Taste and Olfactory Stimuli and Behavior in Fishes
TineValentincic
Department of Biolo gy, Un iversity of Ljublj ana, 1000 Ljubljana, Slovenia
ABSTRACT
Whe n earching for food different fish specie use the same sensory mechanisms differentiall y. At one extreme there <Ire ornnivoro u fj he such <IS catfish and carp that, in additio n to visio n, use the taste system to excite and release reflex responses and the olfactory system to excite and di scriminat e chemical stirnuli. On the other extreme Me predatory fishes that detect prey vi ually and, if not condi tio ned differen tially at fry and fingerli ng stages, do not use chemosensory information for food find ing at all.
Depending on the mechanism that they u e to detect and co llec t food, the fishes that have been tudied to date 0 cupy the follow ing ecological ni ches. (1) Bullh ead catfish (Am eiurus me/as) type om nivorous nic he: bull head catfishes use chemica l and ta til e enses to release appeti tive and con urnmatory phase of feedi ng behavior, They also detect prey by the passive electric sense. (2) Channel catfish (lc ta/urus punctatu ) type omnivorou niche: channel ca tf h use vision for predation and, in addition. chemical and tactile enses.(3 )CarpiCyptinu«carp io)and goldfish (Carrasiu auraw ) type omnivorous niche: carp use visual and chemica l sense for food co llec tio n, they have taste- contro lled reflex napping/biting mechanisms and. in additio n, they li se oral food sorti ng to separate edible irom inedible objec ts. (4) Rainbow trout (O ncorhyncus m yk iss) type vi ual hunters niche: farm- rai ed rainbow trout u e vision and/or olfact ion to get excited and sea rch ior food. (5) Exclusively vi ual hunters niche: in nature, Eu ropean huchen tH ucno /1II cI10) and walleye (Stizos ted io n vitreum) consume exclu ively livi ng prey such as fi h and crustaceans that they locate by vision.
Physiologicall y functional olfactory (Cooper and Hasler, 1976; Shoji et al., 1994) and taste organs (Marui et al.. 1983) do not necessari ly indica te that a predatory fish u es either olfac tio n or taste to find food. In nature, visual hunters such as huchen and walleye do not get excited by taste and olfactory stimuli, they neither bi te/snap after ta te stimulatio n nor do they u e olfactio n to discriminate chemic al timuli. In mo t predatory fishes the taste system is used solely dur ing oral food evaluatio n. At fry and early fingerli ng stage. huchen and wa lleye can learn to eat non-liv ing foods such as minced liver and indu trial tarter fed. Juvenil e walleye were condi tio ned in a fir Istep to eat non-living food, to respond to olfac tory stimuli in a second step and, in a third step. to discriminate amino acids, Thus, early learning influences the functio nal expre ion of the nerve networks that enable the use of olfac tory informatio n in the control of feeding in predatory fish.
Key words : Feeding behavio r, Ol factory sti muli , Taste sti muli, Chemo sense, O lfactory learning
G. Von Der Emde et al. (eds.), The Senses of Fish
© Narosa Publishing House 2004
Tine Valentinc ic
COMPLEX FEEDING BEHAVIOR
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Feed ing behavior is compose d of orien ting respon ses, appe ti tive search swimming, reflex turning and con summ atory beh aviors such as reflex snappi ng/b iting, oral manipulati on , mastica- tion and swallowing. In most cases omnivoro us fish de tec t the ir food by either visual or ch emical senses. Visual stimuli ena ble fish to swim directly at food items witho ut th e need of ch emic al stimulation , whereas olfactory or taste stimuli excite fish to swim aro und in sear ch for food (Valentincic and Caprio 1994). In nature, man y carni vorou s fish are exclusive visua l hunters th at do not use che mical stimuli to locat e and bit e at food.
Which ch emic al stimuli enable fish to locat e non-li ving food ? The most likely cand idat es are low-molecular wat er-soluble compounds such as amino acids. Amino acids are th e building blocks of all living organisms. They are present in protein s and free amino acid s are dissolved in th e cytoplasm. Amino acids leak from living organisms and from ca rrion. The concen tration of free dissolved amino acids in wat er ac tually depends on th e bal anc e between loss and upt ake of amino acids from and into living organ isms (Fergusson 1980) . The concentrati on s of free dissolved amino acid s in sea wate r, such as L-alanine and glycine, are grea te r th an 100 nan omolar, where as concentrati on s of un stable amino acids suc h as L-cystein e in natural wat ers may be as low as one nan omolar. Those an imals th at det ect incre ases in amino acid co nce ntrations above natural background concentrations pot entially det ect th e presence of food. Besides the sensitivity for amino acids research ers reported sensitivities of fish che rnore- ceptors for alipha tic acids, nucl eotides and bile salts (Maru i and Cap rio 1992) . The presen ce of th ese substa nces can also indicate food, however th eir precise action on fish behav ior is not kn own.
Two main proc esses distribute che mica l stimuli in wat er: diffusion and currents. At distanc es below 0.1 millimet er diffusion is an extr emely rapid process. In contrast, at distances larger th an centimeters diffusion is an extremely slow process (Table 1; Jacobs 1934) . At th e scale of bacteria and small prot ozoa diffusion is th e rat e-limiting process for stimulus distribu- tion , whereas at the scale of fishes wat er curren ts are th e rat e-limiting mechan ism. In th e world of microorgani sms regularly shaped diffusion grad ients surround th e stimulus source, whereas in the world of fishes chemical stimuli are irregularly distributed in space. On th e fish sca le wat er currents carry around bodies of irregular shapes called eddies (turbulent odor plumes) th at might contain high conc entrati on of ch emical stimuli; eac h eddy is larger th an several cen time - ters. The thin peripheral layer of th e edd y empties very quickl y int o the environment whereas its interior is centimet ers away from th e surro unding wate r; th e high concen tration of che mica l stimuli within the eddy is preserved for seve ral minutes.
Fish swimming through eddies with a speed of> 0.2 m/sec encoun te r high conce n tra tions of chemical stimuli interm ittently (Fig. 1). When a fish swims th rough an eddy, taste and olfactory recep tors are bathed with high conce ntrations of che mical sti muli for a fraction of a seco nd only.
The nasal cili ary pumping mech an isms crea te rapi d water curre n ts between th e olfactory lamellae, but the se mechani sms do not limit th e durat ion of th e olfactory receptor cell expos ure
