Electroreception in elasmobranchs: sawfish as a case study.

The ampullae of Lorenzini are the electroreceptors of elasmobranchs. Ampullary pores located in the elasmobranch skin are each connected to a gel-filled canal that ends in an ampullary bulb, in which the sensory epithelium is located. Each ampulla functions as an independent receptor that measures the potential difference between the ampullary pore opening and the body interior. In the elasmobranch head, the ampullary bulbs of different ampullae are aggregated in 3-6 bilaterally symmetric clusters, which can be surrounded by a connective tissue capsule. Each cluster is innervated by one branch of the anterior lateral line nerve (ALLN). Only the dorsal root of the ALLN carries electrosensory fibers, which terminate in the dorsal octavo-lateral nucleus (DON) of the medulla. Each ampullary cluster projects into a distinctive area in the central zone of the DON, where projection areas are somatotopically arranged. Sharks and rays can possess thousands of ampullae. Amongst other functions, the use of electroreception during prey localization is well documented. The distribution of ampullary pores in the skin of elasmobranchs is influenced by both the phylogeny and ecology of a species. Pores are grouped in distinct pore fields, which remain recognizable amongst related taxa. However, the density of pores within a pore field, which determines the electroreceptive resolution, is influenced by the ecology of a species. Here, I compare the pore counts per pore field between rhinobatids (shovelnose rays) and pristids (sawfish). In both groups, the number of ampullary pores on the ventral side of the rostrum is similar, even though the pristid rostrum can comprise about 20% of the total length. Ampullary pore numbers in pristids are increased on the upper side of the rostrum, which can be related to a feeding strategy that targets free-swimming prey in the water column. Shovelnose rays pin their prey onto the substrate with their disk, while repositioning their mouth for ingestion and thus possess large numbers of pores ventrally around the mouth and in the area between the gills.