Cupulae in shark neuromasts: composition, origin, generation.

Cupulae on the surface of the sensory epithelium of canal neuromasts of the shark lateralis system have been demonstrated. They are mounds of mucoid material secreted primarily by the supporting cells of the neuromast epithelium. Individual columns of mucus in fixed, stained sections produce striations perpendicular to the neuromast surface. The hair of the sensory cell is embedded in mucus at the bases of coalescing columns produced by a group of surrounding supporting cells. There is evidence that mucus production is continuous and is accompanied by regeneration of secretory cells and elimination of necrotic cells at the distal surface. It follows that cupular material is being discarded either intermittantly or continually, along with trapped cell debris and other detritus.

Shark pit organs: response to chemicals.

Nerve fibers from pit organs and canal neuromasts are distinguished by the nature of their electrophysiological response to mechanical and chemical stimulation. Pit organs respond to touch but have a relatively high threshold compared with canal neuromasts. They respond readily to sodium and potassium chloride solutions, the rate of discharge increasing with the concentration of the solution. Order of effectiveness with 1 molar solutions of monovalent cations is as follows: potassium, rubidium > sodium, ammonium > cesium, lithium. Anions are ineffective. Divalent cations such as calcium and magnesium are inhibitory. Responses to acid, sugar, and quinine are either very slight or inhibitory.