Cuttlefish Say It With Skin

Natural History, April, 2000 by Marguerite Holloway

A mesmerizing repertoire of quick changes in skin color and texture qualifies cuttlefishes as masters of communication and disguise.

A fleet of tiny creatures swim the length of a shallow tank at the Marine Biological Laboratory at Woods Hole, Massachusetts, passing above yellow sand, then brown sand, then variegated pebbles, and finally a bed of white shells. These newly hatched animals, each no larger than a thumbnail, undergo instant and seemingly magical transformations as they travel the route, their skin color shifting from yellow to a tasteful khaki, to mottled black and white, to a uniform soft white. Protean masters of disguise, the apogee of marine crypsis, these young creatures are cuttlefishes, and according to their devotees, we should be so lucky.

Virtual reality pioneer Jaron Lanier is among the many people entranced by the fact that cuttlefishes seem to wear their thoughts on their skin. "This is what virtual reality is about, the ability to directly express your images," he says with obvious envy. Indeed, he asserts, the purpose of technology "is to turn people into cuttlefish." A single cuttlefish can become speckled, ocellate, stippled, lineate, whorled, black, white, brown, gray, pink, red, iridescent--all in different combinations and all in less than a second. It can hold zebra stripes for hours or send waves of color flickering across its skin. It can make half its body white while the other half displays lines. Its skin can pucker into riffles and spines and bumps, then suddenly go smooth as polished stone. And that's just the common cuttlefish (Sepia officinalis); each of the hundred or so other species has its own repertoire of quick changes.

Cuttlefishes--which, like snails and scallops, are mollusks--are highly advanced invertebrates, a distinction they share with several cephalopods, a class that includes squids and octopuses. But while cuttlefishes are closely related to these other ink-spurting, color-flexing creatures, they remain more mysterious, largely because they are for the most part so hard to observe in their natural habitats. Researchers have recently ventured into the field with night-vision video cameras and other high-tech equipment, but most of what we know about cuttlefishes still comes from the laboratory, where, often clustered together in tanks, these cephalopods--which are probably solitary by nature--continue to yield surprises. In the past few years, scientists have found that cuttlefishes can detect water motion in the same way that fishes do, that their large eyes (which have W-shaped pupils) can see polarized light, and that their reproductive behaviors and modes of communication are quite complex.

Cuttlefishes range enormously in size, from the two-inch Metasepia pfefferi to the three-foot-long Sepia apama. But all have eight arms and two tentacles, the latter usually remaining retracted unless the cuttlefish is feeding. Like other cephalopods, cuttlefishes seem to grow quickly, mate once, and then die: most live for no more than eighteen months. Like squids and octopuses, cuttlefishes have a funnel for jet propulsion, but unlike the other two, they also have an internal, oval-shaped bony chamber that fills with gas. The result is that, as well as being able to swim and squirt backward and forward, they can gracefully hover, rise, and fall.

Found in tropical and temperate oceans everywhere except in the Americas, cuttlefishes favor mostly near-shore environments such as coral reefs, mangrove swamps, and fields of sea grass and algae. Although they are an important source of food for many people, no data have been compiled on whether they have been endangered by over-harvesting, and demand for them has been growing in Asia and the Mediterranean. Still, even if they can't always escape nets, cuttlefishes can stay hidden from their other predators: sharks and teleosts (a range of bony, jawed fishes). This ability probably evolved between 370 million and 190 million years ago, when teleosts began taking over coastal environments and forcing some mollusks into deep water, where hydrostatic pressure made their shells cumbersome. So the cephalopods (with the exception of the chambered nautilus) shed their shells and slowly moved back to compete with, and to avoid, the fish.

The secret to cuttlefishes' capacity to fade into the background lies in several special types of muscle groups and cells: papillae, which allow the animals to deform their skin so they can assume the texture of seaweed or a bumpy rock; chromatophores, which contain pigment; and two kinds of reflecting cells, iridophores and leucophores, which influence color. In the common cuttlefish--the most extensively studied species because it is rather easily found in the eastern Atlantic Ocean, the English Channel, and the Mediterranean Sea--the chromatophores are yellow, red, orange, and dark brown to black, with a density of about 30,000 per square inch. The muscles around these sacs of pigment expand or contract in response to "messages" from the brain as it processes visual information. This neural control allows cuttlefishes and other cephalopods to transform their appearance rapidly; chameleons, on the other hand, control coloration through hormones traveling in the blood--a much slower process.