A sexually dimorphic basal anal-fin ray expansion in the weakly discharging electric fish Gnathonemus petersii

Male Gnathonemus petersii (Mormyridae) exhibit two structural, sexually dimorphic characters: anal-fin ray bone expansion and an indentation of the posterior ventral body wall (formerly described as anal-fin indentation). Females lack this bone expansion, but may show a slight indentation. Morphometric data on both characters were obtained from radiographs of 414 fish (males and females) ranging in size from 60 to 276 mm Ls. Both body wall indentation and bone expansion began to develop in males of about 120 mm Ls. At 160–180 mm, the sex ratio of fish with expansion to fish without expansion was 50 : 50. Androgens seem to affect the expression of both these sexual dimorphisms. Bone expansion may provide increased bone surface for muscle attachment and thus facilitate the anal-fin reflex during courtship behaviour.