Incidence and host-parasite relationship ofLeidya Bimini(Crustacea, Isopoda, Bopyridae) in the Brachyuran Crab Pachygrapsus transversus from Bermuda

Abstract

Abstract Pachygrapsus transversus (N = 1814) from intertidal locations in eastern Bermuda were examined from 1973–78 for the branchially infesting bopyrid isopod Leidya bimini. In two main collecting areas, Whalebone Bay(N = 1280) and Ferry Reach (N = 489), the incidence of infestations with all life history stages was 9.0 and 29.5%, respectively. This difference is explained by the larger numbers of older crabs in the Ferry Reach samples. The mean width of crabs infested with mature female isopods was similar for both locations (14.4 versus 16.0 mm, respectively). Five-hundred and eleven isopods were recovered from 1814crabs: 106 (20.7 %) cryptoniscids, 45 (8.8 %) sexually undifferentiatedjuveniles, 152(29.7%) males, 112(21.9%) immature females, and 96 (18.8%) mature females. Limited data suggest that the main period for cryptoniscid invasion of crabs is from April through July. None of the 149 ovigerous crabs, among 857 females from the two main collecting areas, harbored mature isopods, indicating castration. It was further shown that a preponderance of nonovigerous infested crabs (= 10mm wide) had little or no gonad development as compared to uninfested animals. Morphometric data on the life history stages of Leidya are compiled and some female developmental stages are illustrated. Mature isopods produce at least two broods of embryos, with as many as 21 000 per brood in the larger females. While female isopod size may be controlled by host size, evidence indicates that maturation occurs independently, i.e. the parasites mature at a smaller size in smaller hosts. Mature as well as some ofthe large immature female parasites produce a relatively subtle lateral swelling of the crab§ carapace. As two mature female isopods have never been found in a single crab, this swelling was always unilateral. The height of the body on the infested side is also increased. Left and right branchial chambers are equally susceptible to invasion and development ofthe isopod. Recognizable gill damage and sometimes damage to maxillary epipodites occurred in 52.5% of infested crabs, particularly in the larger ones harboring the larger parasites (i.e. crabs= 15mm wide). Forty three Pachygrapsus gracilis sympatric with P. transversus at Ferry Reach were devoid of bopyrids.