Fish o' the Day - Lake Tanganyika scale-eating cichlid

You may know that the Great Rift Valley Lakes of East Africa support an incredible diversity of cichlids. We'll undoubtedly be talking more about those later. For now, we're going to look at one very remarkable species. The Lake Tanganyika scale-eating cichlid, Perissodus microlepis, has become a textbook example (literally) of the manner in which frequency-dependent selection can affect the evolution of a species. As the image above (courtesy of A. Meyer) illustrates, the mouth of these remarkable little fish take a decided twist to the left or right. This apparently support their feeding habit of approaching another unsuspecting fish from behind and plucking off the scales on which Perissodus feeds. The left-handed and right-handed forms cycle in frequency - left-handed forms dominate for a while, then are gradually replaced by the right-handed forms. Why? Well, when left-handed forms are the most frequent, potential prey species must be on their guard for attacks on their right side. This makes them more vulnerable to attacks by the rarer right-handed scale-eaters. This selective advantage leads to a gradual shift in the gene frequencies.