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Water‐soluble chemical compounds are detected by olfaction (smell) or gustation (taste). For olfaction, teleosts have paired nares on the rostrum lined with olfactory epithelium. Hagfish and lampreys (Agnatha) are unique with only a single nare (Evans et al. 2004). Water passing through the nares stimulates receptors in the olfactory tracts, which send signals to the olfactory bulbs within the forebrain (telencephalon) (Roberts and Ellis 2012). Some teleosts have nasal sacs and accessory nasal sacs that actively pump water over the epithelium (coinciding with opercular movement) (Hara 1975). Some rely heavily on olfaction and have large olfactory pits extending from the rostrum to the eye, e.g. moray eels and true eels (Anguilliformes). Others rely on visual cues and lack nasal sacs, e.g. pufferfish (Tetraodontidae) (Hara 1975). In some species, males have a larger olfactory capacity (Hara 1975).

Figure A1.7 Otoliths visible on lateral radiograph of a red drum (Sciaenops ocellatus).

Source: Image courtesy of Shane Boylan, South Carolina Aquarium.

Taste buds are epidermal and can be found in the oral cavity, lips, head, barbels, body wall, fins, and esophagus (Evans et al. 2004; Roberts and Ellis 2012). In some fish, the external taste buds outnumber intraoral taste buds by as much as 90% (Hara 1975). Fish have up to three anatomically different taste buds (Reutter et al. 1974). The taste cells are receptive to amino, nucleic, and organic acids (Oike et al. 2007). Fish do have aversive and preferential responses to some chemical stimuli but extensive research on gustatory preferences has not been done (Oike et al. 2007).