Mar Biol (2007) 151:751–758 DOI 10.1007/s00227-006-0521-z 123 RESEARCH ARTICLE Diet of Balaenophilus spp. (Copepoda: Harpacticoida): feeding on keratin at sea? F. J. Badillo · L. Puig · F. E. Montero · J. A. Raga · F. J. Aznar Received: 16 June 2006 / Accepted: 11 October 2006 / Published online: 11 November 2006  Springer-Verlag 2006 Abstract Evidence about the diet and the type of association, commensalistic or parasitic, of species of Balaenophilus is not conclusive. In this study, we addressed these questions based on patterns of micro- habitat selection, SEM and immunohistochemistry analyses of gut contents of the two species in the genus, B. unisetus and B. umigamecolus. We also made histo- pathological descriptions of the sites where the latter species occurred. Heavy infections of B. unisetus were found on the baleen plates of 18 out of 20 Wn whales, Balaenoptera physalus, captured oV the NW coast of Spain. Gut contents were packed as food pellets that were composed mostly of baleen tissue, as indicated by both SEM and immunohistochemistry. Individuals of B. umigamecolus appeared in 43 out of 52 loggerhead sea turtles from the Western Mediterranean. In six tur- tles analysed in detail, copepods occurred mostly in the hinge region between limb scales and on the skin of the cloacal region; rarely under whitish skin lesions on the neck and hindlimbs. No food pellets could be found in 20 individuals examined, but two of them had pieces of tissue in the mouth that resembled turtle skin. The his- topathologic analysis was also compatible with mild host reaction to the erosion of epidermis produced by B. umigamecolus. The occurrence of Balaenophilus spp. as ectoparasites of phylogenetically unrelated hosts which, however, provide similar keratin-rich hab- itats, might suggest that the exploitation of this food resource (an ecological novelty among crustaceans) is in the origin of these associations. This hypothesis is further supported by the record of another putative species of Balaenophilus on the skin of sirenians. Introduction Harpacticoids are considered primarily as marine epi- benthic copepods, but many species have successfully exploited a wide variety of other marine, brackish and freshwater habitats (Huys and Boxshall 1991). Harpac- ticoids have also been able to associate with host spe- cies from as many as nine metazoan phyla (Huys and Boxshall 1991; Boxshall and Halsey 2004). From the viewpoint of the evolution of life history strategies, an interesting question concerns the type of the relation- ship, namely, commensalistic or parasitic, that harpac- ticoids have established with their hosts (Ho 2001). Parasitism has sometimes been inferred from putative adaptations of the feeding apparatus (e.g. Kim 1991) or pathological eVects (e.g. López-González et al. 2000). In many cases, however, the distinction between com- mensalism and parasitism is far more diYcult to estab- lish, and this has lead to a varied nomenclature, including terms such as ‘semiparasite’, or simply ‘asso- ciate’ (Ho 2001). The basic problem is that harpactic- oids are versatile feeders on many food types, including microalgae (Carman and Thistle 1985), bac- teria (Rieper 1978, 1982), and metazoan tissue (López Communicated by S.A. Poulet, RoscoV. F. J. Badillo · J. A. Raga · F. J. Aznar (&) Marine Zoology Unit, Cavanilles Institute of Biodiversity and Evolutionary Biology, University of Valencia, P.O. Box 22085, Valencia 46071, Spain e-mail: Francisco.aznar@uv.es L. Puig · F. E. Montero Department of Animal Biology, Plant Biology and Ecology, EdiWci V, Autonomous University of Barcelona, Bellaterra, Barcelona 08193, Spain