The fish gustatory system provides the final sensory evaluation in the feeding process. Unlike other vertebrates, the gustatory system in fish may be divided into two distinct subsystems, oral and extraoral, both of them mediating behavioural responses to food items brought in contact with the fish. The abundance of taste buds is another peculiarity of the fish gustatory system. For many years, morphological and electrophysiological techniques dominated the studies of the fish gustatory system, and systematic investigations of fish taste preferences have only been performed during the last 10 years. In the present review, basic principles in the taste preferences of fish are formulated. Categories or types of taste substances are defined in accordance with their effects on fish feeding behaviour and further mediation by the oral or extraoral taste systems (incitants, suppressants, stimulants, deterrents, enhancers and indifferent substances). Information on taste preferences to different types of substances including classical taste substances, free amino acids, betaine, nucleotides, nucleosides, amines, sugars and other hydrocarbons, organic acids, alcohols and aldehydes, and their mixtures, is summarised. The threshold concentrations for taste substances are discussed, and the relationship between fish taste preferences with fish systematic position and fish ecology is evaluated. Fish taste preferences are highly species-specific, and the differences among fish species are apparent when comparing the width and composition of spectra for both the stimulants and the deterrents. What is evident is that there is a strong similarity in the taste preferences between geographically isolated fish populations of the same species, and that taste preferences are similar in males and females, although at the individual level, it may vary dramatically among conspecifics. What is noteworthy is that taste responses are more stable and invariable for highly palatable substances than for substances with a low level of palatability. Taste preferences as a function of pH is analysed. There is a good correspondence between development of the gustatory system in fish ontogeny and its ability to discriminate taste properties of food items. There is also a correspondence between oral and extraoral taste preferences for a given species; however, there is no correlation between smell and taste preferences. Taste preferences in fish show low plasticity (in relation to the diet), appear to be determined genetically and seem to be patroclinous. Fish feeding motivation and various environmental factors like water temperature and pollutants such as heavy metals and low pH water may shift fish taste preferences. Comparisons between bioassay and electrophysiological data show that palatability is not synonymous with excitability in the gustatory system. The chemical nature of stimulants and deterrents in various hydrobionts is outlined. The significance of basic knowledge in fish taste preferences for aquaculture and fisheries is emphasised.

Taste preferences in fishes

Summary
(1) Based on data from the literature, the phenomenon of homing in salmonids is examined with special reference to the Atlantic salmon. Wild, native fish demonstrate an excellent homing ability, judged from percent return to the home river (1–3 %) and straying to non-native watersheds (less than 3 % of returning fish).

(2) The homing ability in wild fish is shown to be closely related to the existence of reproductively isolated populations between and within watersheds, as demonstrated by data from salmonid ecology and biochemical genetics.

(3) Two main hypotheses have dominated the literature on salmonid homing during recent years: (a) An ‘imprinting’ hypothesis based on a process of learning of stream odours during seaward migration, coupled with sun-orientation for open sea navigation, (b) A ‘pheromone’ hypothesis related to odours from fish and based on inheritance and the seasonal migrating schedules of discrete populations.

(4) The olfactory sense has been demonstrated as mandatory for salmonids, both in near range and open sea navigation. According to genetic, sensory and ecological aspects of homing, the pheromone hypothesis is therefore concluded to be the most appropriate.

(5) Fish produced from artificially fertilized eggs, released within native systems or transplanted, demonstrate a reduced homing ability. Since hatchery-raised fish demonstrate a survival in sea equivalent to that of wild fish, a genetic disturbance of navigational ability has been suggested, resulting from the production of population hybrids by man.

(6) Studies made in the fields of behaviour, electrophysiology and chemistry strongly suggest that population-specific fish odours are involved in home-stream recognition by salmonids.

(7) An evaluation of ‘imprinting’ experiments related to artificial organic compounds reveals that: (a) the odorant properties of the applied chemicals must be questioned, (b) imprinting related to olfaction may be based on a weak theoretical foundation, (c) returns obtained in census experiments may be adequately explained through ecological interpretations, and (d) behavioural preferences obtained from exposure to non-natural compounds may be founded on mechanisms not associated with homing.

(8) A logical link between the use of olfaction and the role of genetics in salmonid homing is emphasized, together with its practical implications for salmonid management.

Homing and olfaction in salmonids: a critical review with special reference to the atlantic salmon

Introduction Ollaetion Morphological basis an overview Peripheral olfactory organ Central projections Registration of olfactory activities Electroolfactogram (EOG) ExtraceUular recordings of action potentials Electroencephalogram (EEG) Cross-adaptation Responses to chemical stimuli Amino acids Steroid hormones Bile salts Prostaglandins (PGs) Other compounds Gustation Morphological basis an overview Taste buds Gustatory nerves and central projections Recordings of gustatory responses Responses to chemical stimuli Amino acids Carboxylic acids Nucleotides Bile salts CO2 and pH Toxins and other compounds Future prospects and research needs Summary and conclusions Olfaction Gustation Acknowledgements References

The diversity of chemical stimulation in fish olfaction and gustation

Background
In this review, the position of behavioural ecotoxicology within the available means to assess the status of marine environments is described as filling the gap for the needed “early warning” signals. A few examples of studies performed since the 1960s are discussed to highlight the sensitivity of these approaches in investigating the effects of chemicals, including priority pollutants and emerging contaminants, relative to conventional toxicity tests measuring survival.

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Behavioural ecotoxicology, an “early warning” signal to assess environmental quality

This report gives a detailed description of the fine structure of the olfactory mucosa in man. Using a special biopsy instrument and technique, fresh biopsies of olfactory epithelium were taken under local anaesthesia from eight normal volunteers. Transmission electron microscopy reveals that human olfactory epithelium has four major cell types: ciliated olfactory receptors, supporting cells, basal cells and microvillar cells. The ciliated olfactory receptors, as in other mammals, are bipolar neurons; the dendrite tip, modified to form the olfactory vesicle, bears 10–30 cilia that lack dynein arms. The supporting cells, markedly different from the goblet cells of respiratory epithelium, are not specialized for mucus secretion. Instead they are equipped to contribute materials to, and remove materials from, the surface mucus. The basal cells are stem cells that serve to replace epithelial cells and receptors lost during normal turnover or injury. In addition to ciliated olfactory neurons, supporting cells and basal cells, the human olfactory mucosa contains a distinct fourth cell type, the microvillar cell, of unknown function. The apical pole of the cell sends a tuft of short microvilli into the nasal cavity; its basal pole gives rise to a slender cytoplasmic process that resembles an axon. If microvillar cells prove to be sensory cells, the current concept of the human olfactory epithelium will have to be revised to include two morphologically distinct classes of receptors.

The fine structure of the olfactory mucosa in man.

Olfactory responses to amino acids in rainbow trout, Salmo gairdneri.

1.

Gustatory responses to amino acids and derivatives obtained from the palatine nerve (VIIth cranial nerve) of rainbow trout (Salmogairdneri) were studied.




2.

The response to an amino acid was characterized by its fast-adapting, phasic nature that returned to baseline within 5 s during continuous stimulation.




3.

Complete recovery of the response took place 120 s after stimulation when tested with two identical stimuli given successively (Figs. 2, 3).




4.

Among common amino acids tested, onlyL-isomers of proline, hydroxyproline, alanine, leucine, and phenylalanine were stimulatory (Table 1), demonstrating that the facial taste system is more narrowly-tuned to amino acid stimuli than the olfactory system. The arginine derivative,L-α-amino-β-guanidinopropionic acid (L-AGPA) andL-argininic acid, and betaine were also effective.




5.

Concentration-response relations, when plotted semi-logarithmically, were generally sigmoidal, saturating at higher concentrations (>1 mM) (Fig. 5). The threshold concentration forL-proline, the most effective amino acid tested, was estimated to be 50 nM. The thresholds for other stimulatory chemicals ranged between 1 and 100 μM.




6.

L-AGPA, though having higher threshold (10 μM), could induce responses three times the magnitude of that ofL-proline at 1 mM.




7.

The stimulatory activity of heterocyclic-imino acids having 4-, 5-, and 6-membered rings and also of thioproline and hydroxyproline suggests that the receptor recognizes primarily the imino acid region (Fig. 6).




8.

Taste responses to amino acids were independent of pH, except that arginine analogues were only active at basic pHs (Fig. 7 and Table 3). larginine was active only at pHs higher than 8.5. The palatal chemoreceptors were stimulated by waters with pHs below 7.0, suggesting the existence of receptors for pH and/or CO2 (Fig. 7).

Toshiaki J. Hara

Papers

Olfactory responses to amino acids in rainbow trout, Salmo gairdneri.

Gustatory responses of the rainbow trout (Salmo gairdneri) palate to amino acids and derivatives

Structure-activity relationships of amino acids in fish olfaction.

Further studies on the structure-activity relationships of amino acids in fish olfaction

Effects of pH on the olfactory responses to amino acids in rainbow trout, Salmo gairdneri.