You can't always identify a fish by its color

by B. N. Sullivan

You know that old cliché: You can't judge a book by its cover. In a sense, the same can be said for many fish species. Countless species exhibit different colors and patterns as they develop. In fact, sometimes juveniles and sub-adults look markedly different from the adults of their kind.

This is confusing to divers trying to learn the names of the fishies they see underwater, and in some cases it even has tricked scientists. Taxonomic records of fish species have many, many examples of fishes that were thought at first to be two distinct species, but later turned out to be different color phases of the same species.

The two individuals in the photo at right are an example of this phenomenon. They are both Whitespotted Filefish (Cantherhines macroceros), a Caribbean species common on reefs around Belize and the Cayman Islands. They appear in two distinct color phases: one with spots, and one without.

Of course, this happens on land, too. Just think of Bambi, for instance. As a fawn, Bambi had spots on his back -- spots he outgrew as he matured into an adult deer.

The fishes in the photo at least resemble one another, but some fish species undergo really radical color changes as they mature. Not only that, many reef fishes even change sex! Some species begin life as females, and later turn into males, a condition called protogyny. Then there are the protandrous species, which begin life as males and ultimately become females. There are even a few species that can change sex in both directions. Talk about gender-bending!

Like a lot of land animals, many fish species are sexually dimorphic, that is, the two sexes differ in appearance. The males are usually (but not always) the ones that are larger and more brightly colored. I've always thought that sexual dimorphism made obvious sense; if the two sexes look different, than each should be more readily able to spot a potential mate.

It turns out, though, that mate attraction is a much more complex process, probably less dependent on visual cues like color and size, than on subtle biochemical sensing (i.e., scent, or the fishy equivalent thereof). In any case, when critters -- including fishes -- need to find a mate, they know what to look for.

And that reminds me of a conversation we had one time with ichthyologist Jack Randall of the Bishop Museum in Honolulu. We described to him a peculiar individual fish we saw regularly at a certain patch of the reef at Puako. The fish was a Yellow Tang (Zebrasoma flavescens) with an odd color pattern. Usually that species is solid yellow, but this individual had irregular white blotches on it, such that it looked like it had been splashed with bleach! Dr. Randall told us that similarly patterned Yellow Tangs had been noted occasionally, and that it was presumed to occur because of some mutation to the genes that control pigmentation.

Another diver who was present said he had seen a Yellow Tang with similar white blotches several miles south of where we regularly saw 'our' little mutant. We mused aloud about what might result if we could get those two fishies with the aberrant color patterns to mate. Maybe we could capture one and release it near the other in the hope that they would produce some interesting offspring.

But then Jerry asked, "What if both are males or both are females?" Both sexes of Yellow Tangs look exactly alike. There's no way to tell them apart visually.

Jack Randall just grinned and winked. "Don't worry," he said. "They know."