Say Hello to My Little Friends! Investigating the Influence of Exaiptasia diaphana Symbiotic State on the Feeding Ecology of the Nudibranch, Berghia stephanieae

Abstract

Mutualistic cnidarians are crucial to the success of biodiverse and economically valuable marine ecosystems, with coral reef ecosystems in particular holding immense ecological and economic importance. However, natural stressors, such as predators and anthropogenic stressors like climate change, pose serious risks to these ecosystems. The interactions between these stressors must be understood to provide a holistic view of how cnidarian ecosystems are being impacted. This study uses a model mutualistic cnidarian, Exaiptasia diaphana (Aiptasia), and a model specialist invertebrate predator of cnidarians, Berghia stephanieae (Berghia), to investigate how bleaching events impact the quantity and functionality of nematocysts in cnidarians, the feeding behaviors of invertebrate predators of cnidarians, and the stable isotope composition of invertebrate predators of cnidarians. This study utilized unbleached and bleached E. diaphana to quantify unfired and fired nematocyst content in both E. diaphana variants, in feeding trials to examine B. stephanieae feeding behavior in the presence of both E. diaphana variants, and in stable isotope analysis to assess how different E. diaphana variants influence the tissue composition of B. stephanieae. Bleaching was found to decrease nematocyst quantity but did not impact nematocyst functionality. Offering bleached E. diaphana as a prey item did not influence invertebrate cnidarian feeding preferences nor did consuming bleached E. diaphana immediately alter B. stephanieae’s stable isotope composition. However, mixing models indicate that Symbiodinium is a diet item that is assimilated within B. stephanieae’s tissues, but the proportion of tissue that Symbiodinium accounts for is unknown. This indicates that Symbiodinium found within E. diaphana are not an attractant, but are an additional source of nutrients, for B. stephanieae, which could have further reaching effects unobserved in this study. While mutualistic cnidarians are adversely impacted by bleaching events, this study provides evidence that invertebrate predators of cnidarians may possess limited resilience to bleaching events. Continued investigation into the effects of bleaching at multiple trophic levels is necessary to determine the comprehensive impacts of such events and to coordinate future conservation actions effectively.