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Climate change induced heat stress has increased coral bleaching events worldwide. Differentially regulated immune genes are one of the primary responses to heat stress suggesting that immune activation is critical. However, the cellular immune mechanisms of coral bleaching is currently unknown, and it is still not known if the immune response documented during heat stress is a consequence of bleaching or is directly caused by the heat stress itself. To address this question, we have used two model system sea anemones (Order: Actiniaria): Exaiptasia diaphana and Nematostella vectensis . E. diaphana is an established sea anemone model for algal symbiont interaction, while N. vectensis is an established sea anemone model that lacks the algal symbiont. Here, 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. We also found that the cellular immune activity was not affected by the presence of the Symbiodiniaceae. Our results suggest 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. 

Abstract Venomous animals can deploy toxins for both predation and defense. These dual functions of toxins might be expected to promote the evolution of new venoms and alteration of their composition. Cnidarians are the most ancient venomous animals but our present understanding of their venom diversity is compromised by poor taxon sampling. New proteomic data were therefore generated to characterize toxins in venoms of a staurozoan, a hydrozoan, and an anthozoan. We then used a novel clustering approach to compare venom diversity in cnidarians to other venomous animals. Comparison of the presence or absence of 32 toxin protein families indicated venom composition did not vary widely among the 11 cnidarian species studied. Unsupervised clustering of toxin peptide sequences suggested that toxin composition of cnidarian venoms is just as complex as that in many venomous bilaterians, including marine snakes. The adaptive significance of maintaining a complex and relatively invariant venom remains unclear. Future study of cnidarian venom diversity, venom variation with nematocyst types and in different body regions are required to better understand venom evolution.