Friend, food, or foe: sea anemones discharge fewer nematocysts at familiar anemonefish after delayed mucus adaptation

For decades, it has been hypothesized that anemonefishes are able to live within the stinging tentacles of host sea anemone species because the chemical composition of their mucus layer inhibits or lacks the trigger for firing host nematocysts. However, there is very little molecular evidence for this, beyond suggestions that glycans in the mucus could be key. In this study we assessed these hypotheses by testing Bubble-tip anemone ( Entacmaea quadricolor ) nematocysts in response to three different mucus sources, before and after anemonefish association. We also profiled the corresponding mucus lipid and glycan composition of anemonefish. Host sea anemones significantly reduced nematocyst firing at acclimated anemonefish mucus compared to mucus from unacclimated individuals. Changes in anemonefish mucus glycan composition became distinguishable three weeks after introduction relative to an anemonefish that was not living in association of a host sea anemone. The glycan composition reverted back to a pre-acclimated composition when profiled 24 hours after anemonefish removal from a host sea anemone. Triggering fewer nematocysts through glycan profile alterations may be an important adaptation that has enabled anemonefish to live long-term in a sea anemone host. However, the delay in mucus response indicates it is not the initial mechanism used by anemonefish to enter a host sea anemone without being stung. ### Competing Interest Statement The authors have declared no competing interest.