The predominantly facultative nature of epibiosis: experimental and observational evidence

Abstract

Epibiosis is d spatially close association between 2 or more organisms belonging to the same or different species. Through direct and indirect interactions, this association has major effects on the species involved and on community dynamics. When the effects are predominantly beneficial for epibiont and basibiont, coevolution can be expected to lead to associational specificity. Circumstantial evidence, however, suggests that many epibionts are non-specific substratum-generalists. In this article, we investigate the commonness of specificity in epibiotic associations. In a first approach, we investigated the in situ recruitment preferences of potential epibionts when choosing between artificial and living substrata. After exposure for 3 wk in early summer, an early successional community had established, comprising cyanobacteria, diatoms, sessile colonial ciliates and red algae. All species recruited on almost all substrata available. However, artificial substrata were usually preferred over living surfaces. Consequently, the species studied are classified as facultative epibionts. An analysis of a list of over 2000 epibiotic associations corroborated these results: the majority of described 'epibionts' are not basibiont-specific and generally occur on non-living substrata as well. Also, basibiont species usually bear more than 1 epibiont species. Relative to each other, epibionts and basibionts are characterised by a typical set of life history traits. We conclude that specific and obligate epibionts are rare. Their scarcity is discussed in view of multilevel antifouling defences and presumptive evolutionary transitions from epibiosis towards endoparasitism or endosymbiosis.