Why Are Flounders Flat? Because it Works.

Flounders are famously fishes that start out with typical-fish body shapes and morph into bottom-dwelling flatfishes that live sideways, with both eyes on the same side. As weird as this sounds, they’re highly successful survivors and predators. But why are flounders flat? And how do they get flat? 

Peacock Flounder (Bothus lunatus)
Matching the corals below, a highly colorized Peacock Flounder (Bothus lunatus) passes above the reef.

FLOUNDERS ARE THE FISHES THAT PABLO PICASSO MIGHT HAVE DREAMED UP – all the parts are there, just arranged differently. And Peacock Flounders obviously would have been from his Blue Period.

Somehow, what starts out as a normal fish in the larval stage transitions from vertical, bi-lateral and opposite-eyed into a flat-as-a-pancake adult that swims, eats and hangs out on its side, one side always down, one always up. As part of the process, the eye on the lower side moves to join the other on the upper side.

Add to this strange architecture a spectacular ability to manipulate color patterns and disappear into the background and you have amazingly singular fishes.

Peacock Flounder (Bothus lunatus)
More typically, this Peacock Flounder adds blue rosettes to its mimic of a sandy bottom. The long pectoral/dorsal fin on this suggests it’s a male, as opposed to the fish at top. 

WHY ARE FLOUNDERS FLAT? 1: THE FLATFISH CONTINUUM

Flounders are members of the flatfish order (Order Pleuronectiformes) that includes more than 800 species ranging in size from about two inches (Tarphops oligolepis, a flounder found near Japan and Korea) to eight feet or more (Atlantic halibut, Hippoglossus hippoglossus, found in the North Atlantic).

They all generally follow the same flatfish rules of body design. And, flounders, soles, turbots, plaices and halibuts, of course, are much sought after as tasty food fishes.

It should  be mentioned that “flounder” is both a specific and a loose term. A Peacock Flounder is a flounder, for sure. Additionally, in some references any flatfish may be grouped into the flounder category for descriptive purposes. Sometimes, a halibut is a part of the flounder community, if not simply a “flounder.”

Pacific Halibut
Among the largest of the flatfishes, Pacific Halibuts can grow to more than 8 feet in length, five feet in width, and 500 pounds in weight.

WHY ARE FLOUNDERS FLAT? 2: LIVING LIFE SIDEWAYS

The singular Eyes-On-The-Same-Side part of these fishes’ unique make up is a flounder’s universal given. Perhaps less appreciated is the “side” part of the equation:

A flounder may look like a regular fish squashed down to a deflated football shape, with dorsal fin reaching upward from its back and its eyes on top of its head.

In fact, it spends its adult life lying on its side. The second eye has not only migrated to its first-eye side, the “dorsal fin” is actually an adapted pectoral fin. Those fringy-looking things that seem to encircle his flat shape are his original dorsal and anal fins.

WHY ARE FLOUNDERS FLAT? 3: THEY’RE AMBUSH EXPERTS

But with this shape and positioning, flounders tend to thrive. Their flatness gives them a narrower profile for hiding from predators. They’re demersal fishes – living near the bottom – and their flat shapes help them speedily bury themselves under the sand with only their eyes protruding to scan the surrounding waters.

Their stalked eyes function independently with 180-degree-rotation, giving them excellent binocular vision. Lying in wait, they’re experts at ambushing passing delectables.

Winter Flounder (Pseudopleuronectes americanus)
Flounder’s two stalked eyes give them 360 degree vision. This Winter Flounder (Pseudopleuronectes americanus) matches a dark bottom with brown hues.

And also at hiding, which can be advisable. Juvenile flounders are preyed on by crabs and shrimps as well as other fishes. Adults are vulnerable to a range of piscivorish fishes, from groupers and moray eels to stingrays and sharks. They’re also subject to predation by sea birds, seals and sea lions.

As for the flatties themselves, the optimum diet for a 12-14 inch fish like a Peacock Flounder focuses on small, minnow-like fishes like silversides, herrings and anchovies. They’ll also dine on crustaceans, worms, small octopuses and other prey. Researchers studying eyed flounders have found specimens with four-inch jacks and mantis shrimps in their stomachs.

HOW DO FLOUNDERS GET FLAT? 1: THE CHANGE PROCESS

As larvae, flounders start out life with traditional bi-lateral fish anatomy – upright swimmers with eyes and fins on each side. But in a matter of weeks, in a transition taking multiple days in some species and as little as one in others, they morph into something else:

  • Muscle and bone shift into the flattened shape.
  • One eye moves across the head, slipping through a notch to relocate near the other on the other side.
  • The new upper side – the “eyed side” – acquires color while the underside – the “blind side” – lightens to a white shade.
  • The internal swim bladder that helps fish control buoyancy disappears.
  • The dorsal and anal fins end up encircling the flat, oval body.
  • The lateral line moves to the top of the back.
  • The pectoral fin on the upper side lengthens to serve as a dorsal fin. It’s longer in males than in females.
Summer Flounder (Paralichthys dentdatus)
With bodies decorated with large ocelli, Summer Flounders (Paralichthys dentdatus) are versatile mimickers.

HOW DO FLOUNDERS GET FLAT? 2: LEFT-EYED VERSUS RIGHT-EYED

All flatfishes share this body design, but in one respect the changes do take different forms. In some species, the eyes end up on the left side of the body – “left-eyed flounders” – and in others on the right – “right-eyed flounders.”

They’re classified into different families but other than left eye/right eye, the difference isn’t significant. However, for amateur naturalists like us it’s a helpful trait for identifying species.

Left-eyed flounders include Peacock, Eyed and Summer Flounders. Species in the right-eyed group include many commercially important fishes – the Atlantic and Pacific Halibuts, Common Dabs and the American Plaice. There’s a third family, the Soleidae, that encompasses soles and tongue fish. They’re left-eyed but with a modestly different alignment.

But not the Lemon Sole or the English Sole  – they’re in the right-eyed family. And the commercially significant Japanese halibut is a lefty.

HOW DO FLOUNDERS CHANGE COLORS? 1 THEY’RE QUICK CHANGE ARTISTS

Peacock flounders are hardly the only fishes that change pigmentation and body markings – hogfishes and scrawled filefishes come to mind. But peacocks are certainly the supreme quick change artists when it comes to changing their colors to mimic their surroundings. And, they can do it within seconds.

The classic laboratory experiment placed a peacock flounder on a checkerboard bottom and famously had it replicate the pattern exactly. Or pretty exactly. Actually, the results weren’t exactly exact and when the fish was moved the pattern didn’t change.

camouflaged juvenile flounder
A juvenile camouflages itself against shells and sediment in a New England cove.

HOW DO FLOUNDERS CHANGE COLORS? 2: IT’S ALL IN THE CHROMATAPHORES

The flounders’ success at mimicking backgrounds lies in myriads of cells called chromatophores located in their skin. They contain black, white, red, yellow, blue and, sometimes, green pigment granules called chromatosomes.

Colors are changed when chromatosomes concentrate or disperse within the chromatophores. Being above a light substrate causes them to aggregate in the center of each cell, giving a paler appearance. Darker colors result when the chromatosomes spread out.

It’s suggested that this quick-change ability is driven by the fish’s superb visual capabilities, coupled with a nerve system that rapidly transmits signals. And, research has found that if one of their eyes is unable to see, they have difficulty making the adjustment.

Research published in 2018 suggested that in additional to optical cues, another fish with color-switching skills – the hogfish – additionally benefits from “dermal photoreception.” That is, they sense light with their skin to blend in with a sandy ocean bottom or other, darker features. Exactly how this “skin vision” works is still unclear, however. And, it’s suggestive when considering flounders, but whether it applies to them is not addressed in the research.

Lemon Sole (Microstomus kitt)
Found in shallow northern European waters, the Lemon Sole (Microstomus kitt) is neither lemony nor a true sole. A right-eyed flatfish, its name probably comes from a French word for “file,” reflecting its rough skin.

MAKING MORE FLOUNDERS

Like many species of fishes, flounders stake out territories and live in harem social structures, with a dominant male claiming a large area and perhaps a half-dozen or so females occupying smaller territories within it.

Researchers studying eyed flounders found that the male seeks to mate with each female in his harem every day at evening time. With mating starts with an elaborate courtship ritual in the water column before they release gametes into the current for fertilization and dispersal.

WHY DO FLOUNDERS GET FLAT? 3: THE JOURNEY

As with all broadcast spawning, the resulting fish larvae are on their own, members of the plankton. The length of the  journey to adulthood varies with the species. Only a small percentage survive the entire journey.

Winter Flounder (Pseudopleuronectes americanus)
The Winter Flounder is not as versatile a quick change artists as the Summer Flounder, but it does change overall coloration to match backgrounds.

The larvae of left-sided flatfishes emerge with protective spines over vulnerable parts of the their bodies, including the head and gills. Right-eyed flatties are less endowed with spines and have a shorter transition. In any event, the spines disappear during the transition from tradition bi-lateral fish to sideways-living flatfish.

As larvae living in the plankton, the little fishes have swim bladders that help them deal with buoyancy as they maneuver in the water column. During the transition to demersal lives, the swim bladders disappear.

PRINCIPAL SOURCES:  Marine Biology, Peter Castro, Michael Huber; Reef Fish Behavior, Ned DeLoach; Encyclopedia of Fishes, John Paxton and William Eschmeyer; What a Fish Knows, Johnathon Balcombe; Reef Fish Identification Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Reef Fish Identification, Tropical Pacific, Gerald Allen, Roger Steene, Paul Humann, Ned DeLoach; Marine Life of the North Atlantic, Andrew Martinez; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Peacock Flounder, Eyed Flounder, Florida Museum, University of Florida; Bothus lunatus Flounder, Animal Diversity Web, University of Michigan; Fish FAQ – Can fishes change colour? Australian Museum; THIS COLOR CHANGING FISH CAN “SEE” WITH ITS SKIN, Futurity; Pacific Halibut, Summer Flounder,et.al., NOAA, Flounder, Flatfish, Halibut, et.al, Wikipedia.

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