Epibionts of Asellus aquaticus (L.) (Crustacea, Isopoda): an SEM study

1. This study presents a qualitative and quantitative survey of epibionts infesting two populations of the freshwater isopod Asellus aquaticus (L.). Using scanning electron microscopy, the prevalence, mean intensity, abundance, spatial preferences and distribution of organisms on various external surfaces were ascertained. 2. A diverse community, consisting primarily of protozoa and rotifers, utilised A. aquaticus as a substrate organism. Every individual A, aquaticus supported some form of epizoic life. Peritrich species comprised 89.2% of all epibionts. Carchesium polypinum, Pseudocarchesium aselli, Pseudocarchesium asellicola, Pseudocarchesium simulans and Opercularia hebes dominated the epifauna. 3. Highest epibiont burdens occurred on the mouthparts, the first few ventral segments and the gills and largely comprised the above peritrich species. Most epibionts exhibited some degree of site preference on the host. A distinct gill epifauna existed. Apart from Acineta tuberosa and Vorticella rotunda, which had equal prevalences on dorsal and ventral surfaces in one culture, most species largely avoided the dorsal surface. 4. Epibionts exhibited varying degrees of adaptation to life on a living substrate. They ranged from facultative species such as A. tuberosa, which are capable of colonising abiotic substrata, to Gymnodinoides aselli whose life cycle relies on interaction with A. aquaticus. Highly specialised species tended to exhibit the most pronounced and consistent site restriction. 5. Asellus aquaticus offers a highly tractable system for further study of substrate species-epibiont relationships.