Second record of Eukrohnia kitoui (Chaetognatha) with notes on morphological differences from the closely related E. calliops

Abstract

Nine specimens of the planktonic chaetognath Eukrohnia kitoui, which was originally described from Tokyo Bay 32 years ago, were collected from an underwater canyon in Tosa Bay, Japan. This is the second record of the species. We compared E. kitoui and the closely allied E. calliops based on individual data of the two species reported in the previous and the present studies. Since allometric growth was observed in morphometric characters, specimens smaller than 10 mm in body length were omitted in this comparison. The results revealed that the ratios of tail segment and eye lengths to body length and the numbers of hooks and posterior teeth on the head, which were described as distinguishable features between the species in the previous study, were significantly different between the species. However, the values of these characters overlapped between the species except for the eye/body ratio, indicating that the eye size is the most critical feature distinguishing them. The present specimens showed that the relative position of the transverse musculature to the ventral ganglion, which was another diagnostic character of E. kitoui in the previous study, varied greatly according to growth. Considering the characteristic sampling sites (underwater canyons) and a long absence of occurrence records, E. kitoui is probably a rare species restricted to waters in or near coastal underwater canyons, as observed in E. calliops.