Ultrastructure of the ampullary electroreceptors in lungfish and Brachiopterygii.

Electron microscopy shows that the electroreceptors of Protopterus dolloi (Dipnoi) and Calamoichthys calabaricus (Brachiopterygii) are very similar to the known ampullary electroreceptors of other primitive fish groups. With Protopterus, however, there is one difference: at the apical surface of the receptor cells, instead of a kinocilium, a club-shaped protrusion is found. The club-shaped protrusion has no fixed number nor a fixed arrangement of microtubules (Fig. 4). The presynaptic ribbons are in direct contact with the presynaptic membrane, and may be invaginations of the presynaptic membrane (Fig. 3). The functional significance of the kinocilium and of the club-shaped protrusion is discussed. Furthermore, a possible biological role of the ampullary electroreceptors in fish coming into frequent contact with the water surface (Dipnoi, Brachiopterygii) is considered.