Bleaching in reef invertebrate larvae associated with Symbiodinium strains within clades A–F

Coral reefs are diverse ecosystems relying on the presence of dinoflagellates (genus Symbiodinium), that are found in symbiotic association with multiple phyla and performing the majority of primary production. However, coral reefs are currently threatened by climate change and the increase in seawater temperature, which causes the bleaching phenomenon. While bleaching has been well documented for adult host organisms, it is still poorly understood in larval stages. We offered Symbiodinium types within clades A–F to the larvae of Mussismilia hispida (scleractinian coral), Berghia stephanieae (nudibranch) and Tridacna crocea (giant clam) and manipulated the temperature to 26, 29 or 32 °C. Samples were taken at 0, 12, 24, 48 and 72 h post-temperature increase, chlorophyll-a (chl-a) was extracted and its content measured in a fluorometer. Symbiodinium type, temperature and time all influenced chl-a content. M. hispida larvae displayed a bleaching threshold at 29 °C; larvae containing Symbiodinium A–F all bleached at 32 °C, but with significantly lower bleaching in larvae associated with type A1. B. stephanieae digested the symbionts; while chl-a content decreased over time equally for all clades, it is not possible to determine if it is related to bleaching. T. crocea larvae at 29 °C bleached for all symbiont types, except for A1. At 32 °C, all types were bleached, but type A1 bleached significantly less. These findings show that type A1 seems to be more thermo-tolerant in larvae of the tested species. This may be related to the fact that strains within this clade are homologous to both M. hispida and T. crocea, as they are found within these adult host’ tissues. Therefore, symbiont type may have an important role in invertebrate larvae development and present relevant implications for recruitment.