Morphological Variation in Japanese Populations of a Freshwater Leech, Helobdella stagnalis (Hirudinea : Rhynchobdellida : Glossiphoniidae)

Abstract

The variations of internal and external morphology and body shape between and within three populations of Helobdella stagnates (Linnaeus, 1758) in northern Honshu, Japan, were examined. The results suggest that (1) the location of internal organs relative to the segmentation is not informative for Helobdella taxonomy, (2) body shape may be useful as a species-level taxonomic character for properly fixed specimens, and (3) a purportedly cosmopolitan species, H. stagnalis may need to be separated into more than one species with restricted distributional ranges.