My buddy Pete and I were hustling along, trying to get across the “barren” sand flat so we could explore the kelp forest. We were only a few feet over the sandy bottom when something caught my eye. I stopped and stared down at the sand, but saw nothing. Pete circled around and hovered next to my side as I sheepishly pointed at what looked like an empty patch of sand.
About the time that I found myself wishing that Pete had never seen me stop, I saw another very slight movement. It was an eyeball on the top of a fish’s head, and this eye was rotating around like a periscope as the fish checked out its surroundings. A few seconds later the outline of the entire fish became obvious to me, and I was able to point it out to Pete.
We were looking at a California halibut, a member of a group of fishes that are commonly called flatfish. And they are exactly that: flat. All have unusual flattened bodies, a design that is very well-suited to life on and along the sea floor.
As Pete and I continued to watch for another 30 seconds or so, a school of four- to nine-inch- (10- to 23-cm-) long jack mackerel began to swarm over the bottom just above the halibut. Mackerel are a perfect meal for a hungry halibut.
Pete and I gave a little ground, and as we were backing away the sand suddenly exploded! The halibut rose out of the sand and swallowed one of the mackerel whole. In a flash it had been flattened by a flatfish.
Reacting to the sudden disturbance, the rest of the mackerel rapidly moved away. But as they did, another halibut, and then another only a few yards away, suddenly appeared out of their hiding places in the sand as they lunged at their chosen mackerel. The school of mackerel quickly ascended and the activity died down just as abruptly as it had begun.
No doubt about it, you don’t witness natural predation like this on every dive. In fact, in many instances you don’t even realize that you are near a flatfish because these fishes are truly masters of the art of camouflage.
Bottom-dwelling Bony Fishes
California halibut (Paralichthys californicus), the species of flatfish that Pete and I encountered on our dive, are closely related to the approximately 600 species known as flounders, halibuts, soles, turbots and sand dabs. Flatfishes are common in both the Atlantic and Pacific and from coldwater to the tropics with about 130 species occurring in waters that surround the Americas. Many of these are important food and game fishes.
Described in several different families, all flatfishes are members of the class known as Osteichthyes, a group that includes all bony fishes. Species range in size from four inches (10 cm) to seven feet (2 m) long, and the Atlantic halibut (Hippoglossus hippoglossus) weighs as much as 720 pounds (324 kg). Most flatfishes are entirely marine and live at shallow-to-moderate depths along continental shelves, but some species live their entire lives in freshwater while still other species enter freshwater occasionally.
The characteristic that tends to separate flatfishes from all other fishes is that as adults, all flatfishes have both eyes on one side of their head. Flatfishes tend to rest on, or swim close to, the sea floor on the side of their body opposite their eyes. The eyes move independently of each other and can rotate nearly 360 degrees, giving these fishes an excellent field of vision. The side with the eyes is usually colored to match the substrate, while the side that faces the sea floor is typically an off-white color.
Adult flatfishes swim in a horizontal attitude rather than in a vertical, back-up/belly-down, orientation as most other fishes do. When they swim, flatfishes tend to glide only an inch (2.54 cm) or so off the bottom while closely following the contour of the sea floor.
Perhaps the most fascinating aspect of this characteristic of having both eyes on one side of the head as adults is that in their larval stage, flatfishes have one eye on either side of their head as do most other fishes. Amazingly, as these fishes mature, one eye migrates over the top of the head to the other side. The mouth also begins to turn to orient to the new position of the eyes. The unusual positioning of the eyes enables flatfishes to lie flat or partially buried in sand bottoms with only their eyes protruding up into the water column.
Another remarkable adaptation that helps flatfishes camouflage themselves is that they can rapidly change the color and pattern of their skin to match their surroundings. Quantifying that skill in strict scientific terms is a tricky task, but to a diver there is no doubt that flatfishes possess this astonishing ability.
When prodded to settle on top of a checkerboard by experimenting specialists, some flatfishes can actually make their bodies take on a checkerboard pattern. However, when blindfolded, those same fishes lack this skill, so it is obvious to specialists that the ability to color match their surroundings is vision-dependent.
There is little question that divers routinely do not see flatfishes because of the fishes’ superb camouflage abilities. But like so many things in diving, once you see one, it gets easier and easier to spot flatfishes on subsequent dives.
Soles and Flounders
Ichthyologists, scientists that study fish, divide flatfishes into two groups: the soles — families Soleidae, Cynoglossidae and Achiridae; and the flounders — families Bothidae and Pleuronectidae. The latter group includes the halibuts, plaice, turbots and sand dabs. Some soles are valuable food fishes, but for the most part the flounders tend to be larger and more sought after by fishermen.
The halibuts are the largest flatfishes and several species, including both the Atlantic and Pacific halibuts, are of great commercial importance. Found in colder waters, both the Atlantic and Pacific halibuts feed voraciously on other fishes. Atlantic halibut are documented to weigh as much as 720 pounds (324 kg), but typical specimens weigh from 20 to 100 pounds (9 to 45 kg). Males are generally much smaller than the females.
Occurring south of San Francisco and reaching 60 pounds (27 kg), the California halibut is a smaller species, but it is still highly prized by all types of fishermen, including spear fishermen. California halibut are slow growers, with 5-year-old fish averaging only 15 inches (38 cm) long. California halibut prey upon anchovies, mackerels and other similarly sized fishes. In turn, they are pursued by myriad sharks, electric rays, sea lions, seals and some inshore dolphins.
California halibut and sand dabs are members of the family commonly called lefteye flounders, because the eye on the right side of the larval fish’s head normally migrates over to the left side as the fish transforms into an adult. There are also 22 species of righteye flounders that occur in California waters, a grouping that includes rock sole, starry flounder, C-O sole and diamond turbot. As the family name suggests, righteye flounders as adults usually have eyes positioned on the right side of the head.
On occasion on both lefteye and righteye flounders, the eyes migrate in the opposite direction, making positive identifications of species rather tricky.
A species known both as the summer flounder and fluke (Paralichthys dentatus) is quite common from Maine to the Carolinas. This species preys upon worms, crustaceans, and other small bottom invertebrates.
About 3,000 tons of American plaice, or sand dab, are taken annually by commercial fishermen. The American plaice is common at depths from 120 to 600 feet (36 to 182 m) on muddy or sandy bottoms, where it feeds on sea urchins, sand dollars, and other bottom life. This species grows to 30 inches (76 cm) and 14 pounds (6 kg).
The most noteworthy flatfish in Caribbean waters is the Peacock flounder (Bothus lunatus). Although they can appear to be dull-brown and rather drab, Peacock flounders can have a number of brilliant, bright blue rings and spots highlighting their bodies.
Perhaps the sand is not the most exciting biome to explore, but in many respects that is because many creatures that inhabit the sand are designed to blend into their surroundings. Certainly that is the case with flatfishes. But when you do encounter a flatfish, it is almost always a lot of fun to see how close you can get and if you can spot another one close by.
Flat Photos
The ability of flatfishes to camouflage and/or partially hide themselves makes these fishes reluctant to move away from a diver unless the fish feels imminently threatened and they seem to be absolutely sure that their cover has been blown. So, as a rule, the challenge for an underwater photographer is to find a flatfish in the first place. Once you see them, if you move slowly in a nonthreatening manner, you are likely to have a good chance to get close enough to get a nice shot.
In many settings you will want to get a few inches to a foot or so (0.3 m) above the fish and shoot at a slight downward angle. If you try to shoot at a level angle, the photograph lacks interest. But a level angle of orientation can work well to get a shot of those periscopelike eyes protruding from the sand, especially if you are able to fill your frame with the eyeballs.
Masters of the Art of Camouflage
Flatfishes have remarkable abilities when it comes to changing colors to match their surroundings. I once watched a flounder at Cocos Island in Costa Rica as it swam across the bottom, changing its pattern and hue as it went along. As I approached the flounder, it quickly settled to the
bottom, stopping with half of its body on the sand and half on a rock. Almost instantly, the half on the sand matched the color and grain of the sand while the portion of the fish’s body that was on the reef matched the reef. No wonder these fish are said to be masters of the art of camouflage.