Sex ratio and sexual dimorphism of the anchovy Anchoa januaria (Actinopterygii, Engraulidae) in a tropical bay in south‐eastern Brazil

Abstract

Sex ratio and morphological traits of a very abundant anchovy Anchoa januaria were described in a tropical bay in south‐eastern Brazil. The aim was to test the hypothesis that sexual dimorphism occurs due to the different reproductive roles of the sexes. A fish sampling programme was carried out between September 1998 and August 1999 at six sites: four sandy beaches and two lower‐river sites. Population structure at river sites comprised adults only, ranging from 60 to 80 mm total length (LT), while at sandy beaches both juveniles and adults were found, ranging from 32 to 80 mm LT. Well‐balanced ‘spawning school’ at river sites during reproduction were detected, while female‐dominated schools occurred in the bay feeding areas. Males had relatively longer pectoral fins, slightly larger hearts and more somatic mass than females. Females outnumbered males at sizes >67 mm LT and had significantly longer intestines and heavier livers than males. The largest size reached by females was probably related to a higher growth rate as they have a larger intestinal absorbing area for nutrients. The prediction of higher energetic investment in reproduction by females that should have larger organs associated with food acquisition and processing to produce energy‐rich eggs was confirmed for A. januaria in Sepetiba Bay.