HYBRIDIZATION OF SYMPATRIC PATIRIELLA SPECIES (ECHINODERMATA, ASTEROIDEA) IN NEW-SOUTH-WALES

Three species of the asteroid genus Patiriella occur sympatrically in New South Wales and the possibility for hybridization among them was examined through a series of cross-fertilization experiments. Patiriella calcar and P. gunnii are morphologically distinct as adults but indistinguishable as larvae. Patiriella exigua is morphologically distinct in both its adult and larval morphologies. The gametes of P. calcar and P. gunnii were reciprocally compatible: laboratory crosses between these species produced viable hybrid juveniles. In crosses between female P. calcar and male P. gunnii, most of the juveniles metamorphosed with an arm number intermediate between that of the parents, whereas crosses between female P. gunnii and male P. calcar produced juveniles with an arm number more similar to the maternal phenotype. Heterospecific crosses with P. exigua resulted in low fertilization rates, and viable hybrids were not produced. This species appears capable of self-fertilization. Because hybrids between P. calcar and P. gunnii were viable, neither gametic incompatibility nor hybrid inviability appears to ensure reproductive isolation between these species. Ecological or habitat segregation and temporal separation in breeding may isolate these species in the field. The results demonstrate that if gamete surface recognition molecules are involved in fertilization of P. calcar and P. gunnii, then they are not strongly species specific, at least at the sperm concentrations used in this study. Reproductive isolation between these species has evolved despite their gametic compatibility. In contrast, P. exigua is isolated from its congeners because of gametic incompatibility and several features characteristic of its reproduction and development. The implications of these findings for reproductive isolation and speciation of Patiriella and for the evolution of reproductive isolation in free-spawning marine organisms are discussed.