The biochemical estimation of age in krill: laboratory calibration and field comparisons

Abstract

Euphausiids play a key role in the marine ecosystem as a link between primary producers and top predators. Understanding their demographic structure is an essential tool to assess growth and recruitment as well as determine how changes in environmental condition might alter their condition and distribution. Since age determination of crustaceans cannot be accomplished using traditional approaches, we evaluated the potential of biochemical products of tissue metabolism (termed lipofuscins) to determine the demographic structure of Euphausiids in field collections. Lipofuscin was extracted from neural tissues (eye and eye-stalk), quantified using their fluorescent intensity, and normalized to protein content to allow comparisons across animal sizes. Age calibration of lipofuscin accumulation was accomplished using Euphausia pacifica eggs collected in the field and reared in the laboratory for over 1 yr. Lipofuscin content extracted from neural tissues of laboratory-reared animals was well correlated with the chronological age of animals. Field populations contained variable levels of lipofuscin dependant on size. Based on comparison with laboratory accumulation rates, sub-adults and adults in the Northeast Pacific were older than 100 days and younger than 1yr based on lipofuscin age estimation. Comparative measures of the Antarctic krill, E. superba, showed much higher lipofuscin levels, suggesting a much longer lifespan than that of the temperate species, E. pacifica. These regional comparisons suggest that biochemical indices allow a practical approach to estimate population age structure, and combined with other measures can provide estimates of vital rates (i.e. longevity, mortality, growth) for krill populations in dynamic environments.