Fish Hemoglobin
Hemoglobin (Hob) is a respiratory pigment that vastly
Increases the binding power of the blood for oxygen. For example, in the Port Jackson shark (Heterogonous portjacksoni) at 20°C, 93% of the oxygen carried by the blood is reversibly bound to the hemoglobin while 7% is physically dissolved in the plasma at saturation.In colder environments the plasma percentage may increase (12% in the Antarctic nototheniid Tresnatomus bernacchii at -1.5°C). Indeed, the Antarctic crocodile ice fishes (family Channichthyidae) carry no hemoglobin in their blood at all. The ice fishes survive because (1) their metabolic oxygen requirements are low and the environmental dissolved oxygen is high in the consistently cold Antarctic waters; (2) their sluggish activity levels are adequate to catch sufficient quantities of the plentiful krill and small fish; and (3) special cardiovascular adaptations (e.g., comparatively large heart and blood volume with relatively low resistance capillaries) promote efficient movement of their blood.
Despite this evidence that some fish can get along without hemoglobin, its importance to most fishes is difficult to overstate. However, hemoglobin is not just a single type of molecule but really a class of structurally similar molecules that vary in their structure and in their affinity for oxygen under different conditions. This variability wills be-come evident in the next sections on (1) hemoglobin structure; (2) the role of hemoglobin in blood oxygen affinity; and (3) factors affecting blood oxygen affinity.
Hemoglobin structure. Fish hemoglobin is of two basic types,monomeric and tetramer. Monomer hemoglobin’s consist of single-home polypeptide molecules, each with a molecular weight of about17,000. They are characteristic of lampreys and hagfishes (Agatha).
Tetramer hemoglobin’s are characteristic of all “higher” fishes. They are composed of four chains of amino acids (two cm chains and two 0chains), much like mammalian hemoglobin’s, and have molecular weights of approximately 65,000. There are many different kinds of tetramerichemoglobins, and several kinds may be found in one fish! For example, four kinds of hemoglobin are found in rainbow trout blood, two in American eel (Anguilla prostrate) blood, and three in goldfish (Crassus aerates) blood.The significance of synthesizing more than one hemoglobin type appears to be related to the different functional properties of each, so different combinations of hemoglobin types reflect adoptions to different environments or ways of life.
Multiple hemoglobin’s are especially adaptive in migratory species which experience considerable environmental variation. For example, thecatadromous American eel has one hemoglobin which has a high oxygen affinity in saltwater and one with a high affinity in freshwater conditions.Poluhowich (1972) suggests that the polymorphic hemoglobin’s assist in the acclimation of these eels to environments of different salinity by maintenance of an approximately constant blood oxygen affinity.
The goldfish hemoglobin’s are functionally differentiable by their responses to temperature. Goldfish acclimated to 2°C had two different hemoglobin’s, while others held at 20°C and35°C featured three. Because the observed concentration of the third hemoglobin did not exceed 12.5% of the total concentration in any individual, its physiological importance may be minor, and a warm-temperature function for this component has yet to be demonstrated. The third Hob cane made to appear and disappear with temperature changes from 3°C to23°C and vice versa, respectively, within 3 hours. Thus, this rapid seethe-sis of the third hemoglobin in goldfish probably stems from rearrangement of the chi and i3 subunits in other hemoglobin’s rather than from synthesis of a new hemoglobin or production of a new type of erythrocyte.
Hemoglobin polymorphism for activity levels has also been hypothec-sized for species of suckers (Catostomidae). The desert sucker (Catostomus clarkia) possesses a pH-insensitive hemoglobin which maintains a high 02 affinity even when the 02 affinities of other hemoglobin’s are drastically reduced from in-creases in circulating lactic acid from violent muscular activity. This species typically lives in fast water. In the same stream, however, lives the Sonora sucker (Catostornus insignias), which does not possess this hemoglobin. This species is therefore found mainly in the slower-water portion (e.g., quiet pools) of their streams.
Changes in hemoglobin types with age have also been demonstrated in fishes. Coho salmon, for example, show changes with the progression from eleven to fry to presold stages. Giles and Vanstone (1976)believe these changes are controlled genetically and may be related to known changes in the hemopoietic origin of the erythrocytes during development. Certainly the pattern of Hob’s seems more fixed in de-eloping coos than in goldfish, as exposure of the salmon fry and pre-smelts to extremes of temperature, salinity, and dissolved oxygen produced no detectable variations