In the shallow waters off the coast of Indonesia’s Saonek Island, a frill-mouth sole (Brachirus heterolepis) lies in wait on the ocean floor. The ambush predator’s splotchy coloring is a near-perfect match for the surrounding coarse sand and its flattened body barely protrudes from the seabed. Except for the occasional swivel of an eye, it’s motionless. Only when an unsuspecting fish or crustacean passes within striking distance does it erupt from its hiding spot and reveal its true shape.
Like all flatfish in the order Pleuronectiformes, which also includes turbots and flounders, frill-mouth sole owe their impressive camouflage abilities to an array of pigment-filled cells, called chromatophores. By expanding and contracting various cells—each a particular color—the fish can adjust their skin to blend in with their backdrop. But there are limits to this ability. A seminal 1977 study showed that the chromatophores are distributed in permanent arrangements, which means that the fish have a fixed set of available patterns to choose from. When an individual moves into a new habitat and needs to alter its coloration, neural signals direct each chromatophore to expand or contract, producing the pattern from the set that most closely matches its current background.
For some flatfish, like the tropical eyed flounder (Bothus ocellatus), this color change happens stunningly quickly—in as few as two seconds. For others, especially those that live in colder water, it can take several days to complete an adjustment. But at both ends of the spectrum, the fish can respond more quickly to a substrate pattern they have encountered before than they can to a novel environment. That may be one reason why flatfish like the frill-mouth sole tend to return to the same breeding and hunting grounds year after year, making it slightly easier for photographers like Alex Mustard to find and document these masters of disguise.
Saonek Island, Indonesia
Dr. Alex Mustard has been taking underwater photographs for more than 30 years and has worked as a full-time underwater photographer for the past 12. His work has been displayed in exhibitions around the world and has won numerous awards, including seven wins in the BBC Wildlife Photographer of the Year competition and four category wins in the British Wildlife Photography Awards. His 2007 book “Reefs Revealed,” won the International Grand Prize for the best book of underwater photographs.