Marine protozoan epibionts on the copepod Lepeophtheirus salmonis, parasite of the Atlantic salmon GREGORIO FERNANDEZ-LEBORANS 1 , MARK FREEMAN 2 , REGINA GABILONDO 1 , & CHRISTINA SOMMERVILLE 2 1 Departamento de Zoologı ´a, Universidad Complutense, Madrid, Spain, and 2 Institute of Aquaculture, University of Stirling, Stirling, UK (Accepted 28 May 2004) Abstract Two species of ciliate suctorian protozoa belonging to the genus Ephelota, E. gemmipara and E. gigantea, were found as epibionts on the marine copepod Lepeophtheirus salmonis (salmon louse), an ectoparasite of marine salmonid fish, including the Atlantic salmon, Salmo salar. Epibionts were distributed over the cephalothoracic shield, genital segment, abdomen and caudal branches of the copepods. Individuals from both species possessed two types of tentacles: long, prehensile, pointed tentacles and short, adhesive, capitate feeding tentacles. Both species contained a highly ramified and lobated macronucleus. E. gemmipara showed a rounded cellular body attached to a stalk possessing longitudinal and transversal striations but lacking a suprastylar extension. E. gigantea had an umbrella-shaped cellular body significantly larger than in E. gemmipara, the stalk showing only longitudinal striations but possessing a conspicuous suprastylar extension. This is the first time that the presence of ciliate epibionts has been recorded at species level on the copepod Lepeophtheirus salmonis. Statistical data about the distribution of both species on the surface of the copepod are detailed and a new geographical distribution for E. gigantea is proposed. Keywords: Ephelota, epibiosis, Lepeophtheirus salmonis, parasite, salmon Introduction Epibiosis is a facultative association of two organisms: the epibiont and the basibiont (Wahl 1989). The term epibiont includes organisms that, during the sessile phase of their life cycle, are attached to the surface of a living substratum, while the basibiont lodges and constitutes a support for the epibiont (Threlkeld et al. 1993). Both concepts describe ecological functions (Wahl 1989). Epibiosis between ciliated protozoa and crustacea is widely reported and occurs across most crustacean orders. However, ciliated protozoa from the peritrichs, suctoria and chonotrichs are more frequently reported as epibionts of crustacea (Morado and Small Present address: Mark Freeman, Laboratory of Fish Diseases, Department of Aquatic Biosciences, The University of Tokyo, Tokyo, Japan. Correspondence: Gregorio Fernandez-Leborans, Departamento de Zoologı ´a, Facultad de Biologı ´a, Pnta 9, Universidad Complutense (UCM), 28040 Madrid, Spain. E-mail: greg@bio.ucm.es Journal of Natural History, 2005; 39(8): 587–596 ISSN 0022-2933 print/ISSN 1464-5262 online # 2005 Taylor & Francis Ltd DOI: 10.1080/00222930400001525