In the Caribbean, there are many fishes (different species of fish) as well as many fish (different individual animals). Here is a list of some of the fishes you might find there; on our trip to Hospital Point we actually encountered these and these and these and many others. As it turns out, one of us (Ian) did about half a PhD on fish pigmentation, and is in a minor position to make various pronouncements about these animals. While zebrafish and their relatives were the subjects of Ian´s work (if you´re dying to read it, you can see it here and here. Drink some coffee first), some of the rules still apply. For example, in several individuals we saw at Hospital Point, we saw a classic sexual dimorphism in their pigment patterns. This is a fancy way of saying that the boys and the girls look different. Zebrafish boys are a little skinnier than the girls, and they also typically have a brighter, yellowish tail, anal fin, and the whole bottom of the animal. Take a look at the image to the right: the zebrafish on the top is a female, and the one on the bottom is a male. See the yellow tinge? Here´s another picture to help you out.
Why would males and females look different? Indeed, the mere existence of fancy-looking adornments on other males, such as peacocks, was something that bugged Charles Darwin enough to write much of a book about it. He proposed that while individuals are struggling against the elements to survive, individual males are also struggling against one another to get access to mates. That males look different from females is largely a result of such a struggle, particularly because the females themselves don´t have to struggle in that manner at all. Smart guy, that Darwin.
In the lab, the sperm produced by really yellow males seemed to be the best, and there is quite a venerable evolutionary argument that claims prettier, brighter males are better mates and healthier somehow. This is the so-called "good genes hypothesis", where the line of thinking is that if you´re a brightly colored male, you must have all sorts of energy to blow on pretty adornments, so you must be healthy and tough.
However, there is mounting evidence for a slightly different take: brighter colors and the like might have more to do with how the female nervous system (or perhaps the sensory organs themselves) is wired to perceive external signals than as true indicators of how tough the male bearing them might be. Ian´s friend Mike Ryan at the University of Texas has been breaking ground in this field for decades now, starting, interestingly enough, with a frog he found in Panamanian ditches, the Tungara frog (we haven´t seen any, but we really haven´t been looking, either). Mike´s former student and Ian´s good buddy Ray Engeszer then found that male and female zebrafish actually perceive the same things quite differently, giving a potential window into how female choice might influence what the males would look like. If the females see yellow brighter than other colors, for example, then males lucky enough to be more yellow would tend to have more mates. Moreover, Ray found that it is impossible to correlate what the fish are seeing and what humans can see, which makes designing future studies tricky and the findings of prior studies suspect. It turns out that trying to see what other animals see can be very, very hard.
Intracacies of why the males look different from the females aside, we´ve been noticing several strong examples of really bright individuals with the same sort of yellowish or reddish tinge one sees in the boy zebrafish and other individuals that are much more drab. We bet the brighter ones here in the Caribbean are boys, too.