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Intraspecific and Interspecific Differentiation of Fish
7. lntraspecific and lnterspecific Differentiation of Fish ....................................................................................... ....................................................................................... ............................................................................................
7.1. lntrapopulation Variability 7.2. Interpopulation Variability 7.3. InterspecificVariability
222 224 227
Genotypic studies involve large-scale examination of the structure within a population. Originally, they were based on haemagglutination, i.e. defining the blood type (Sindermann and Mairs, 1961; Altukhov, 1969, 1974; Limansky, 1970) and later involved the study of polymorphism of proteins, including haemoglobin, albumin, transferrin, a-,f3- and y-globulins and enzymes (Tsuyuki et al., 1965; Salmenkova, 1973; Altukhov, 1974, 1983; Kirpichnikov, 1978, 1987; Dobrovolov, 1980; Chikhachev, 1984; Jamieson, 1974; Jamieson and Birley, 1989; Lukyanenko et al., 1991). Study of protein polymorphism can reveal the genetic structure of a population, interpopulation invasion and the nature of local fish stocks and shoals, and has made it possible to evaluate their degree of displacement. For example, the separation of the Baltic cod from the Atlantic stock has been c o n h e d (Jamieson, 1974). The genetic results also provide a better understanding of the distinctiveness of allied species, and allow their taxonomic status to be defined more exactly from associated morphological characters (Dobrovolov, 1980; Smith et al., 1990; Lukyanenko et al., 1991). Additional progress in this field has been achieved from studies of DNA (Mednikov et al., 1977;Ginatulina et aZ., 1988; Cherkov and Borchsenius, 1989; Smith et al., 1989; Bentzen et al., 1996). Other recent work in fish genetics is containedin a volume edited by Matthews and Thorpe (1995). The genotypic aspect of research into intra- and interspecific differentiation should be regarded as an independent field, with a less direct bearing on ecology. Ecology emerges only when the adaptive significance of the various characteristics is explored and related to the environmental variables. In the initial stages of research, the adaptive importanceof genetic characters such as allozymes is not easy to establish with certainty.
222
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
Much progress has also been achieved in the phenotypic approach to studies of intra- and interspecific differentiation of fish. It is this approach in the context of a ‘species within its area’ that has been vigorously explored by ecologists (Volskis, 1973). Each species needs to be regarded as a discrete ecological unit, not an abstract speculation but a distinct component of the biosphere in which it plays a special and versatile part.
7.1. INTRAPOPULATION VARIABILITY
Variabilities within and between species represent adaptation evolved to provide a dynamic balance of a population, race or species inhabiting a changeable environment. This concept has been recognized and developed by many authors (Nikolsky, 1965; Polyakov, 1975; Shatunovsky, 1980). Following from it, any growing instability of the environment leads to an increase of intra- and interpopulationvariability. Shatunovsky (1980) reported that the growth rate, oxygen uptake and mortality rate of steelhead trout and Baltic cod changed during the lifetime of one generation (Figure 77), the fish composing the population becoming heterogeneous. We have already referred to heterogeneity manifesting itself as variations in the fatness of various species from the Azov and Black Seas (Shulman, 1967, 1972a). Another manifestation is the varying content of lipid fractions and their unsaturation in populations of Black Sea horsemackerel and Pacific humpback salmon (Chapters 2 and 3). The physiological and biochemical characteristics become more variable during periods of enhanced functional activity, for example, in the periods before spawning and during spawning, when the coefficient of variation in the lipid content may amount to 80%. This phenomenon is widespread, for example, the replenishment of glycogen in the liver or the large increase in non-esterified fatty acids in the blood of cod during refeeding after starvation. The variation between individual fish is greatest where the curve is steepest and least when values have settled to a steady state (Black and Love, 1986). Indeed, a plot of the variations in the results can be as revealing as a plot of the results themselves, showing, as it does, the metabolic activity at each stage of the experiment. Such heterogeneity fosters optimum functioning of the population, and guarantees long-lasting and efficient spawning in those fish that exhibit intermittent maturation and batch reproduction. It results in balanced productivity and recruitment of the stock and in the more efficient use of the nutritive base. If living conditions become hard, individual variability will rise within the population (Nikolsky, 1974; Polyakov, 1975), offering a better chance for the population to survive. Shatunovsky (1980) recorded
223
INTRASPECIFIC AND INTERSPECIFIC DIFFERENTATION
100
1
2
3
4
5
6
7
8
910111213
YearS
Figure 77 Effect of different rates of growth of individual fish on other properties (schematic): 1, fast growth rate; 2, middle rate; 3, slow rate. (After Shatunovsky, 1980.)
heterogeneity in growth rate between different generations of White Sea flounder. Individual variability depends significantly upon the age structure of a population. Of especial importance is the divergence between individuals in protein accumulation, which accounts for the productivity, quality of reproductive products and feeding mode followed by individuals (Shatunovsky, 1980). It was convincingly shown that narrowing the
224
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
nutritive base may lead, through depressed protein growth, to emergence of monocyclic dwarf forms (as in smelt), altering the character of the originally polycyclic population (Kriksunov and Shatunovsky, 1979).
7.2. INTERPOPULATION VARIABILITY
Whereas individual variability promotes stability of the population, the interpopulation or intraspecific variability performs a similar function for species within a particular biosphere. An increase in intraspecific variation is a sign of biological progress. In northern seas, fish such as herring, cod, haddock and flounder show intraspecies population differences in life span, structure, age at maturity and various physiological and biochemical characteristics (Shatunovsky, 1963, 1970;Love, 1970,1980; Storozhuk, 1971;Lapin, 1973). In contrast, fish from the Black, Azov and Mediterranean Seas have shorter life spans and display less distinct intraspecifc differences. Moreover, within one sea population, fish of one species do not differ much in age structure, fecundity or spawning character. The most pronounced difference in the latter group of species is the rate of lipid accumulation in sprat from different sites in the Black Sea and anchovy in the Azov Sea (Shulman, 1972b). Comparison of anchovy from the Azov, Black and Mediterranean Seas shows more marked differences in the levels of accumulated lipid and the growth rate (Chapter 2). Anchovies arranged in the following order - Mediterranean (Engruulis encrusicholus rnediterruneus), Black (E.e. ponticus) and Azov (E.e. mueoticus) Seas - show a decrease in linear growth and somatic production rate (Figure 78). The most likely explanation is the shortening of the growth period during the annual cycle resulting from reduced time of intensive feeding at the optimum ambient temperature. In the Mediterranean Sea, the water is warmest and so favourable for growth over most of the year. The Black Sea is not as warm, and the favourable conditions last only from May to October, while in the Azov Sea the favourableperiod is even shorter. Lipid accumulation differs from somatic growth, since food plankton increasesin abundance from the Mediterranean, via the Black to the Azov Sea, a situation that promotes the greatest accumulation of lipids in Azov fish. Species that live in northern seas may be structurally differentiated by environmental conditions, which vary considerably from the centre to the periphery of the area. Crucial factors are the temperature and salinity of the water and the character of the nutritive base. Populations of cod, herring and flounder exist in two forms, oceanic and coastal (Shatunovsky, 1980). The food supply of oceanic populations of cod, herring and haddock in the north-eastAtlantic varies considerably from year to year. In the North Sea the
225
INTRASPECIFIC AND INTERSPECIFIC DIFFERENTATION
Black Sea race
AZOVSearaCe
o+
1+
2+
3+
o+
1+
Age
2+
MediterraneanSea race
3+
o+
1+
2+
3+
Figure 78 Production (calories) in different races of European anchovy: solid line, total calories; dotted line, protein calories; broken line, lipid calories. (After Shulman and Urdenko, 1989.)
food supply remains more or less stable, so there are smaller year-to-year variations in fatness of fish at the end of their feeding period. The best-nourished cod are those confined to the small area of the Faroe Bank.There they are very corpulent compared with other stocks, possessing the highest concentrationof protein in their musculature of any cod examined and carrying relatively enormous, creamy livers. The ground appears to provide a good food supply as a result of upwelling of oceanic currents. In this stock, even the small percentage of lipids found in the musculature of cod is larger. The carbohydrate content of the musculature is high, leading to a lower post-mortem pH. This engenders a tougher texture after cooking and the ready breakdown of connective tissue, which causes the fillets to ‘gape’. This unique stock of cod was illustrated by Love (1970, Figure 62), and its properties were described fully by Love et al. (1974,1975) and Love (1986). The colour of the skin of many fish species changes according to that of the sea bottom. In the black volcanic larva off north Iceland, cod have a very dark skin, while on the white shell bottom at the Faroe Bank,a l l species of fish are almost devoid of pigmentation. However, Love (1974) has shown that, although live Faroe Bank cad and the darker Aberdeen Bank cod, when placed together in an aquarium of intermediate colour, tended to adopt that colour, they were still easily distinguished after 8 months of living together. The range of adaptation to environmentalcolour appears to be limited within each population. Conditions on fishing grounds are critical in the nutrition of the fish and can vary greatly. As
226
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
noted previously, studies on the bile of cod have shown that all the fish from one ground can be actively feeding,while those from a nearby ground are all starving. The growth rate of cod populations decreases consistently from high in the North Sea to lesser in the Baltic to least in the White and Barents Seas. However, North Sea cod do not usually live longer than 8 years, while Arctic cod exist that are more than 25 years old. A similar difference in life span is found in herring from the North and Baltic Seas (Shatunovsky, 1980). The level of liver lipid is greater in cod from the Baltic Sea and the Arctic sector of the Norwegian Sea, when compared with those from the North Sea. However, those from the White Sea have lower levels coupled with a slower growth rate, so the relationship of low growth-high lipid is not universal. Characteristics of the protein and lipid metabolism of a population are directly related to those of the reproductive system - the period of maturation, absolute and relative fecundity and the weight and content of organic matter in gonads and their products (Shatunovsky, 1980). Different groups of North Sea herring differ somewhat in content of dry matter in individual eggs (Blaxter and Hempel, 1966). There are comparable differences in Baltic and White Sea flounder, and the spring brood of Baltic and AtlanticScandinavian hemng (Shatunovsky, 1963).The weights of mature ovaries are 2650% more in Baltic populations of cod, herring and flounder than those recorded in oceanic and North Sea stocks (Shatunovsky, 1980). More rapid growth and accelerated maturation are characteristic of populations that have stable and sufficient food supplies, such as North Sea cod and herring from the central region of the area. Slow growth but accelerated maturation both occur in marginal populations with a plentiful food supply; among such are White Sea cod and Black Sea flounder. In Mediterranean sprat, a good food supply exists at the boundary of the population area,giving high somatic and generativeproduction. The duration and scope of generative production are longer in southern forms that have multiple spawning (Koshelev, 1984). Their generative production is 20-308 of their body weight, compared with 6 1 0 % from northern forms (Shulman and Urdenko, 1989). The southern forms are younger at maturity than the northern, and their abundant yield of sexual products reduces their somatic growth. Living at lower temperatures, the northern forms create more somatic material and have the longer life span. Geography and climate strongly influence many species of fish. For example, the northern (Baltic) race of sprat spawn in the summer and attain maximum lipid content in winter (Mankowski er at., 1961; Biryukov, 1980). In contrast, lipid is maximal in summer in sprat of the Black and Mediterranean Seas. A similar discrepancy in the timing of peak fatness is found in different races of herring (Marti, 1956;Wood, 1958; Krivobok and Tarkovskaya, 1960; Shatunovsky, 1980), cod (Dambergs, 1963; Love, 1970; Shatunovsky, 1980); flounder (Dambergs, 1963; Shatunovsky, 1963, 1970); and North Atlantic horse-mackerel (Podsevalov and Perova, 1973;Chuksin et d.,1977).
INTRASPECIFIC AND INTERSPECIFIC DIFFERENTATION
227
Marked interpopulationdistinctions in characteristics such as fatness have been observed in a number of freshwater fish - silver carp, bighead, bleak, wild goldfish and mongolian grayling (Marti, 1988; Konovalov, 1989; Lapin and Basaanzhov, 1989; Lapina, 1991). The phenotypic distinctionsbetween fish of different populations discussed in this section are often more profound than interspecific ones.
7.3.
INTERSPECIFIC VARIABILITY
This factor reveals physiological and biochemical distinctions between phenotypes of related species. Differences between forms that belong to different families, orders and even classes are probably not relevant. Distinctions should be named ‘specific’ only between allied species of the same genus. Species specificity is relevant to ecology, as it involves the identification of characters peculiar to a given species but different from others, however closely related. Such knowledge points to the understanding of the adaptive significance of the characters, and the nature of the microevolutionary processes involved in the emergence of the particular form. Precise analysis of the species specificity of such characters will require comprehensive examination of their variability within the species, accurate determination of the structural and functional polymorphism and the extent of any interspecific transgression. An example is the thermal resistance of isolated tissues and proteins (including enzymic proteins), studied by Ushakov (1963), detailed in Chapter 2. The study revealed a distinct relationship between the thermal stability of the tissues and proteins, and the temperature conditions of the environment where the organisms lived. Andreeva (1971) found small differences in the shrinkagetemperaturesof the collagens of cod and whiting which depended on the place of capture, warmer habitats leading to slightly higher shrinkage temperatures, i.e. greater stability. However, Lavdty et al. (1988) could find no shrinkage difference between the collagens of two groups of turbot maintained for a long period at temperatures6-10°C apart. Possibly the adaptation reported by Andreeva had been established at an early larval stage of the fish. The studies reported in Chapter 2, on five related species of Azov goby, brought to light a close relationship between the degree of unsaturation of lipids and the concentrationof ambient oxygen. Species specificityof phenotypes has been little studied through the physiological or biochemical approach because of insufficient knowledge of their adaptive role. While the many papers on protein polymorphism are important in the context of population genetics, they cannot yet contributeto the progress
228
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
of the present subject. An exception is the encouraging interpretation of the haemoglobin and serum protein polymorphism observed in related species of sturgeon (Lukyanenko et al., 1991). Looking into the variability shown at levels higher than species, one immediately encounters difficulties. Many published accounts are mere descriptions, but what is required is knowledge of the nature of the evolutionary processes and the environmental factors that engender them. The task is not easy, because the divergence and convergence of physiological and biochemical characteristicswithin a large taxon are so great and the forces that form them so varied that a clear picture can hardly be drawn yet. We warn against adopting too superficialan approach when tackling this problem, since environmental aspects have usually been treated as of secondary importance.
7.1. lntrapopulation Variability 7.2. Interpopulation Variability 7.3. InterspecificVariability
222 224 227
Genotypic studies involve large-scale examination of the structure within a population. Originally, they were based on haemagglutination, i.e. defining the blood type (Sindermann and Mairs, 1961; Altukhov, 1969, 1974; Limansky, 1970) and later involved the study of polymorphism of proteins, including haemoglobin, albumin, transferrin, a-,f3- and y-globulins and enzymes (Tsuyuki et al., 1965; Salmenkova, 1973; Altukhov, 1974, 1983; Kirpichnikov, 1978, 1987; Dobrovolov, 1980; Chikhachev, 1984; Jamieson, 1974; Jamieson and Birley, 1989; Lukyanenko et al., 1991). Study of protein polymorphism can reveal the genetic structure of a population, interpopulation invasion and the nature of local fish stocks and shoals, and has made it possible to evaluate their degree of displacement. For example, the separation of the Baltic cod from the Atlantic stock has been c o n h e d (Jamieson, 1974). The genetic results also provide a better understanding of the distinctiveness of allied species, and allow their taxonomic status to be defined more exactly from associated morphological characters (Dobrovolov, 1980; Smith et al., 1990; Lukyanenko et al., 1991). Additional progress in this field has been achieved from studies of DNA (Mednikov et al., 1977;Ginatulina et aZ., 1988; Cherkov and Borchsenius, 1989; Smith et al., 1989; Bentzen et al., 1996). Other recent work in fish genetics is containedin a volume edited by Matthews and Thorpe (1995). The genotypic aspect of research into intra- and interspecific differentiation should be regarded as an independent field, with a less direct bearing on ecology. Ecology emerges only when the adaptive significance of the various characteristics is explored and related to the environmental variables. In the initial stages of research, the adaptive importanceof genetic characters such as allozymes is not easy to establish with certainty.
222
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
Much progress has also been achieved in the phenotypic approach to studies of intra- and interspecific differentiation of fish. It is this approach in the context of a ‘species within its area’ that has been vigorously explored by ecologists (Volskis, 1973). Each species needs to be regarded as a discrete ecological unit, not an abstract speculation but a distinct component of the biosphere in which it plays a special and versatile part.
7.1. INTRAPOPULATION VARIABILITY
Variabilities within and between species represent adaptation evolved to provide a dynamic balance of a population, race or species inhabiting a changeable environment. This concept has been recognized and developed by many authors (Nikolsky, 1965; Polyakov, 1975; Shatunovsky, 1980). Following from it, any growing instability of the environment leads to an increase of intra- and interpopulationvariability. Shatunovsky (1980) reported that the growth rate, oxygen uptake and mortality rate of steelhead trout and Baltic cod changed during the lifetime of one generation (Figure 77), the fish composing the population becoming heterogeneous. We have already referred to heterogeneity manifesting itself as variations in the fatness of various species from the Azov and Black Seas (Shulman, 1967, 1972a). Another manifestation is the varying content of lipid fractions and their unsaturation in populations of Black Sea horsemackerel and Pacific humpback salmon (Chapters 2 and 3). The physiological and biochemical characteristics become more variable during periods of enhanced functional activity, for example, in the periods before spawning and during spawning, when the coefficient of variation in the lipid content may amount to 80%. This phenomenon is widespread, for example, the replenishment of glycogen in the liver or the large increase in non-esterified fatty acids in the blood of cod during refeeding after starvation. The variation between individual fish is greatest where the curve is steepest and least when values have settled to a steady state (Black and Love, 1986). Indeed, a plot of the variations in the results can be as revealing as a plot of the results themselves, showing, as it does, the metabolic activity at each stage of the experiment. Such heterogeneity fosters optimum functioning of the population, and guarantees long-lasting and efficient spawning in those fish that exhibit intermittent maturation and batch reproduction. It results in balanced productivity and recruitment of the stock and in the more efficient use of the nutritive base. If living conditions become hard, individual variability will rise within the population (Nikolsky, 1974; Polyakov, 1975), offering a better chance for the population to survive. Shatunovsky (1980) recorded
223
INTRASPECIFIC AND INTERSPECIFIC DIFFERENTATION
100
1
2
3
4
5
6
7
8
910111213
YearS
Figure 77 Effect of different rates of growth of individual fish on other properties (schematic): 1, fast growth rate; 2, middle rate; 3, slow rate. (After Shatunovsky, 1980.)
heterogeneity in growth rate between different generations of White Sea flounder. Individual variability depends significantly upon the age structure of a population. Of especial importance is the divergence between individuals in protein accumulation, which accounts for the productivity, quality of reproductive products and feeding mode followed by individuals (Shatunovsky, 1980). It was convincingly shown that narrowing the
224
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
nutritive base may lead, through depressed protein growth, to emergence of monocyclic dwarf forms (as in smelt), altering the character of the originally polycyclic population (Kriksunov and Shatunovsky, 1979).
7.2. INTERPOPULATION VARIABILITY
Whereas individual variability promotes stability of the population, the interpopulation or intraspecific variability performs a similar function for species within a particular biosphere. An increase in intraspecific variation is a sign of biological progress. In northern seas, fish such as herring, cod, haddock and flounder show intraspecies population differences in life span, structure, age at maturity and various physiological and biochemical characteristics (Shatunovsky, 1963, 1970;Love, 1970,1980; Storozhuk, 1971;Lapin, 1973). In contrast, fish from the Black, Azov and Mediterranean Seas have shorter life spans and display less distinct intraspecifc differences. Moreover, within one sea population, fish of one species do not differ much in age structure, fecundity or spawning character. The most pronounced difference in the latter group of species is the rate of lipid accumulation in sprat from different sites in the Black Sea and anchovy in the Azov Sea (Shulman, 1972b). Comparison of anchovy from the Azov, Black and Mediterranean Seas shows more marked differences in the levels of accumulated lipid and the growth rate (Chapter 2). Anchovies arranged in the following order - Mediterranean (Engruulis encrusicholus rnediterruneus), Black (E.e. ponticus) and Azov (E.e. mueoticus) Seas - show a decrease in linear growth and somatic production rate (Figure 78). The most likely explanation is the shortening of the growth period during the annual cycle resulting from reduced time of intensive feeding at the optimum ambient temperature. In the Mediterranean Sea, the water is warmest and so favourable for growth over most of the year. The Black Sea is not as warm, and the favourable conditions last only from May to October, while in the Azov Sea the favourableperiod is even shorter. Lipid accumulation differs from somatic growth, since food plankton increasesin abundance from the Mediterranean, via the Black to the Azov Sea, a situation that promotes the greatest accumulation of lipids in Azov fish. Species that live in northern seas may be structurally differentiated by environmental conditions, which vary considerably from the centre to the periphery of the area. Crucial factors are the temperature and salinity of the water and the character of the nutritive base. Populations of cod, herring and flounder exist in two forms, oceanic and coastal (Shatunovsky, 1980). The food supply of oceanic populations of cod, herring and haddock in the north-eastAtlantic varies considerably from year to year. In the North Sea the
225
INTRASPECIFIC AND INTERSPECIFIC DIFFERENTATION
Black Sea race
AZOVSearaCe
o+
1+
2+
3+
o+
1+
Age
2+
MediterraneanSea race
3+
o+
1+
2+
3+
Figure 78 Production (calories) in different races of European anchovy: solid line, total calories; dotted line, protein calories; broken line, lipid calories. (After Shulman and Urdenko, 1989.)
food supply remains more or less stable, so there are smaller year-to-year variations in fatness of fish at the end of their feeding period. The best-nourished cod are those confined to the small area of the Faroe Bank.There they are very corpulent compared with other stocks, possessing the highest concentrationof protein in their musculature of any cod examined and carrying relatively enormous, creamy livers. The ground appears to provide a good food supply as a result of upwelling of oceanic currents. In this stock, even the small percentage of lipids found in the musculature of cod is larger. The carbohydrate content of the musculature is high, leading to a lower post-mortem pH. This engenders a tougher texture after cooking and the ready breakdown of connective tissue, which causes the fillets to ‘gape’. This unique stock of cod was illustrated by Love (1970, Figure 62), and its properties were described fully by Love et al. (1974,1975) and Love (1986). The colour of the skin of many fish species changes according to that of the sea bottom. In the black volcanic larva off north Iceland, cod have a very dark skin, while on the white shell bottom at the Faroe Bank,a l l species of fish are almost devoid of pigmentation. However, Love (1974) has shown that, although live Faroe Bank cad and the darker Aberdeen Bank cod, when placed together in an aquarium of intermediate colour, tended to adopt that colour, they were still easily distinguished after 8 months of living together. The range of adaptation to environmentalcolour appears to be limited within each population. Conditions on fishing grounds are critical in the nutrition of the fish and can vary greatly. As
226
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
noted previously, studies on the bile of cod have shown that all the fish from one ground can be actively feeding,while those from a nearby ground are all starving. The growth rate of cod populations decreases consistently from high in the North Sea to lesser in the Baltic to least in the White and Barents Seas. However, North Sea cod do not usually live longer than 8 years, while Arctic cod exist that are more than 25 years old. A similar difference in life span is found in herring from the North and Baltic Seas (Shatunovsky, 1980). The level of liver lipid is greater in cod from the Baltic Sea and the Arctic sector of the Norwegian Sea, when compared with those from the North Sea. However, those from the White Sea have lower levels coupled with a slower growth rate, so the relationship of low growth-high lipid is not universal. Characteristics of the protein and lipid metabolism of a population are directly related to those of the reproductive system - the period of maturation, absolute and relative fecundity and the weight and content of organic matter in gonads and their products (Shatunovsky, 1980). Different groups of North Sea herring differ somewhat in content of dry matter in individual eggs (Blaxter and Hempel, 1966). There are comparable differences in Baltic and White Sea flounder, and the spring brood of Baltic and AtlanticScandinavian hemng (Shatunovsky, 1963).The weights of mature ovaries are 2650% more in Baltic populations of cod, herring and flounder than those recorded in oceanic and North Sea stocks (Shatunovsky, 1980). More rapid growth and accelerated maturation are characteristic of populations that have stable and sufficient food supplies, such as North Sea cod and herring from the central region of the area. Slow growth but accelerated maturation both occur in marginal populations with a plentiful food supply; among such are White Sea cod and Black Sea flounder. In Mediterranean sprat, a good food supply exists at the boundary of the population area,giving high somatic and generativeproduction. The duration and scope of generative production are longer in southern forms that have multiple spawning (Koshelev, 1984). Their generative production is 20-308 of their body weight, compared with 6 1 0 % from northern forms (Shulman and Urdenko, 1989). The southern forms are younger at maturity than the northern, and their abundant yield of sexual products reduces their somatic growth. Living at lower temperatures, the northern forms create more somatic material and have the longer life span. Geography and climate strongly influence many species of fish. For example, the northern (Baltic) race of sprat spawn in the summer and attain maximum lipid content in winter (Mankowski er at., 1961; Biryukov, 1980). In contrast, lipid is maximal in summer in sprat of the Black and Mediterranean Seas. A similar discrepancy in the timing of peak fatness is found in different races of herring (Marti, 1956;Wood, 1958; Krivobok and Tarkovskaya, 1960; Shatunovsky, 1980), cod (Dambergs, 1963; Love, 1970; Shatunovsky, 1980); flounder (Dambergs, 1963; Shatunovsky, 1963, 1970); and North Atlantic horse-mackerel (Podsevalov and Perova, 1973;Chuksin et d.,1977).
INTRASPECIFIC AND INTERSPECIFIC DIFFERENTATION
227
Marked interpopulationdistinctions in characteristics such as fatness have been observed in a number of freshwater fish - silver carp, bighead, bleak, wild goldfish and mongolian grayling (Marti, 1988; Konovalov, 1989; Lapin and Basaanzhov, 1989; Lapina, 1991). The phenotypic distinctionsbetween fish of different populations discussed in this section are often more profound than interspecific ones.
7.3.
INTERSPECIFIC VARIABILITY
This factor reveals physiological and biochemical distinctions between phenotypes of related species. Differences between forms that belong to different families, orders and even classes are probably not relevant. Distinctions should be named ‘specific’ only between allied species of the same genus. Species specificity is relevant to ecology, as it involves the identification of characters peculiar to a given species but different from others, however closely related. Such knowledge points to the understanding of the adaptive significance of the characters, and the nature of the microevolutionary processes involved in the emergence of the particular form. Precise analysis of the species specificity of such characters will require comprehensive examination of their variability within the species, accurate determination of the structural and functional polymorphism and the extent of any interspecific transgression. An example is the thermal resistance of isolated tissues and proteins (including enzymic proteins), studied by Ushakov (1963), detailed in Chapter 2. The study revealed a distinct relationship between the thermal stability of the tissues and proteins, and the temperature conditions of the environment where the organisms lived. Andreeva (1971) found small differences in the shrinkagetemperaturesof the collagens of cod and whiting which depended on the place of capture, warmer habitats leading to slightly higher shrinkage temperatures, i.e. greater stability. However, Lavdty et al. (1988) could find no shrinkage difference between the collagens of two groups of turbot maintained for a long period at temperatures6-10°C apart. Possibly the adaptation reported by Andreeva had been established at an early larval stage of the fish. The studies reported in Chapter 2, on five related species of Azov goby, brought to light a close relationship between the degree of unsaturation of lipids and the concentrationof ambient oxygen. Species specificityof phenotypes has been little studied through the physiological or biochemical approach because of insufficient knowledge of their adaptive role. While the many papers on protein polymorphism are important in the context of population genetics, they cannot yet contributeto the progress
228
THE BIOCHEMICAL ECOLOGY OF MARINE FISHES
of the present subject. An exception is the encouraging interpretation of the haemoglobin and serum protein polymorphism observed in related species of sturgeon (Lukyanenko et al., 1991). Looking into the variability shown at levels higher than species, one immediately encounters difficulties. Many published accounts are mere descriptions, but what is required is knowledge of the nature of the evolutionary processes and the environmental factors that engender them. The task is not easy, because the divergence and convergence of physiological and biochemical characteristicswithin a large taxon are so great and the forces that form them so varied that a clear picture can hardly be drawn yet. We warn against adopting too superficialan approach when tackling this problem, since environmental aspects have usually been treated as of secondary importance.