Reproductive isolation between two cryptic sponge species in New Zealand: high levels of connectivity and clonality shape Tethya species boundaries

Although commonly reported among widespread marine taxa, hybridization has only been recorded for one species of marine sponge to date. Sponges have evolved an array of sexual and asexual reproductive strategies, but it remains unclear if the lack of reported hybrids in this phylum is due to their reproductive diversity (which promotes reproductive isolation) or instead due to a lack of data. In this study, we aimed to determine whether hybridization occurs between two cryptic species of Tethya burtoni ( T. burtoni sp. A and T. burtoni sp. B) that live sympatrically in central New Zealand. We also examined how both small-scale population structure (for five locations) and asexual reproduction contributed to instances (or lack thereof) of hybridization for these sponges. Using 11 microsatellite markers, we found no evidence of hybridization between species. Both species exhibited differences in distribution, where one species was present at all five locations, but the other was only detected at three locations. For these three locations (all within 20 km of each other), both species exhibited high levels of gene flow. Asexual buds did not appear to disperse far, and groups of clonal individuals were found within areas of < 900 cm 2 . Asexual reproduction, therefore, did not play an obvious role in connectivity between populations, but instead appeared to be important for population maintenance. While specific mechanisms for reproductive isolation, such as gamete recognition, between both T. burtoni species remain unknown, we suggest that such mechanisms likely exist due to the strong differentiation found between both species (FST = 0.191, P < 0.0001). Further, the observed high levels of both gene flow and asexual reproduction may also act to reduce the potential for hybridization by maintaining genetic diversity and increasing the chance of mating with a conspecific individual.