Sexual dimorphism in Stylocheiron indicum Silas and Mathew (Crustacea: Euphausucea)

Abstract

Many species of euphausiids are found sexually dimorphic, the characters being manifested with several of their body organs. This phenomenon has been investigated in some species in detail and as a result it has been found that the morphological variations between the two sexes are more pronoimced in species of the genera namely Thysanoessa, Nematoscelis, Nematobrachion and Stylocheiron. The organs thus found to vary in males and females are the antennules, their flagellae, photophores, lateral denticles on carapace, abdominal segments, eye or even the preanal spine. Considering die manifestation of sexual dimorphic characters in diverse body organs in euphausiids an attempt has been made here to study the degree of sexual dimorphism in Stylocheiron indicum Silas and Mathew. Ir this species differences were observed with some of the body parts in the adult males and females. In order to assess the magnitude of such differences a few morphological characters were cwnpared in both the sexes statistically. Of the characters examined it was found that the proportion of length to depth of the sixth abdominal segment, and the proportion of the width of the upper part of the eye to the width of the lower part of the eye were significantly different in the two sexes.