The "nasal barbel" of the halfbeak Dermogenys pusilla (Teleostei: Zenarchopteridae) - an organ with dual function

Abstract

Members of the taxon Zenarchopteridae (Beloniformes) possess external paired olfactory organs each consisting of a small cone-like papillae also called nasal barbel. We examined the structure of these barbels in the viviparous halfbeak Dermogenys pusilla using scanning (SEM) and transmission electron microscopy (TEM). Nasal barbels are covered by a typical epidermis characterized by ridged surface cells. Further, the epidermis contains goblet cells and small taste buds. The epidermis is interspersed with small depressions or pits (sensory islets) which contain the olfactory epithelium. Typically, taste buds consist of spindle-shaped dark cells with numerous apical microvilli, light cells with a thick microvillus each, basal cells and a rich nerve fiber plexus between receptor and basal cells. The olfactory epithelium at least contains two types of receptor cells, i.e. ciliated cells with a strikingly variable microtubular pattern and microvillous cells, and supporting and basal cells. An olfactory organ with an open groove and an elongated papilla is considered as synapomorphy of the Beloniformes (does not hold for the Adrianichthyoidei). Comparison of these olfactory organs suggests that D. pusilla and very probably all Zenarchopteridae may have the most uniform and least elaborated olfactory organs.