Attrition or adaptation? Bleaching stress drives changes in sea anemone cnidae and venom

Abstract Bleaching stress has long been known to have deleterious impacts on cnidarian health and fitness. An area that has received little attention is how bleaching may impact their venom ecology, specifically, cnidae and venom. Using the tropical sea anemone, Isactinia citrina , this study found that significant Symbiodiniaceae loss impacted the relative abundance and size of cnidae, and the profile and composition of venom. The cnidoms from both the tentacles and the body column within bleached sea anemones were shown to have a significantly greater proportion of large basitrichs than their photosymbiotic counterparts. While the volume of these basitrichs was reduced in the tentacles of bleached sea anemones, basitrichs in the body column were significantly larger. Mass spectrometry revealed distinct differences in the profiles of venom collected from bleached and photosymbiotic sea anemones. After comparing the elution intensity of prominent venom components, it was shown that bleaching influenced venom composition. The drivers behind the changes observed in this study remain unclear, as some of the responses appeared counterintuitive if interpreted solely as consequences of energetic depletion. For example, the observed shift towards larger cnidae seems unlikely to arise from an energy deficit and may instead reflect an attempt to enhance defensive capabilities. Together, these findings suggest that physiological decline may not be the primary driver of observed changes. Rather, changes to venom ecology may represent purposeful adaptations intended to enhance survivorship during bleaching stress, therefore warranting further investigation.