New insights on the life style of the epibiotic amphipod crustacean Tritaeta gibbosa (Crustacea, Amphipoda, Dexaminidae)

Tritaeta gibbosa is an epibiotic amphipod that lives in self-constructed invaginations in the tissue of various hosts as diverse as sponges, soft corals, ascidians and echinoderms. Here, some aspects of the relationship of T. gibbosa to different hosts are described. The main focus, however, is the colonisation of the holothurian Ocnus planci. The association between this holothurian species and T. gibbosa was studied by means of histology and scanning electron microscopy. The investigations revealed that T. gibbosa does not damage the host tissue. Only some stretching, thinning, thickening, and displacement of the holothurian cuticle, epidermis and dermis was observed. This suggests that the invagination is formed by pressure caused by a specific behaviour of T. gibbosa. The amphipod takes advantage of the properties of the mutable connective tissue characteristic for echinoderms. The majority of invaginations are situated at the holothurian's anterior end. Furthermore, most invaginations are perpendicularly oriented to the long axis of the holothurian body. This seems to improve the exploitation of food particles coming from the tentacles of O. planci. The amphipod specimens are subadult to adult and there is a strong bias toward females as is generally the case in benthic T. gibbosa populations. Although some new information about the ecology of T. gibbosa is added by this study, some general questions such as the tissue reactions of the various hosts, the ontogeny and juvenile behaviour, and the type of association to the host still need investigation.