VARIATION IN THE LIFE-SPAN AND SIZE OF THE FRESH-WATER PEARL MUSSEL

1. Within the distribution area of the freshwater pearl mussel (Margaritifera margaritifera L.), the maximum observed life span attained in a population varies from 30 to 132 years and the maximum shell length from 80 to 145 mm. For both life-history traits, the growth constant k is a major determinant. The growth constant describes the shape of the average individual growth curve, the lower its value, the slower is the asymptote approached. Maximum size and life span increase as the growth constant declines and the maximum age almost equals the time necessary to complete the growth pattern. 2. With increasing latitude, from North Spain up to the polar circle, the growth constant declines, whereas age and size increase. This must be attributed to the close relationship between the growth constant and metabolism. As temperature decreases (towards the north), metabolic rate declines and the rate of growth decreases; the asymptote is approached slowly and this leads to a long life. Simultaneously, there is probably a shift in the anabolic-catabolic-balance increasing the asymptotic size and hence the maximum size. 3. The growth constant and maximum life span also respond to the productivity of the habitat. There is a negative correlation between growth constant and eutrophication, and the maximum life span is reduced as the concentration of nitrate increases. 4. Implications for Bergmann's rule and for the theory of ageing are discussed. It is concluded that this variability is a case of phenotypic plasticity with no adaptive value because: (i) plasticity must be attributed to unavoidable physiological responses; (ii) between populations the relationships between maximum life span, maximum size and reproductive output are positive, whereas trade-offs would be expected if variation in life-history tactics was the source of plasticity; and (iii) the demographic consequences of plasticity may be highly disadvantageous.