Specimens of the Arctic sympagic amphipod Gammarus wilkitzkii, which were collected in the ice-covered areas near Spitsbergen, Norway, were infested with protozoan epibionts in densities of 499 to 3346 individuals per amphipod. The epibionts belong to the five ciliate genera: Ephelota, Cryptacineta, Acineta and Podophrya (suctorian ciliates), and Epistylis (peritrich ciliate). In this study we present the first observations of epibionts on ice-associated crustaceans and provide a detailed description of morphological and taxonomical aspects of the different ciliate genera. Cryptacineta has not been found earlier in the marine environment. This ciliate showed the highest density values (215–2571 individuals per amphipod), followed by Ephelota (2–1302 ind./amphipod). The number of individuals of Acineta, Podophrya, and Epistylis did not surpass 240 ind./amphipod. Epibionts colonized all appendages and the entire body surface, but were most numerous on the anterior body part of G. wilkitzkii. The body length of the gammarid and the number of epibionts of Ephelota, Podophrya, and Epistylis were positively correlated. The highest density of epibionts was found on the anterior body parts with the antennae bearing up to 613 individuals. In contrast, the posterior body showed only little burden. The number of epibionts along the caput-telson axis of the amphipod body shows a decrease towards the posterior end of the amphipod. The highest degree of infestation was found on females, followed by juveniles and eventually, males. When grouping the 37 anatomical units (including left and right appendages) to 8 body “regions,” the pereiopods, as a whole, showed the highest density (39.25%), followed by the gnathopods (22.29%), and antennulae and antennae. Basibiont got infested with the sessile ciliates in the benthic and pelagic environment during the ice-free season and carried them along back to the sympagic ecosystem when colonizing the newly formed ice. Epibionts are therefore considered as indicators for bentho-sympagic coupling processes.
