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.M. Sokolova, V.J. Berger J. Exp. Mar. Biol. Ecol. 245 2000 1 –23
1. Introduction
Shell colour polymorphism is a distinctive feature of the populations of many marine and terrestrial gastropods e.g. Clarke, 1978; Raffaelli, 1982; Cain, 1983, 1988; Rehfeld,
1997. In all gastropod species studied so far in this respect, a direct genetic control of shell coloration has been demonstrated e.g. Murray and Clarke, 1966, 1976a,b; Barker,
1968; Cain, 1983, 1984; de Matos, 1984; Kozminsky et al., 1995; Ekendahl and Johannesson, 1997. In many cases, variation in shell colour is related to environmental
gradients such as climate Currey and Cain, 1968; Cowie, 1990; Chang and Emlen, 1993; Honek, 1993, insolation Heath, 1975; Heller and Volokita, 1981; Heller and
Gadot, 1987; Etter, 1988; Berger et al., 1995, wave exposure Etter, 1988 and salinity Sergievsky, 1992; Sokolova et al., 1995, 1997. Such variation has proved to be stable
and repetitive through time and space Owen, 1963; Currey and Cain, 1968; Wolda, 1969; Cain, 1971; Goodhart, 1973; Murray and Clarke, 1978. This suggests an adaptive
value to shell colour in gastropods and has stimulated numerous experimental works on the adaptive significance of shell coloration.
In gastropods, shell colour may have three functions: communication, crypsis and thermoregulation Endler, 1978. For these functions, it is the shell coloration itself
which is selectively important. Several cases of so-called physiological selection Cain and Sheppard, 1961 on shell colour have also been documented, where the shell colour
has been correlated with selectively valuable physiological traits including responses to temperature Kavaliers, 1989, salinity Sergievsky, 1992, 1995, metabolic rates
Steigen, 1979 and fecundity Wolda, 1967. It is suggested that in these cases, correlation between individual physiology and shell colour polymorphism is a result of
pleiotropic effects of genes responsible for the shell colour or a linkage between them and genes determining certain physiological features Raffaelli, 1979, 1982.
L . saxatilis Olivi Gastropoda: Prosobranchia is an abundant species in the
intertidal zone of North Atlantic, White and Barentz Seas Reid, 1996. Extreme variability of shell coloration in L
. saxatilis is well documented and partially responsible for the complicated synonymy of this species including 35 names of shell colour morphs
Reid, 1996. Shell colour is inherited in L . saxatilis Kozminsky et al., 1995; Ekendahl
and Johannesson, 1997 and hence, shell colour variability directly implies genetic variation in the population. In general, shell colour polymorphism was shown to be of
adaptive value in L . saxatilis populations and related to crypsis Atkinson and Warwick,
1983; Byers, 1990 and or thermoregulation Berger et al., 1995. Other studies Sokolova et al., 1995, 1997; Sokolova, 1997 also suggested that physiological
selection on shell colour may be the case in some populations of this species. Particularly, a clinal variation in the proportion of main shell colour morphs
phenotypes of L
. saxatilis was observed along the gradient of environmental salinity in White Sea estuaries Sokolova et al., 1995, 1997; Sokolova, 1997. Towards the head of
an estuary, the relative abundance of brown tessellated unbanded morphs increased 2 to 5–10-fold as compared to the adjacent marine sites, and the frequency of other colour
phenotypes mostly purple tessellated unbanded and plain purple unbanded pro- portionally declined. This pattern was found in the three studied White Sea estuaries
separated by a distance of over 100 km and was stable over time, during several years of
I .M. Sokolova, V.J. Berger J. Exp. Mar. Biol. Ecol. 245 2000 1 –23
3
the study Sokolova, 1997. Previous studies Sokolova et al., 1997 have suggested that the phenotypic differentiation along a salinity gradient cannot be explained by selective
pressures imposed by the differences in crypsis and or heating properties of the shell colour morphs involved and probably implies physiological selection.
In order to test the hypothesis about physiological selection as a driving force shaping pheno- geno-typic structure of L
. saxatilis populations along a salinity gradient in White Sea estuaries, we investigated physiological responses of different shell colour
morphs of L . saxatilis to salinity variation and also to the combination of low salinity
and subzero temperatures which may be expected in White Sea estuaries in winter. Particular attention was given to the phenotypes which are most common in the studied
populations of L . saxatilis and show the greatest and the most consistent difference in
abundance between the marine and estuarine sites purple tessellated unbanded and brown tessellated unbanded. Since it has been shown that the nature and the
mechanisms of salinity adaptations depend on the degree of environmental disturbance Kinne, 1964, 1971; Precht and Plett, 1979; Berger and Kharazova, 1997, we analysed
responses of different shell colour morphs of L
. saxatilis to moderate and extreme salinity changes. This allowed a comparison to assess the relationship between
genetically determined shell colour polymorphism and physiological variation with respect to salinity in more detail.
2. Materials and methods
