Species-specific gut bacterial communities in mesopelagic calanoid copepods: relationships with marine snow feeding and gut morphology

Marine planktonic copepods are highly diverse in the mesopelagic zone, where marine snow is among the major food sources. Although marine snow composition varies with origin, whether this influences copepod feeding ecology remains poorly understood. We investigated the gut microbiota of mesopelagic copepods, Pleuromamma xiphias, Spinocalanus magnus, and Scottocalanus securifrons via high-throughput 16S rRNA gene amplicon sequencing and compared them relative to their gut morphology and feeding habits. Among gut bacteria, Synechococcus and Planctomycetes, both aggregate-associated bacteria, were the most abundant in all samples, suggesting copepod feeding on marine snow. Photobacterium, which has chitinases and includes luminous species, was significantly more abundant in Sc. securifrons, which has a red-colored gut, than in Sp. magnus and P. xiphias, both of which have semi-transparent guts. Phylogenetic analysis of the LuxA gene revealed that Photobacterium sequences from the Sc. securifrons samples formed a distinct clade within the genus but did not correspond to known species, suggesting a possible relationship between bacterial bioluminescence and the red coloration of the gut. Gut microbiota composition differed among the three species at phylum and genus levels, with Sp. magnus showing the most specialized profiles reflecting its phytodetritus-based diet and P. xiphias exhibiting more diverse composition consistent with its generalist feeding. These results indicate that gut microbiota differences are associated with feeding habits and gut morphology, potentially reflecting types of marine snow ingested. Future studies should compare gut microbiota with bacterial communities of individual marine snow particles to clarify how specific bacterial groups relate to copepod feeding habits.