Dietary niche partitioning within detritivorous copepods (Calanoida; Scolecitrichidae) based on the ultrastructure of photosensory organs and enteric bacterial flora

Abstract Scolecitrichidae (Copepoda: Calanoida) is essentially a detritivorous taxon feeding on carcasses, fecal pellets and secretion matters in the process of sinking. The ultrastructure of photosensory organs of two scolecitrichid species (Lophothrix frontalis and Scottocalanus securifrons) was observed and detrital foods probably detected by these organs were presumed based on DNA metabarcoding of the enteric bacterial flora. The frontal eyes of L. frontalis comprise only one ventral eye without lenses or pigment granules. In contrast, S. securifrons has one ventral and two lateral eyes, pigmented red because of pigment granules and two cuticular lenses. In addition, the two Gicklhorn’s organs are entirely occupied by enlarged rhabdoms. For DNA barcoding of the enteric bacterial flora, six scolecitrichid species (photosensory organs were detected from three of which including L. frontalis and S. securifrons) were investigated. The enteric bacterial flora of scolecitrichids whose photosensory organs were detected has significantly lower β-diversity (Welch’s t test, P < 0.001) and higher frequency of Vibrionaceae (Welch’s t test, P < 0.01) than those of other scolecitrichids.