Comparison of the ectosymbionts and parasites of an introduced crab,Charybdis japonica,with sympatric and allopatric populations of a native New Zealand crab,Ovalipes catharus (Brachyura: Portunidae)

The success of biological invaders is often attributed to escape from specialist enemies in their natural range, such as predators and parasites. For enemy escape to have direct consequences in competitive interactions, invaders need to be less vulnerable to enemies than native competitors in the region they invade, but first the presence of these enemies must be established. We investigated the macroparasite and ectosymbiont fauna of the recently introduced portunid crab Charybdis japonica, and compared it with sympatric and allopatric populations of the native New Zealand portunid, Ovalipes catharus. A total of 468 crabs (350 O. catharus and 118 C. japonica) were collected from six harbours throughout New Zealand (Whangarei, Waitemata, Nelson, Lyttelton, Dunedin, and Bluff) and the identity, incidence and prevalence of ectosymbionts and parasites were compared among the different populations. Charybdis japonica and O. catharus harboured different ectosymbionts. Serpulid polychaete tubes occurred on the exoskeleton of 85.4% of C. japonica examined, but were absent from O. catharus. The bryozoan, Triticella capsularis occurred on 97.4% of O. catharus but was not found on C. japonica. Few endoparasites were present in either species. An unidentified juvenile ascaridoid nematode occurred in the hindgut of 5.9% of C. japonica, but was not found in sympatric populations of O. catharus. A second, unidentified species of ascaridoid nematode occurred in 7.1% of O. catharus from Nelson, but was not present in specimens from the five other harbours sampled. Melanised lesions were observed in the muscle tissue of almost half (46.6%) of the C. japonica examined. Histological examination showed these to be of two types: (1) spherical bodies resembling melanised trematode metacercariae; and (2) lesions consistent with wound repair. Lesions were not observed in O. catharus. Although the identity of parasites and epibionts carried by each species differed, both C. japonica and O. catharus had relatively low parasite species richness. We could not test whether the introduced portunid, C. japonica, is any less vulnerable to parasite enemies than the New Zealand portunid, O. catharus.