Parasitic helminths of jumbo squid Dosidicus gigas (Cephalopoda: Ommastrephidae) in open waters of the central east Pacific

Abstract

During 1981–1989, 849 jumbo squid, Dosidicus gigas, from four open ocean regions of the east Pacific (from 11°N to 22°S) were examined for parasitic helminths. The samples were collected from the Peruvian (9–21°S and 82–87°W), east equatorial (2°N–6°S and 84–87°W), west equatorial (1°N–3°S and 96–100°W) and Nicaraguan (9–11°N and 88–91°W) regions. Nine species of parasitic helminths were found, with a total prevalence of infection of 75.5%. Trematoda: metacercaria of Didymozoidae indet (prevalence: 13%, intensity: 1–35, abundance: 0.51); Cestoda: Pelichnibothrium speciosum (75.2%, 1–63 000, 664.5); Phyllobothrium sp. (1.2%, 1–2, 0.02); Tentacularia coryphaenae (6.6%, 1–5, 0.16); Nematoda: Anisakis simplex (9.2%, 1–16, 0.23); Anisakis physeteris (24.2%, 1–26, 1.22); Porrocaecum sp. (29.4%, 1–17, 0.30); Contracaecum sp. (0.5%, 1–41, 0.22); Spinitectus sp. (0.4%, 1–3, 0.01). All these parasites occurred in the larval stage. The composition of helminths and quantitative infection indexes were similar for males and females of similar mantle length (ML). Four size groups of Dosidicus were recognised. The helminths of the first size group, 30–89 mm ML comprised metacercaria of Didymozoidae and larvae of Porrocaecum sp. In the second size group, 90–139 mm ML, the prevalence of didymozoids increased to 47.5% and the majority of the parasite fauna (four species) occurred at this stage. All nine species were present in the third size group, 140–359 mm ML, but with a sharp decrease in the prevalence of didymozoids (5.8%). In the fourth size group, 360–431 mm ML, didymozoids were absent while the prevalence and intensity values for the other helminth species were maximal. The helminth fauna of similar sized squid (190–300 mm ML) from different regions was similar and the levels of infection corresponded closely. However, a comparison of ontogenetic infection dynamics between the Peruvian and east equatorial waters showed that the prevalence and intensity data for the main and secondary helminth species differed. This may support the hypothesis of isolated populations in these two regions. D. gigas is a paratenic (transport) host for the helminth species studied, with scombroid and xiphoid fishes, sharks and marine mammals as the definitive hosts.