Colonization of marginal habitats, demographic responds of the long-lived ocean quahog Arctica islandica

Background/Question/Methods

Marginal populations are often characterized by a reduction of genetic diversity and thus reduced population viability and evolutionary potential compared to their habitat of origin. Limits of colonization potential can be influenced by the individual’s demographic response to environmental variation. Therefore the question arises of whether or not individuals of both marginal and original populations exhibit similar trends in key life history attributes such as growth, reproduction and mortality. The ocean quahog Arctica islandica (Bivalves) provides a unique model system to study the consequences of colonization of marine species because it occurs in the geographically isolated, brackish Baltic Sea with a strong gradient in salinity and temperature and its native habitat, the North Sea. A. islandica is the longest-lived animal reported, with a maximum life span > 400 years, which makes it especially relevant for understanding life-history patterns in long-lived organisms. This study compares the demographic variables of about 1100 individuals from a marginal (Baltic Sea) and an original (Iceland) population, with the goal of contributing to our understanding of whether selection molds differing life history strategies of individuals in geographically peripheral locations with huge differences in abiotic parameters as compared to their habitat of origin.

Results/Conclusions

A. islandica from the Baltic Sea population showed a much shorter maximum lifespan of just 36 years, compared to the 242 years for the Iceland population. The Baltic Sea population also had lower age-specific growth rates, leading to smaller individuals. The average shell weight of Baltic Sea individuals was half that of the Iceland individuals. The female to male sex ratio was indistinguishable between populations but in the Icelandic population the sex ratio at greater ages was biased toward females. Males in both populations became mature at smaller sizes (< 20 mm) than females (> 30 mm). Consequently, the greater limits on life span, growth rate and shell weight of the Baltic population, but the similar physiological responses in e.g. antioxidant capacity and cell turn over rates in both populations, suggest a more constrained and less adapted life history of individuals of the Baltic Sea due to harsher abiotic conditions as compared to their native habitats.