WFSC 448 – Fish Ecophysiology
(Week 5 – 28 Sep 2015)
Major point: Physical and chemical properties of water dominate lives of
aquatic organisms in ways largely alien to terrestrial vertebrates
CHEMORECEPTION: OLFACTION AND GUSTATION
Nearly all aspects of life—prey or food location, predator detection, species and
mating cue assessment, parental behavior, migration, etc., depend on
chemoreception.
Chemoreception includes what we typically deem olfaction (smell) and gustation
(taste).
Olfaction (smell) results from stimulation of sensory receptor cells in the olfactory
organs, which are innervated by the olfactory nerve (cranial nerve I).
Teleost brain showing major brain lobes and nerves: (I) olfactory (II) oculomotor
(III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), auditory (VIII),
glossopharyngeal (IX), and vagus (X).
Gustation (taste) is mediated by taste buds (sensory receptor cells) innervated by
the facial, glossopharyngeal and vagus nerves (cranial nerves VII, IX, andX). Taste
buds are found in the mouth cavity, as well as in the gill cavity, on the gill arches,
and sometimes on the body epidermis.
A third type of chemoreception is often recognized, usually referred to as
a common or general chemical sense, mediated by sensory receptors located on
exposed body surfaces of a fish, and better developed in scale-less, bottom-living
forms.
Taste buds are not involved; instead, the receptors are free nerve endings supplied
by the spinal nerves. It is low in sensitivity compared to smell and taste. But
importantly, it is yet another evolutionary channel that can be crafted into
adaptation.
Structure/function of chemosensory organs
The fish “nose” is composed of two channels
arrayed in a manner that they appear as
nostrils. The paired channels are called nares.
Each naris is ciliated to create water flow over
a bed of sensory cells. Fluid ramming and
branchial pumps (in some taxa) have evolved
as well. This cell bed is a series of folded
tissues (lamellae) covered with olfactory
epithelia.
The point of lamellae is to increase surface area. For olfaction this means a larger
number of receptors exposed to a larger laminar layer of water containing
chemicals to be detected.
The narial fold is like a hood scoop, acting to
increase ventilation of fluid through a designed
architecture, much like a racing car uses a hood
scoop.
The shape of the chemoreception components
varies greatly among fishes. For example, hagfishes and lampreys have only a
single medial naris. Scale-less fishes tend to be more dependent on body
chemoreception, and the number and composition of lamellae varies by fish
species and habitat. Olfactory morhpology also varies ontogenetically. E.g.,
As a contrast among habitats, consider the three-spine stickleback, which relies
mostly on vision to make its way in the world and accordingly has a minimized
olfactory chamber. Whereas, most eels rely heavily on olfactory cues and have
large, elongate olfactory chambers with numerous (>60) lamellae.
In more derived fishes the nares have
both channel openings on the body
surface.
Hagfishes (Myxini), a very few teleosts
(some uranoscopids and the
bathydraconidgenus Gymnodraco),
and, of course, the lungfishes (Dipnoi)
have internal nares (exhalent
openings into the pharynx)
comparable to those of terrestrial
vertebrates.
Anterior nares
Olfactory sensitivity/acuity of the sense of smell is very high in many species of
fishes, both with respect to the (1) perception of very low concentrations of odors
and to (2) discrimination of two or more odors in a mixture.
 Early research on thresholds involved behavioral experiments in which
individuals trained by reward or punishment were conditioned to select or
avoid water that held the chosen chemical and then were tested on more
and more diluted concentrations.
 More modern methods involve conditioned heart rate,
electroencephalograms, recordings from nerve tracts, and other
electrophysiological methods. Various published sources for a number of fish
species and chemical substances show wide variation.
 One calculation of olfactory sensitivity shows that olfactory acuity
corresponds to 1 ml of alcohol dissolved in a lake of a volume 58 times as
great as that of Lake Constance (bounded by Germany, Austria, and
Switzerland; 207 square miles), which equates to one or two molecules of
alcohol in the olfactory chamber at any one time.
 The European eel (Anguilla anguilla), with its highly developed olfactory
chambers and numerous olfactory lamellae, can detect phenyl-ethyl alcohol
at concentrations of 3 x 10-20 molar or about the same concentration
detectable by dogs.
 Gustation, the sense of taste, is important to fishes primarily in the location
and identification of possible food sources. Depending on the ecology of the
species, it can be a very highly developed sense. For example, in certain
minnows the sensitivity to various sugars and salts has been found to be 512
and 184 times greater, respectively, than that of humans.
Unlike olfactory sense receptors, taste receptors are not restricted to
a single location. In addition to positions throughout the mouth, gill arches and
skin, catfishes in particular but also many other taxa evolved well-developed
barbels (sometimes longer than the fish itself) that bear dense concentrations of
taste receptors and can be controlled with musculature.
What other taxa can you think of with evolved sensory filaments?
What elements of natural history would you imagine select for elaboration of
barbels?