Mob rulers and part-time cleaners: two reef fish associations at the isolated Ascension Island

Abstract

Isolated oceanic islands may give rise not only to new and endemic species, but also to unique behaviours and species interactions. Multi-species fish interactions, such as cleaning, following, mob-feeding and others are understudied in these ecosystems. Here we present qualitative and quantitative observations on cleaning and mob-feeding reef fish associations at the isolated Ascension Island, South Atlantic Ocean. Cleaning interactions were dominated by juveniles of the facultative fish cleanersBodianus insularisandPomacanthus paru, with lesser contributions ofChaetodon sanctaehelenae, Thalassoma ascensionisand the cleaner shrimpLysmata grabhami. Two types of feeding mobs were consistently identified: less mobile mobs led by the surgeonfishAcanthurus bahianusandA. coeruleusand the more mobile mobs led by the African sergeantAbudefduf hoefleri. This is the first record ofA. hoeflerifrom outside of the Eastern Atlantic and also the first report of this species displaying mob-feeding behaviour. The principal follower of both mob types was the extremely abundantMelichthys niger, but the main aggressor differed:Stegastes lubbocki, a highly territorial herbivore, was the main aggressor ofAcanthurusmobs; andChromis multilineataa territorial fish while engaged in egg parental care, was the principal aggressor towardsAbudefdufmobs. Our study enhances the scarce information on reef fish feeding associations at the isolated Ascension Island and at oceanic islands in the Atlantic in general.