Chemical compounds that are naturally present in the oceans, lakes, and rivers have benefited the survival and reproduction of fishes. Chemical cues associated with feeding derive from the food items. For example, amino acids indicate a source of protein, and bile acids released in the wastes of a prey may also indicate a food source.
As humans, we think of taste comprising of sweet, bitter, salty, sour, and umami. In the fishes’ world, palatability is conveyed by the taste of bile acids, amino acids, nucleotides, small peptides, quaternary ammonium compounds, organic acids, glucuronide conjugates, as well as quinine, strychnine, and tetrodotoxin. For amino acids, the response threshold is normally 10−7–10−8 M but decreases to 10−9 M for channel
Just as bitter stimulates aversive responses to food in humans, there are aversive taste stimuli forf fish For example, algae may be protected against consumption by fish even when quinine and other alkaloids are tasted. In the predatory fish Ariopsis felis, the gustatory system mediates an aversive response to a potential prey, it, Aplysia californica. This mollusk is chemically defended by releasing ink containing an unpalatable pigment originating from red seaweed consumed by the sea hare. This adaptation is also seen for the marine toxins tetrodotoxin, which defends the pufferfish (Tetraodontidae), and saxitoxin, a shellfish toxin originating from algal blooms. Both stimulate specialized gustatory receptors in fishes. Thus, gustation is an indicator of palatability, and has an adaptive component for the prey that emits unpalatable chemical signals. This ability to taste these compounds protects the fish from consuming prey and plants containing poisons.
Fish utilize olfaction to drive a variety of biologically important behaviors including feeding,
mating, and predator avoidance. Both the olfactory and gustatory systems are involved in feeding, and therefore some amino acid and bile acid gustatory stimuli also stimulate the olfactory system. The olfactory repertoire of fishes is diverse and includes amino acids, bile salts, nucleotides, steroids, prostaglandins, and polyamines.
The fish olfactory system responds to many different l-amino acids which are associated with feeding responses. In cyprinids, gadids, and silurids, feeding behaviors such as biting and snapping are mediated by the lateral olfactory tract and are abolished when this tract is severed, there by removing odor output from the lateral OB, which processes amino acid stimuli. In sea lampreys, amino acid responsiveness is limited to basic amino acids. In salmonids, olfactory imprinting to a home stream is needed for spawning migration and may involve an amino acid compound; however, the odors that are utilized for imprinting are still under investigation. The olfactory system of cyprinids responds to nucleotides such as adenosine triphosphate, which may signal fresh food to a predatory fish. In addition, polyamines, which are utilized in a variety of cellular functions and have been shown to be indicative of tissue decomposition, are olfactory stimuli to cyprinids.
The olfactory system also responds to pheromon-species-specific signals for innate, intraspecific chemical communication. Danger is conveyed through alarm pheromones, compounds released by damaged skin cells from fish that have been preyed upon. These communicate predation to surrounding conspecifics. Olfactory stimuli such as bile salts, prostaglandins, and steroids mediate migration and spawning behaviors in some fishes. Cyprinids communicate during mating by emitting and responding to prostaglandins and 17α, 20-β-dihydroxy-4-pregnen-3-one. The reproductive hormones and hormonal metabolites released into the water by con-specifics convey information during mating. Some fishes, such as gobiids, respond to steroids such as etiocholanolone and its conjugates, but not to prostaglandins. Lampreys synthesize and release bile alcohols such as petromyzonol sulfate,
and its derivatives, which are utilized for migration and spawning.
Environmental calcium (Ca2+) and sodium (Na+) also elicit olfactory sensory responses in fish. In fishes that migrate from saltwater to freshwater and vice versa, or inhabit areas that experience shifts in salinity, it is necessary to detect changes in environmental concentrations of ions, such as Ca2+, in order to regulate internal homeostasis of these ions and avoid detrimental osmotic conditions. The gilthead seabream, which inhabits estuarine waters, exhibits olfactory sensitivity to changes in environmental Ca2+ concentrations. Moreover, the goldfish (Carassius auratus), which
lives in freshwater habitats, possesses a calcium sensing receptor in the olfactory epithelium and exhibits olfactory sensitivity to changes in waterborne Ca2+ concentrations. In this freshwater fish, the observed olfactory sensitivity to Ca2+ may play a role in internal calcium homeostasis.
Clearly, fish are well aware of their chemical environment. The compounds that have been described represent diverse chemical structures, and originate from many components of the aquatic environment; yet fish have found a way to sense these, and to respond appropriately.
© 2021 jitendra saraf