In the following excerpt from Concealing Coloration in Animals, new this month, Judy Diamond and Alan B. Bond describe the amazingly adaptive camouflage of the cuttlefish.
Concealment is often at least as much an effect of pattern as of color. Animal coloration usually appears as distinctive markings like the narrow stripes on grassland birds and insects, the spots on jungle cats, the leaf-sized patches of muted color on animals of the forest floor, or the fine mottled speckles on sandpipers and bottom-dwelling fishes. Such patterns echo the texture of the environment, matching the animal’s surroundings in the size and shape of color patches, as well as in the particular hues and intensities of coloration. To envision how pattern contributes to concealment, think of a digital image of an animal against its background. The image is composed of pixels of specified colors, and adjacent pixels that are similar in color will appear to the eye as patches of varying size. How well the animal blends into its surroundings is a function of the match between the color of the patches in the background and those on the animal, but blending in is also affected by the relative sizes of the patches, regardless of their color. Animals process these two kinds of visual information, color and patch size, in separate parts of their nervous system, and the information is subsequently integrated to assess the degree of resemblance to the background.
How animals integrate the information about color and patch size is most impressively illustrated by cuttlefish. Cuttlefish, which are free-swimming relatives of octopus and squid, are masters at generating patterns to match their environment. They have large, complex, sensitive eyes and a sophisticated control system that allows them to rapidly modify the patterns they display. When they detect changes in their surroundings, they can change the arrangement of light and dark patches on their body accordingly. At the Marine Biological Laboratory at Woods Hole in Massachusetts, Roger Hanlon studied how cuttlefish create their remarkable patterns. In one experiment, Hanlon and his colleagues placed cuttlefish in transparent aquaria on top of complex backgrounds, such as checkerboards, stripes, or spots. They photographed the cuttlefish to determine how well they matched the background patterns, and they timed how rapidly the animals could change from one pattern to another.
The change was immediate. Hanlon and his team found that cuttlefish use three major body pattern types for camouflage: uniform patterns on backgrounds with little or no contrast; mottled patterns on substrates with small, highly contrasting patches like small checkerboards; and disruptive patterns that break up their outline on substrates with high contrast and long, defined edges, like large checkerboards. In the lab, cuttlefish will camouflage on any substrate, natural or artificial, regardless of whether or not there is a predator in the tank.
Hanlon asked whether the cuttlefish had preferences among background types. To ensure that previous experience had no influence in their choice, the cuttlefish were raised in the lab on backgrounds of a solid color. First, the cuttlefish were placed on a substrate that was half white and half uniform gray. In this case, the animals clearly preferred the darker substrate. When they were presented with soft sand versus sand that had been glued to plastic so the animals could not burrow into it, the cuttlefish preferred the soft sand. When cuttlefish were given a choice of three different artificial backgrounds—uniform gray, small black-and-white checkerboard, and large checkerboard—they showed no preferences, and they still showed no preferences when given three natural backgrounds: sand, small shells, and large shells. Subsequent experiments showed that cuttlefish readily integrate multiple cues from a mixed substrate (like shells of different sizes) producing a mixed camouflage pattern.
Cuttlefish are unusual because they generate their own patterns even on backgrounds they have never encountered before, and they can achieve this with lightning speed. They change their color patterns with organs called chromatophores, bags of pigment with muscle fibers radiating out around them. Each chromatophore contains one type of pigment, either dark brown, orange, or yellow. Because the organ operates with muscular contractions, color change in cuttlefish happens as fast as you can snap your fingers. A similar system occurs in most of the cuttlefish’s relatives, including octopuses and squids. Many fish have pattern-matching capabilities, particularly flounders and other flatfish that inhabit sandy ocean bottoms, but their chromatophores work on a different basis and are significantly slower. No other animal has the versatility or speed of pattern matching as the cuttlefish.