Flounders - Facts and Information for Scuba Divers

swimming peacock flounder
A peacock flounder swims over the reef; a rare sight! Click below for more images. © istockphoto.com

A pair of rounded eyes poked out of the sand. One eye pointed directly at my dive group while the other focused in the opposite direction. I couldn't believe my luck! My dive group was making its way between two coral heads, and I had just spotted a peacock flounder buried beneath the sand. I called my dive group to a halt, and then slowly finned my way closer to the flounder so as not to frighten it away.

Dive guides across the globe know that flounders can be some of the most difficult creatures to spot, but with their unusual body shape, interesting life cycle, and superior camouflage skills, they are also among the most rewarding animals to find. Realizing that it had been seen, the flounder took off across the sand like a flying carpet, then buried itself up to its eyeballs in the sand once again. Back on the boat, I explained to my divers why flounders are so special.

Flounders Start Out Like Any Other Fish:

Flounders can be easily identified by their flat oval shape. They are most commonly observed buried partially in the sand. Two eyes sprout on short stalks from a flounder's head, and are often the only part of the flounder visible. Flounders have a very unusual shape, but what many divers do not realize is that they begin life looking just like typical fish.

In the larval stage, flounders swim in an upright position, have eyes on either side of their heads, and have two normal pectoral fins.

During a flounder's transition from the larval to juvenile stage it undergoes a metamorphosis from a relatively normal, free-swimming fish to a flat, bottom-dwelling creature.

During its metamorphosis, a flounder begins to swim on one side. Its dorsal and anal fins elongate to line the edges of the fish's body, and the pectoral fin on the fish's upward-facing side grows taller.

The flounder loses its swim bladder, leaving it unable to float effortlessly in the water column. The most astonishing part of the flounder's transformation is that the eye on the bottom side of the fish migrates to the upward side of the fish. A flounder can move its eyes independently. Depending upon the species of flounder, either the left or the right eye migrates. Flounder species are often distinguished as either left eye (left side up) or right eye (right side up) flounders.

Flounders Are Experts in Camouflage:

Many fish can alter their colors and patterns to mimic the environment. Few fish, however, do so as effectively as the flounder. One study found that flounders have six different systems of markings, each of which can be controlled independently. By darkening and lightening these markings, a flounder mimics its environment.

In "Reef Fish Behavior" by Paul Humann and Ned Deloach, the authors site an experiment conducted in the 90's by researchers at the University of San Diego. "Five specimens rotated through a series of five bottom patterns -- yellow sand, coarse gravel, one-centimeter and two-millimeter checkerboard patterns, and a solid gray sheet -- [the flounders] closely matched the background patterns in 91 percent of the trials.

Even more impressive, most changes were complete in just two to eight seconds."

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Divers have observed this behavior in flounders for years. A flounder that is disturbed will jet across the floor, settle in a new position, and disappear into its background in a matter of seconds. Flounders visually identify the characteristics of their environment, and use their observations to adjust their colors and patterns. A flounder whose eyes are covered by sand will not change patterns. Interestingly, a flounder's coloration may also indicate its emotional state. Flounders that are threatened or frightened will often pale as they flee a perceived threat.

A flounder's camouflage allows it to hide from both enemies and prey.

Flounders are primarily lie-in-wait predators, capturing prey by opening their large mouths quickly to create suction and draw prey into their jaws. They feed on small fish, and occasionally shrimp or crustaceans. Flounders have also been observed to stalk prey.

Spotting a Flounder Underwater:

Flounders can be found in both tropical and temperate waters at nearly any depth. They have been observed near shallow reefs and, astoundingly, at the bottom of the Mariana Trench, over 35,000 feet below sea level. Juvenile flounders may be as small as a coin, and adult flounders may grow to be over two feet long, depending upon the species.

Divers who wish to spot a flounder should concentrate their efforts on the ocean floor. Swim slowly and look for small movements; flounders who sense a diver will often jet away and rapidly camouflage with their new environment. They bury themselves in the sand using fanning movements of their fins, and these small disturbances in the sand may also alert a divers to a flounder's presence.

The Take-Home Message About Flounders:

Flounders are interesting and difficult to spot. They undergo an unusual metamorphosis from the larval to juvenile stages and are adept at camouflage. Once a flounder is spotted, observe its color and pattern changes, eye movements, and burial techniques, but be careful -- even once a flounder is seen, it is easily lost again, often disappearing from view right in front of a diver's nose.

Next Fish Article: Yellowhead Jawfish

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Gibb, Natalie. "Flounders - Facts and Information for Scuba Divers." ThoughtCo, Jul. 13, 2013, thoughtco.com/flounders-facts-and-information-2963113. Gibb, Natalie. (2013, July 13). Flounders - Facts and Information for Scuba Divers. Retrieved from https://www.thoughtco.com/flounders-facts-and-information-2963113 Gibb, Natalie. "Flounders - Facts and Information for Scuba Divers." ThoughtCo. https://www.thoughtco.com/flounders-facts-and-information-2963113 (accessed November 19, 2017).