Morphology and distribution of pit organs and canal neuromasts in non-teleost bony fishes.

The morphology of the pit organs and canal neuromasts of the actinopterygian Polypterus and the lungfishes Lepidosiren, Neoceratodus and Protopterus are described using light microscopy, scanning electron microscopy and transmission electron microscopy. Pit organs resemble canal neuromasts in their hair cell polarization, but they differ in size, shape, hair cell density and in the length of the kinocilia. Pit line neuromasts in Lepidosiren, Polypterus and Protopterus are arranged in dense lines, with numerous neuromasts between pores in the canal, and are an order of magnitude smaller than canal neuromasts in Polypterus. We suggest that the evolutionary transformation of canal neuromasts into superficial neuromasts occurred through the evolutionary reduction of the lateral line canals, either as the result of selection for changes in neuromast function, or as the result of non-adaptive reduction of dermal bone around the lateral line canals and their neuromasts. Protopterus (a lepidosirenid lungfish) is apparently unique among bony fishes in that it has multiple neuromasts between adjacent pores in lateral line canals on the head. An analysis of the distribution of this character among major gnathostome clades indicates that multiple canal neuromasts between pore positions is a primitive gnathostome character. Latimeria, Neoceratodus and actinopterygian fishes are characterized by an alternative character state, the presence of one canal neuromast between adjacent pores.