Heat stress is increasing hexacorallian immune cell function

In vertebrates, heat induced immune activation is well documented. Yet, the effect of heat induction on the immune activation is less clear in invertebrates. Rising sea water temperatures have increased coral bleaching events worldwide. Previous gene expression research on coral heat stress induced bleaching found that different immune genes were differentially regulated and suggested immune activation. The cellular mechanisms of immunity involved in coral bleaching is currently unknown, and it is still not understood if immune response is a consequence of bleaching or is directly affected by the heat stress itself. To address this question, we have used two species of hexacorallian sea anemones as model systems: Nematostella vectensis- lacking symbiotic algae, Symbiodiniaceae, and Exaiptasia diaphana, which can be reared with and without Symbiodiniaceae. We examined the effect of increased temperature on phagocytic activity, as an indication of immune function. Our data shows that immune cell activity increases during heat stress, while small molecule pinocytosis remains unaffected. We observed an increase in cellular production of reactive oxygen species with increasing temperatures, a marker of heat stress induced bleaching in corals. We also found that the cellular immune activity was not affected by the presence of the Symbiodiniaceae. This suggests that the immune activity observed in heat-stress induced bleaching in corals is a fundamental and basic response independent of the bleaching effect. These results establish a foundation for improving our understanding of hexacorallian immune cell biology, and its potential role in coral bleaching.