Two commensals on a host: habitat partitioning by a ciliated protozoan and a chironomid on the burrowing mayfly, Ephemera danica

SUMMARY 1. Two taxonomically unrelated species, the ciliated protozoan Scyphidia sp. and the chironomid Epoicocladius flavens (Malloch), share a commensal mode of life on nymphs of the burrowing mayfly Ephemera danica Müller in a small river in eastern England. 2. Both commensals were present all through the year, with an average level of infestation of 25.1% and 54.7% for Scyphidia sp. and E. flavens, respectively, of the total E. danica population. 3. Scyphidia sp. occurred more frequently on younger, smaller nymphs of E. danica and was totally absent from hosts of more than 10 mm in length. In contrast, E. flavens preferred hosts of larger body size, usually more than 10 mm. Change in dominance of infestation between the two commensals occurred in the host size range of 6–8 mm and this pattern of habitat partitioning on the basis of host size was consistent throughout the study period. 4. Amongst E. danica individuals colonized by E. flavens, larger hosts supported heavier commensal load. A similar trend, though weak, was also observed among hosts harbouring Schyphidia sp. 5. Both commensals are considered to benefit from the association with E. danica in that they gain enhanced mobility and security in otherwise inhospitable habitats. Although plausible on an evolutionary time scale, competition alone cannot constitute a proximate cause for the clear partitioning of habitat observed in the two commensal species.