The adipose fin of salmonids, once widely regarded as vestigial and lacking in function, was shown to be important to swimming efficiency in juvenile brown trout Salmo trutta. Examination with confocal microscopy of adipose fins of S. trutta stained with various antibodies targeting the nervous system revealed several large nerves entering the fin and anastomosing throughout its length. The branching nerves form a plexus with specific patterns of fine terminal branches in the leading and trailing edges. A network of astrocyte-like cells (ALCs) that is linked through cell processes to nerves and structural collagen reacted positively with antibodies to glial cells. No other fish fins, including other adipose fins, have been shown to exhibit this type of neural architecture. Many vertebrate mechanoreceptors rely on collagen deformation to stimulate responses in afferent nerves; similarly, the adipose fin also may function as a mechanosensor, where passive mechanical deflection by water currents stimulates afferent nerves.