Taxonomic status of Stylocheiron armatum Colosi 1917 (Crustacea : Euphausiacea) — A critical evaluatton

Abstract

Stylocheiron armatum originally described by Colosi (1917) closely resembles Stylocheiron carinatum G.O. Sars (1883). Colosi described the species based on morphological differences observed with the elongated third coiroopod and the male copulatory organ. The species was established partly on Sars' (1883), Ortmann's (1893) and Hansen's (1910) S. carinatum and also on the specimens collected from the Indian Ocean and the Caribbean Sea. However, the later workers without perhaps examining the tropical euphausiids of the Indian Ocean, synonymised S. armatum with S. carinatum stating that the differences between the two species in the petasma and the elongated thoracic leg were too small to give the status of species to the former. In the course of a detailed study of Euphausiacea of the southeastern Arabian Sea, the author had the opportunity to examine vast collections of euphausiids and this has enabled him to have a closer look on the systematic position of S. armatum- The study has revealed that the morphological characters between the two species are so distinct tltat a re-thinking on the validity of the S. armatum is necessary. The material e5wmined by the author had characters more akin to S. armatum- In the present communication a critical evaluation of the status of S. armatum is made. A revised key, incorporating some new characters, for the identification of species and ' forms' of genus Styloclieiron is also presentel.