Ontogeny and sexual dimorphism of the sonic motor nucleus in the oyster toadfish

Male oyster toadfish, Opsanus tau, produce a courtship boatwhistle call and have a larger sound-producing organ than females, who do not boatwhistle . We investigated the possibility of sexual dimorphism in ontogeny of the sonic motor nucleus (SMN) of the toadfish. Brain weight increases for life though the increase decelerates with increasing fish weight. Neuron number, ranging from 760 to 2,888 in the SMN, increases rapidly to about 3 years, more slowly to about 8 years, and then levels off. There are no sexual differences in regressions of brain weight and SMN neuron number against fish size or age. Neuronal soma size in the SMN increases from 8 to 35 micron in average diameter and 67 to 916 micron2 in area over a period of at least 7 years. Males have larger neurons than females (P less than .01). However, males can be separated by inspection into populations with large and small soma sizes. Neuron size is not different between females and males with small somas. Neurons in males with large somas are larger than neurons in females and neurons in males with small somas. Such male dimorphism is reminiscent of other behavioral and morphological dimorphisms , which have led to characterization of males into territorial and satellite forms in certain mating systems.