ABSTRACT
INTRODUCTION Th ere are three living genera of lungfi shes, or dipnoans: Protopterus (the African lungfi sh), Lepidosiren (the South American lungfi sh), and Neoceratodus (the Australian lungfi sh). In these fi shes, the lateral line system comprises two diff erent sensory modalities: mechanoreception and electroreception. Th e organs responsible for mechanoreception are called neuromasts, and those responsible for electroreception are termed electrosensory ampullary organs, or ampullae. Both types of receptors are formed ontogenetically from placodal material, as demonstrated experimentally in the axolotl by Northcutt et al. (1994, 1995). Th e mechanosensory neuromasts of the lateral line system consist of sensory hair cells, supporting cells, and mantle cells, and they occur within canals as well as superfi cially in lines on the head and trunk. Th e electrosensory organs of the lateral line system occur as ampullae, which are concentrated on the snout but also spread over the head and trunk. Th e sensory epithelium of these ampullae is similar to that of the mechanoreceptive neuromasts, but the electrosensory ampullae have a more narrow apical part, with a single cilium and no stereovilli. Th e three genera of lungfi shes vary in several respects. One notable diff erence is that the lepidosirenid lungfi shes (Protopterus and Lepidosiren) are able to aestivate during dry periods. When their habitat literally dries up annually, adult individuals burrow into the mud and wait for the rainy reason to recreate their fl oodplains. Th is is known as their aestivation phase, as opposed to the preceding active or aquatic phase. In the West African lungfi sh, Protopterus annectens, the neuromasts remain intact, but their apical parts, with the sensory hair cell bundles, become enclosed in small cavities. Also during aestivation, the electroreceptive ampullae in Protopterus annectens become overgrown with epidermal cells, leaving a small ampullary lumen with the apical cilia of the sensory cells. Th e mechanoreceptive neuromasts and the electroreceptive ampullary organs of the lateral line system in lungfi shes will be described in greater detail below in sections on their morphology, their distribution in the three genera, and their development. Unless noted otherwise, all descriptions are based on adult, i.e. sexually mature, lungfi shes.
