Abstract: Publisher Summary This chapter discusses the causes of speciation in living oysters and examines the genera of living oysters. Several levels of speciation occur in the oysters, Crassostrea virginica , Crassostrea. gigas , Ostrea edulis , and Ostrea. lurida. These species have widespread distributions, some 2000 to 5000 miles north to south. The speciation in living oysters occurs because of geographic isolation. Geographic speciation leads to the isolation of a population from its parental species in the form of geographical isolates. The isolates then acquire, during a long and continued period of isolation, characters, that promote or guarantee reproductive isolation when external barriers break down. Physiological races, spawning at different temperatures in different localities, are known in Crassostrea virginica and Ostrea edulis but these show no morphological divergence. The chapter outlines the genetic differences between two genera of oysters––that is, Ostrea and Crassostrea by using serological and chromatographic technique. A brief discussion of the chromosome cytology and karyotypes is presented in the chapter for an assessment of the potential of oysters for chromosomal evolution.

Abstract: Publisher Summary This chapter analyzes the physiological mechanisms involved in the migration of marine and amphihaline fish. It reveals that the combination of external and internal factors is particularly evident in downstream migration, in the regions of young Atlantic salmon. In a given sector of a Pyrenean stream, young salmon acquire the characteristics of the smolt—either at the end of the first, the second, or the third year—which clearly demonstrates the essential intervention of a physiological preparation; the speed at which it occurs depends on genetic traits inherited from the parents, because they are apparently all submitted to the same environmental conditions. However, in the spring, if the level of the stream remains the same when smoltification has been accomplished, migration only occurs sporadically, individually, or in small groups; but as soon as the water level rises—however, slightly as a result of a spring flood— then an absolute rush takes place, carrying down thousands of individuals. Fishes respond to a temperature gradient by choosing a given temperature (thermopreferendum)—which appears to be the optimal one for a given physiological state—and there exists—at least in the sockeye salmon ( Oncorhynchus nerka )—a behavioral thermoregulation, the consequence of which is to ensure a maximal bio-energetic yield.

Abstract: Publisher Summary This chapter describes the gustatory system in fish and the component of the communis system that serves the sense of taste, in contrast to those communis neurons that serve less specialized visceral sensations. Taste is primarily a close-range sense and its function is the identification of nutrients and the avoidance of noxious substances. A few fish, such as catfish, are able to use the sense of taste at a distance and can orient by means of extended barbels to intensity gradients. The chapter discusses the structure and distribution of taste buds in all classes of vertebrates. It reviews both anatomical and physiological aspects of the gustatory system of fish, but it focuses on the anatomy of the taste receptors. The chapter describes the origin, development, and location of the taste buds. It discusses the sites of first appearance and the rate and manner of distribution of the taste buds in a developmental series of the catfish, Ameiurus melas .

Abstract: Publisher Summary This chapter describes the alimentary canal and digestion in teleosts. It recognizes several types of teleosts on the basis of the different foods taken by them and their feeding habits: plankton feeders, herbivores, omnivores, carnivores, and specialists. The various degrees of specialization in feeding habits lead to the distinction of europhags, stenophags and monophags. Many cell types of the teleost digestive tract have been examined electron-microscopically, such as oral epithelial cells, chloride cells, scattered chemosensory cells, the cells of taste buds, intestinal goblet cells, and epithelium cells. Histo- and cyto-chemical techniques have been used in the study of the teleostean alimentary canal. Because the inner lining of this canal is mucified, emphasis is given to the mucus-secreting cells, which appear to contain either neutral or acidic mucopolysaccharides. In many teleosts, alkaline phosphatase and acid phosphatase activity has been detected in the brush-border of intestinal and cecal epithelia. Feeding rates have been measured by observing food intake under laboratory conditions with restricted or excess rations. The chapter discusses three methods by which the digestion rate in fish is estimated––namely, (1) by measuring the time interval between food intake and defecation, (2) by measuring the stomach contents at various time intervals after feeding, and (3) by using X-rays for observing the progress of digestion with time.