Presence of abdominal appendages in females of Caprella equilibra Say, 1818 (Peracarida, Amphipoda): is Metacaprella Mayer, 1903 a valid genus?

Abstract

Detailed morphological studies of the caprellid abdomen are necessary to understand the evolution of this group. According to the diagnosis of the genus Caprella, the abdomen of males is provided with a pair of uni- or bi-articulate appendages and a pair of lobes, and females only have the pair of lobes and lack appendages. However, during a monitoring study of Caprella equilibra in southern Spain, we found that 7% of the females were provided with abdominal appendages, revealing that there is intraspecific variation of the female abdomen, even in the same population. The intrageneric and even intraspecific evidence of morphological variation of the abdominal appendages in Caprella and Metacaprella indicates that the external morphology of these appendages is not an ontogenetically and morphologically stable character to diagnose a genus. Consequently, this study supports the evidence that the abdominal appendage is a polymorphic and symplesiomorphic character in Caprella and Metacaprella, and that Metacaprella is not a valid genus.