Existing knowledge on reproductive isolating barriers between sea urchin species is reviewed. Experiments involving artificial production of hybrids between congeneric echinoid species have shown that in most cases hybrids are viable and capable of backcrossing. Only species separated for > 5 million yrs show complete post-zygotic isolation. Each potential prezygotic isolating barrier appears to be incapable of completely preventing gene flow between sympatric species. different habitat preferences exist in many, but not all, sympatric species. Annual reproductive cycles are too environmentally labile to isolate entire species. Lunar reproductive rhythms may be a form of temporal isolation in some diadematid species, but they are generally lacking in other echinoids. Gametic isolation is bidirectional and complete in a few pairs of congeneric species, but as a rule it allows one-way gene flow between congeneric species. There is no correlation between pre-zygotic isolation and the time since separation of the species. Bindin, a reproductive molecule involved in gamete incompatibility, shows evidence of strong selection in genera that contain sympatric species, but appears to be evolving neutrally in genera that do not. however, the cause of selection, where it exists, is more likely to be some form of intraspecific process, such as sexual selection, rather than reinforcement to avoid hybridization. Even though no single barrier seems to be either absolute or universal, the combination of several barriers is potentially capable of reducing the probability of hybrid production in nature, which may explain why there is little credible evidence of natural hybridization or introgression between sea urchin species.
