Differential life history responses of two Idotea species (Crustacea: Isopoda) to food limitation

Abstract

Organisms have to allocate assimilated energy appropriately among different life his- tory processes in a way that maximizes individual fitness. Energy allocation schemes are shaped by the patterns of food availability that organisms experience in their environment. Therefore, species that have to cope with frequent and unpredictable food limitation are likely to exhibit adaptive life history responses that allow for persistence under adverse conditions and optimal use of temporarily available resources. We compared life history responses of 2 idoteid isopod species with contrasting life styles to different degrees of temporary food deprivation. The coastally distributed Idotea baltica (Pallas) usually has unlimited access to food in benthic macroalgal belts. The oceanic I. metallica Bosc inhabits objects floating at the sea surface and frequently experiences food limitation in oligotrophic open oceans. I. baltica was severely affected when food was temporarily limited, as indicated by a statistically significantly decreased population growth rate (λ). I. metallica was less affected by tem- porary starvation. When food was limited, energy was allocated in I. metallica from development and reproduction to survival, enabling the isopods to outlast periods of unfavourable food conditions. In this species, a reduced λ reflected slower gene propagation rather than a reduced fitness. A high lipid content allows I. metallica to outlast periods of starvation. Freshly hatched I. metallica have higher starvation resistance than juvenile I. baltica, making the demographically important juvenile stage less vulnerable to unpredictable food availability. The plastic responses of I. metallica to variable food levels represent efficient adaptations to life under conditions of low and unpredictable food availabil- ity and provide insight into the nutritional conditions with which organisms have to cope during extended rafting voyages at the surface of oligotrophic oceans.