MEPS 598:99-112 (2018)  -  DOI: https://doi.org/10.3354/meps12601

ABSTRACT: For marine species that hybridize, the maintenance of separate lineages requires natural barriers that limit hybridization zones or species distributions in which hybrid progeny cannot survive across the entire range of the parent species. We examined this potential in the sea urchin species Pseudoboletia maculata and P. indiana, which have overlapping distributions in the Pacific and readily hybridize, yet have maintained separate lineages. We examined the role of developmental thermal windows in native and hybrid progeny reared across a temperature gradient (8 to 37°C) to determine if post-zygotic processes restrict the environmental isotherm hybridization zone along the eastern Australian coastline. Native and hybrid progeny of Pseudoboletia from Sydney Harbour were reared to late pluteus larvae and scored for development at 3 time points (10, 24 and 48 h post fertilization) to determine the thermal limits for normal early development. While hybrid progeny developed equally well within their thermal windows and at ambient temperature (22°C), they had thermal windows up to 10°C narrower than those of the maternal lineage. The geographic ranges known for the benthic adults coincide with the thermal windows of their progeny. Hybrid progeny were less tolerant of the warmer conditions experienced by the Pseudoboletia species. This indicates that offspring fitness may limit hybridization in the tropical regions. Given the potential for the emergence of new hybridization zones as the oceans warm, our observations highlight the need for a greater understanding of the thermal biology of hybrid offspring when predicting future distributions and the potential for expansion of hybridization zones.

KEY WORDS: Hybridization zones · Introgression · Larval development · Distribution · Ocean warming · Climate change · Echinoids