Bathymetric distribution of parasitic copepods: strategies for adaptation to the deep-sea environment

Abstract The bathymetric distribution and species richness of marine parasites are generally influenced by host-related and environmental factors. While parasite traits such as attachment modes and reproduction strategies are believed to play important roles in shaping these patterns, insights into the influence of these traits remain limited. To enhance our understanding regarding the bathymetric distribution of deep-sea parasites and the biological traits associated with successful colonization of deep-sea habitats, we compiled occurrence data on parasitic copepods parasitizing deep-sea fishes, based on both current and previous records. We found that species richness declined with increasing depth, likely reflecting host distribution patterns. The recorded maximum depths of copepods in the families Chondracanthidae, Lernaeopodidae, Pennellidae and Sphyriidae exceeded 2000 m. These families are characterized by the following traits: suitable attachment sites like gills for efficient nutrient intake; firm attachment modes with limited mobility that enable efficient energy use; reproductive strategies such as the presence of dwarf males or the use of intermediate hosts; and low host specificity. Among all copepods parasitizing fish, a chondracanthid Chondracanthodes deflexus Wilson, 1932 had the deepest occurrence record and was the only species found in the abyssal region (>4000 m). This species exhibited a relatively high intensity (9.6), possibly because of the challenges of locating hosts in an environment with extremely low host density. These results indicate that the colonization of deeper waters by parasitic copepods may have proceeded via a stepwise process involving both the retention and acquisition of traits advantageous for survival under increasingly extreme conditions.