Orientation by bulk messenger sensors in aquatic vertebrates.

All higher animals have evolved sense organs to detect minute amounts of chemicals in their environments. The stimuli they perceive affect their behavior in various ways: in locating of food, in orientation in home ranges or territories, in detection of the opposite sex, and even in the recognition of individuals of the same species. Among some fishes, the chemical sense organs are extremely well developed. The olfactory acuity of the eel (Anguilla anguilla) is such that the animal has been trained to respond to beta phenylethyl alcohol in a solution of 1.77 X 103 molecules per cubic centimeter of water (Teichmann, 1962). This, as the late Dr Teichmann so vividly stated, corresponds to a cubic centimeter of scent being dispersed in 58 times the water volume of the Lake of Constance. At this dilution, only two molecules at best would have been at one time in the nose of his small experimental eels, which had a nasal volume of only one cubic millimeter. Other fishes, however, do not appear to use their noses at all; the stickleback, for example, is anosmic. The other chemical sense, taste, is also very acute. It has been established that certain minnows have thresholds for taste several orders of magnitude lower than for humans (Glaser, 1966). Other fishes, which have many more taste sensors than these minnows, are believed to have taste thresholds that come close to olfactory ones. Sensory acuity is often reflected in the anatomy of the receptor organ. The fishes best able to smell have the olfactory epithelium thrown into the largest number of folds (e.g. the eel, FiGURE 1) and have well developed mechanisms to insure the passage of water over the receptor surfaces. Those best able to taste possess a large number of typical vertebrate taste buds (FIGURE 2) distributed all over their bodies, including the tail. In addition to large numbers of receptor cells for bulk messengers and related to the acuity of performance of smell and taste orientation, there are various adaptations that allow a fish to make comparisons of scent or taste substance concentrations in space and/or time. The hammerhead shark (FIGURE 3), for instance, bears its two nares at the extremities of its peculiarly broadened, wedge-shaped head. Directional detection of scent to the right and left of this shark is further enhanced by its sinuous swimming movements.