I mainly find Trichodina pediculus on Hydra viridis, because this is also the most common Hydra species in Simmelried. Only very occasionally have I found Trichodina pediculus on small water snails of unknown species. However, it is known that Trichodina pediculus also occurs on bryozoans, juvenile and adult fishes, and amphibian larvae.

Trichodina pediculus used to be classified as an ectoparasite because it was assumed that the ciliate could damage the fish skin with its adhesive organelle. However, the ciliate is now classified as an epibiont (Foissner, 1999). In fishes that are sick or stressed due to other causes, however, Trichodina pediculus can develop en masse, leading to skin damage and bacterial infection as a secondary reaction.

Hydra viridis can be colonized by several hundred Trichodina pediculus specimens without any negative effects on the host being observed. Trichodina pediculus moves very quickly across the host’s epidermis without losing its adhesion. Only when the Hydra viridis is slightly squeezed under the coverslip does Trichodina pediculus leave the host.

One might get the impression that Trichodina pediculus “grazes” on the host and is oriented toward the host with its mouth opening. However, this is not the case, because the adhesive organ is located on the dorsal side of the ciliate. This means that Trichodina pediculus glides over the host with its back. The mouth opening with the circular adoral zone is located on the side facing away from the host (s. figs. 3 and 4 a-b).

The composition of Trichodina pediculus’s diet is largely unknown. I myself was able to identify mainly bacteria and small algae in the food vacuoles of specimens that lived on Hydra viridis. I did not observe any cells being detached from the epidermis of Hydra. I therefore consider it likely that Trichodina pediculus lives off the excreted food residues of the host.