Pseudanthessius tortuosus Stock, 1967 (Copepoda: Cyclopoida) from the amphinomid polychaete Hermodice carunculata (Pallas) in the western Caribbean.

The copepod order Cyclopoida contains mainly forms that have been found as symbionts of different vertebrate and invertebrate hosts. One of these groups is the family Pseudanthessiidae, found mainly on echinoderms but very rarely on polychaetes. In this contribution, we report and confirm the occurrence of the pseudanthessiid copepod Pseudanthessius tortuosus Stock, 1967 as a symbiont of the benthic amphinomid polychaete Hermodice carunculata (Pallas) in the western Caribbean. These specimens showed subtle morphological but important size differences with respect to the specimens from the type-locality in the eastern Caribbean. Data are provided on the sex-ratio, the distribution of the copepods on the polychaete body and the finding of pairs in amplexus. The position of the copepods on the body, mainly on the dorsal branchiae, rather than on the lateral ones, allows them to avoid the friction related to the displacement mechanism of the polychaete, involving the lateral branchiae. This is the first record of the species and the symbiotic association in the western Caribbean and in Mexican waters. It is also recorded herein for the first time in the US Virgin Islands, near the type-locality. Hitherto, P. tortuosus has not been recorded since its original description forty years ago. This finding supports the hypothesis that this copepod is associated to H. carunculata throughout the entire Caribbean Basin, but its relation with other amphinomids remains unconfirmed.