Quantitative morphological variation in Sagitta setosa Müller, 1847 (Chaetognatha) and two closely related taxa

This paper reviews the quantitative morphological variation published for Sagitta setosa Müller, 1847 and two other species described within the S. serosa-complex, viz., S. euxina Moltschanoff, 1909 from the Black Sea, and S. batava Biersteker & Van der Spoel, 1966 from the Scheldt Estuary (Netherlands). Data on total (body) length, caudal length, numbers of teeth and hooks, ovary length, and dimensions of fins are compared between these three taxa. Additionally, samples from the North Sea, Mediterranean, and Black Sea are compared to look for geographic differences. Specimens from the Mediterranean were smallest with relatively long caudal segments, and few teeth and hooks, whereas specimens from the Black Sea were largest with relatively short caudal segments and many teeth and hooks. Specimens from the North Sea were intermediate with regards to these characters, but ranges overlapped and there were no obvious differences in allometry. These differences may be ecophenotypic, as the warm and salty Mediterranean Sea and cool and brackish Black Sea are at opposite ends of the environmental spectrum. The dimensions related to the fins showed clearer distinction between samples from different geographical areas, and slight differences in allometry. However, few data were available and little is known about the variance within each geographical area. We found more variation in quantitative characters within S. setosa from different parts of its range than between S. setosa and either S. hatava, or S. euxina. Sagitta batava conformed to S. setosa in terms of all the morphological characters considered. The data for S. setosa derived from Biersteker & Van der Spoel (1966) were atypical and were found to be based on misidentifications of S. elegans. Therefore, we concluded that S. batava cannot be considered a separate taxon. For S. euxina, the data were inconclusive. Quantitative data completely overlapped between S. setosa from the Black Sea and S. euxina, but few data of S. setosa from the Black Sea were available. Because samples were either composed entirely of S. setosa or S. euxina (depending on sampling season and depth) and there was a large variation in body lengths and relative ovary lengths, we consider it possible that these samples represent seasonal variants of one and the same species.