Sexual dimorphism in pelvic fin length of Atlantic cod

Behavioural differences between females and males may result in sexual dimorphism among morphological traits associated with these behaviours. In the broadcast-spawning Atlantic cod (Gadus morhua L., 1758), release of gametes occurs during a "ventral mount" in which the male positions himself directly beneath the female while grasping her with his pelvic fins. Males also display the pelvic fins during agonistic encounters with other males. Based on data obtained from four Atlantic cod populations off Canada and Norway, we find clear evidence of sexual dimorphism in pelvic fin size, the fins being significantly larger in males than in females. Pelvic fin size was, however, not more variable than other morphological traits and was not correlated with body condition or drumming muscles mass (hypothesized to be a secondary sexual characteristic in this species). To our knowledge, this is the first study to demonstrate sexual dimorphism in any external morphological trait in a gadoid fish. Although the observed differences in pelvic fin size may be a product of sexual selection, we identify future work to test this hypothesis and to explore more fully the causes and fitness consequences of this sexual bias in Atlantic cod.