With over 600 species and coming in all sizes, shapes, patterns and colors, Neotropical cichlids of Central and South America are popular pet fish. But most cichlids don't start out with bright bold colors and contrasting patterns across their bodies. So just how does a drab juvenile turn into a vivid adult? Denis Prazdnikov and Fedor Shkil of the Severtsov Institute of Ecology and Evolution, Russia, think that they've discovered the answer: thyroid hormone. This hormone has all sorts of effects, but people are most familiar with its role in controlling metabolic rate. More thyroid hormone makes you have a higher metabolic rate. In fishes, it can also serve other functions throughout their lives. One of the most important is determining where and when the fish produces black and blue colors.
The researchers measured the amount of black pigment in convict cichlids (Amatitlania nigrofasciata) and blue-eyed cichlids (Cryptoheros spilurus) during various stages of development from embryos to adults. They discovered that there were two groups of cells that made the black color – one that is active when fish are very young, and another that is active when they are adults. The team then gave some of the fish an extra boost of thyroid hormone during different stages in their development. This extra dose of hormone caused the juveniles of both species of cichlid to develop a black spotted pattern rather than their normal striped pattern. In most cases, this spotted pattern was replaced with the normal striped pattern when the fish were adults, but if they were given too much thyroid hormone too late in their development, the cichlids were stuck with the spotted pattern for good. But what would happen if the thyroid hormone was prevented from working?
The duo did just that, blocking the thyroid hormone from working during the same stages of development when the other fish were given a boost. In the blue-eyed cichlids, the adults looked just like the younger fish – never developing their vibrant adult blue coloration and keeping their stripes. When thyroid hormone was blocked in the convict cichlids, they developed their adult coloration but with even more stripes than usual. This suggests that thyroid hormone is responsible, at least in part, for the remarkable array of colors and patterns of cichlids – making the underwater world that much more stunning.
