Intruder identity alters the response of territorial damselfish protecting algal farms

Herbivorous damselfish that cultivate algal farms frequently exhibit aggressive territorial behaviour to defend their investment from intruders, particularly against other herbivorous fish and invertebrates. On a coastal coral reef in New Caledonia, the response of the damselfish Hemiglyphidodon plagiometopon to intruders of varying type and level of threat to their algal farm was tested. Responses to live and dead coral fragments, carnivorous whelks, shells, and rocks varied from no response to biting and extrusion from farms. Damselfish elicited the strongest defensive response to the herbivorous sea urchin Echinometra mathaei, rapidly attacking the urchin by biting and lifting it from farms, before extruding it up to 3.5 m away. H. plagiometopon responded in a similar manner to dead urchins (no threat of herbivory) as to live urchins, but typically did not extrude them as far. Ultimately, damselfish responded to intruders in a manner largely consistent with the level of threat posed to their algal farm, with the similar response between live and dead urchins suggesting such strong defensive behaviours may be combination of ‘hard-wired’ and learned behaviours in reaction to the level of realised threat. © 2019, Springer Nature B.V.