MEPS 210:175-184 (2001)  -  doi:10.3354/meps210175

ABSTRACT: A population of the marine isopod Ligia exotica was found in an inland habitat subject to unpredictable aridity. We studied the demography of littoral and inland populations in an attempt to define the variations in life history enabling this inland invasion of L. exotica. Although the 2 populations have similar steady recruitment and age structures, the patterns of size-specific contribution to yearly reproduction differ. The reproductive event itself may cause mortality in females. Both the net reproductive rate and the intrinsic rate of population growth show that both populations are increasing at similar rates. The longer generation time and the greater minimum age/size at reproduction of the inland population indicate that inland females may postpone the onset of reproduction. This delayed maturity may be an adaptive strategy allowing the inland colonization of L. exotica. Inland females are larger at time of reproduction, invest more in reproduction, and produce fewer but larger eggs/juveniles than do littoral females. These differences in the life history traits of L. exotica suggest life history plasticity in response to environmental changes. The production of fewer but larger eggs/juveniles of the inland population as a consequence of delayed maturity suggests that increasing offspring size is more important than higher fecundity in enabling L. exotica to colonize an inland habitat. The smaller individuals of L. exotica, with a lower resistance to desiccation due to high surface-area/volume ratios, have a higher mortality, especially during the juvenile stage. Because of the periodical and unpredictable aridity of the inland habitat, the mortality of inland juveniles is higher than that of littoral juveniles, and the strategy of producing larger offspring may favor inland colonization of L. exotica. For the successful invasion from littoral to inland areas, mortality may be the main selective factor determining life history traits: age and body size at maturation may be determined by age/size-specific survivorship, and the mortality of juveniles would seem to be size-dependent.

KEY WORDS: Life history plasticity · Ligia exotica · Offspring size · Reproductive strategy