As everyone has mentioned, sometimes these are just genetic occurrences. However, there can be nutritional causes. I'm not a nutrition expert, but I do know from experience (sadly) that a tryptophan deficiency will cause spinal deformities in small (inch or less) largemouth bass. So, nutrition is an alternate explanation that may at times be involved. However, I wouldn't expect tryptophan deficiency in wild fish, as that amino acid is quite plentiful in most crustaceans, including zooplankton.
