The colonization of macroalgal rafts by the genusIdotea(sub-phylum Crustacea; order Isopoda): an active or passive process?

Abstract

The association of invertebrate communities with macroalgae rafts has received much attention over recent decades, yet significant gaps in our knowledge remain with respect to the colonization process. Using laboratory-based experiments andin situfield trials in Strangford Lough, Northern Ireland, this study investigated whether members of the known rafting genusIdotea(sub-phylum Crustacea; order Isopoda) could effectively colonize rafts after shore seaweed detachment, or if their presence merely reflected a passive marooning process. Test tank arenas were used to identify traits that may influence the rafting potential of the dominant shore speciesIdotea granulosaand the well known rafterIdotea baltica. When released mid-water,I. granulosainitially ascended and associated with floating seaweed whereasI. balticatended to descend with no clear habitat association. These findings conflict with the differential distribution of theseIdoteaspecies among rafts and shore algae, thus highlighting the complex nature of the potential of organisms to raft. In the field we considered the relative ability of differentIdoteaspecies to colonize tethered rafts composed ofAscophyllum nodosumandFucus vesiculosus, cleaned of all vagile organisms and deployed at locations adjacent to established intertidalIdoteaspecies populations. At the end of the experiment (after 44 days) rafts were inhabited by known rafting and shoreline species, confirming that colonization can occur after algal detachment. Previously considered shoreline species on occasion outnumbered well known rafters suggesting that a wide range ofIdoteaspecies can readily avail of macroalgal rafts as a potential dispersal mechanism or alternative habitat.