflage helps them lie undetected by potential prey until they rush out to grab it, and also makes it difficult for predators to locate them.

Now you know the answer to the mystery of a flatfish's eyes. Aren't you glad I divulged this secret to you? Over the next few weeks I'll describe many of the flatfish species found in our waters off Catalina. Hopefully the Dept. of Fish & Wildlife will get their act together and allow us to control the invasive Asian seaweed so I can film more of them. However, now that the harsh winter months are upon us here in SoCal, I plan to spend time diving and filming in the tropics... perhaps the Bahamas, Aruba and the Philippines.

Interestingly, flatfish are not seen anywhere near as frequently in the tropics and subtropics as they are in temperate waters. I may have to write about sharks, lionfish and other interesting critters from those warmer destinations... as I sit out on a beautiful sandy beach with a cold cerveza in my hand and bountiful bikinis bounding along the beach. Oh, joy! The endless summer for me...

© 2013 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Darwin contemplating the evolution of the flatfish and three photos showing flatfish larva (upper right),
right-eyed C-O sole (lower left) and left-eyed (lower right) fantail sole by Kevin Lee.

This document maintained by Dr. Bill Bushing.
Material and images © 2012 Star Thrower Educational Multimedia