Physiology of Finfish and Shellfish (2+1)

3.3.2.2. Taste

Taste or gustatory chemoreception is generally a “close range” sense in fishes, especially useful in the identification of both food and noxious substances. Where as, the olfactory receptors are localized in the nares (sense of smell), taste buds are commonly located on several exterior surfaces of the fish besides in the mouth.

Many fishes have a well-developed sense of taste , and tiny pit like taste buds or organs are located not only within their mouth cavities, but also over their heads and parts of their body. Taste perception hasn't been extensively studied in bony fishes. Some species can detect some sensations, such as salty, sweet, bitter, and acid stimuli. Taste may be responsible for the final acceptance or rejection of prey items. Bottom fishes such as catfishes have considerable numbers of taste receptors on their skin, fins and barbels. The barbels ("whiskers") of catfishes, which often have poor vision, serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. The high gustatory sensitivity of the barbels extends the usefulness of these chemoreceptors a reasonable distance from the fish presumably to aid in finding food in murky water. A similar distant touch sense might be inferred from the presence of taste buds on the free pelvic fin rays of some codfishes and Trichogaster trichopterus. Other structures bearing recognizable taste buds include the well developed lips of some minnows and the suckers. Taste buds have also been described on the palatal organs in the buccal cavities of various cyprinids, catostomids (Catostomidae is the sucker family of the order Cypriniformes) and salmonids as well as on the gill rakers and arches primarily in freshwater fishes. Some species of naturally blind cave fishes are especially well supplied with taste buds, these often covering most of their body surface.

Gustatory information is transmitted to the brain of the vertebrates by 3 pairs of cranial nerves: the facial (VII), the glossopharyngeal (IX) and the vagus (X). Facial nerve branches, which innervate primarily the taste buds located on the external body surface, together with their central projections constitute the extra-oral gustatory system. This system is implicated in the detection and localization of a food source. The glossopharyngeal (IX) and the vagal nerve branches innervate only those buds located in the oro-pharyngeal region and thus constitute the oro-pharyngeal taste system, which is important for selective food ingestion. The cutaneous taste buds are innervated by branches of the facial cranial nerve VII. Cranial nerves are nerves, that emerge directly from the brain , in contrast to spinal nerves which emerge from segments of the spinal cord . The sensory signals from the “internal” taste buds (eg. those in the pharyngeal cavity and palate) are transmitted to the glossopharyngeal (IX) and vagus (X) nerves.

In species having a palatal organ, the total number of taste buds is considerably augmented and the terminal centres in the visceral sensory column of the medulla are correspondingly enlarged as “vagal lobes”. Just as the olfactory lobes of the brain are grossly enlarged in fishes relying heavily on olfactory sensory information (eg. Atlantic eels, sharks) the facial or vagal lobes of the medulla display a marked enlargement in species which find food by taste.

The functional importance of taste receptors to catfishes has been demonstrated by the fact that surgical removal or sectioning of optic and olfactory senses does not prevent catfish from swimming to a food source in still water. When taste reception is surgically blocked on one side of the catfish, it continuously circles toward the side of intact reception until, eventually the food is found. Surgical removal of the facial lobes of the medulla prevented catfish from locating and ingesting food. In contrast, removal of the vagal lobes did not inhibit finding of food or taking it into its mouth, but prevented swallowing, which is controlled via the glossopharyngeal and vagal nerves. Interestingly, sharks and rays shows no sensory elaboration for taste functions. Elasmobranchs rely primarily on olfaction, vision and electro-reception to locate their food.