The impact of a parasitic gill copepod on the demography of a reef fish host

I examined the demographic impacts of the parasitic copepod Pharodes tortugensis that infects the gill cavities of small reef-associated fish. Laboratory and observational field studies were used to address questions concerning the distribution of the parasite in the British Virgin Islands (BVI), the physical damage and differences in body condition, growth and mortality caused by the parasite. Transmission of the parasite in a free living host population was examined using spatial analysis. P. tortugensis was found infecting three congeneric gobies in the BVI: bridled gobies, Coryphopterus glaucofraenum, colon gobies, C. dicrus, and palid gobies, C. eidolon. During 2001-2004, the parasite prevalence was low at many sites in the BVI. Parasitized gobies were common around Guana Island, but prevalence never reached over 30%. The copepod was aggregated in the host population with few individuals with high infection intensity and most with low infection intensity. Parasites caused extensive damage to the branchial chamber and gill filaments and infected gobies had higher respiration and lower feeding rates than uninfected. Parasitism dramatically affected body condition, growth and survival in bridled gobies: livers were larger in parasitized individuals while gonad size was smaller, somatic growth was reduced by 66%, and the instantaneous rate of mortality increased by a factor of 1.8. Tank experiments verified parasite transmission via short-lived planktonic nauplii. When colon and bridled gobies were pooled as one host population, spatial analysis of the distribution of parasitized and unparasitized individuals showed patterns consistent with contagious transmission, and weak evidence for density-dependent transmission was found in a time-lagged regression of goby density and the parasite prevalence. The spread of the parasite through the host population was slow however, because the rate of reinfection was substantially higher than the rate at which new infections were established. The parasite uses multiple goby species within the reef population and there is evidence of unequal susceptibility to infection among individual hosts but not between host species. The parasitic copepod does affect host demography through increased mortality and decreased fecundity, but is unlikely to contribute strongly to host population regulation because strong density-dependent transmission was not found.