Living on a catfish: nested occupation of ectosymbiotic chironomids on host body

Abstract

Symbiotic species often have preferential attachment sites on their host body and thus some sites may only be occupied if the supposed best sites are already occupied. Alternatively, colonization may be similar in all sites, but removal may differ among them. Both mechanisms might generate a nested pattern. Moreover, the host size can alter the quality of the site for symbiotic occupation; for instance, by increasing the area and anchorage structures or reducing removal. We predicted that the spatial distribution of the ectosymbiotic chironomid Ichthyocladius lilianae Mendes, Andersen and Sæther, 2004 on their host, the armored catfish Pareiorhaphis hypselurus (Pereira and Reis, 2002), would present a nested pattern with regards to body-part preference and variation according to host size. We found that (i) suboptimal sites on the host body were usually only occupied by a symbiont when the optimal sites were already occupied and (ii) sites occupied by larvae on small host body surface were a subset of the sites occupied on large hosts. Our results indicate a nested distribution, yet the mechanisms generating this pattern are unclear. One possibility is that symbionts have preferential sites for attachment on the host body surface and compete for these sites. As an alternative, symbionts may present no preference and colonize equally all sites, but they remain attached to sites that offer better resources or protection from removal.