State-dependent energy allocation in the pelagic Antarctic silverfish Pleuragramma antarcticum: trade-off between winter reserves and buoyancy

Icefishes of the perciform suborder Notothenioidei dominate the Antarctic ichthyofauna. These species originated from a benthic ancestor and do not possess a swimbladder. However, some notothenioids have achieved neutral buoyancy through skeletal reductions as well as storage of lipids to reduce body mass relative to seawater. These adaptations enable them to exploit the highly productive pelagic realm. Mobilizing these lipid reserves in periods of critically low food intake may lead to buoyancy problems. Accumulating and conserving these reserves may slow down the development of somatic and reproductive tissues and hence future reproductive output. We constructed a dynamic state variable model to investigate how ingested energy is partitioned over 3 state variables: lipid reserves, structural protein body mass and egg development. Two forms of the model differed in that lipid reserves were either included in or excluded from the total metabolic energy budget of an individual. The model was parameterised for the Antarctic silverfish Pleuragramma antarcticum, a key species in the pelagic food web of the high Antarctic zone of the Southern Ocean. In Pleuragramma, lipids are stored in unique extracellular lipid sacs, which are thought to serve as buoyancy aids and energy reserves. The model predicts optimal habitat selection and an optimal energy allocation strategy by maximizing future reproductive output. The environment is simulated using vertical gradients in water temperature, optical properties, food availability and predation risk. The form of the model that considers lipids as metabolically inactive reserves best replicates field measurements of fat content and yields high values for fitness in Pleuragramma. Uncoupling fat reserves from metabolism, through the development of extracellular lipid sacs, probably represents a key adaptation in the evolution towards a pelagic lifestyle in a fish species with a low scope for activity.