Qualitative and quantitative insights into the 3D microanatomy of the nervous ganglia ofScrobicularia plana(Bivalvia: Tellinoidea: Semelidae)

The nervous system of bivalves is bilaterally symmetrical and consists of interconnected cerebropleural, pedal and visceral ganglia, which may be partially to totally fused. We studied the microanatomy of the ganglia of Scrobicularia plana using three-dimensional (3D) reconstruction. We also examined whether intersex differences in the neural structure exist. Each type of ganglion had a characteristic 3D shape, and the cerebropleural ganglia shape was slightly asymmetrical. The visceral, pedal and cerebropleural ganglia are progressively smaller in volume, but only the pedal ganglion volume was positively correlated with the animal's length, height or width; suggesting functional implications. As to total surface area, correlations were found for the cerebropleural and visceral ganglia, but it was the visceral that consistently showed strong positive correlations with each biometric parameter. The medulla may often penetrate the cortex and touch the capsule in areas that (contrary to what might be expected) are not connected with emerging nerves. Despite the differences in volume and surface area among ganglia, the volume ratio of cortex/medulla is fairly stable (c. 1.5), suggesting a functional optimum. Finally, we conclude that the ganglia of males and females do not show significant quantitative differences.