Utility of statolith elemental chemistry as a proxy for temperature to elucidate the movements of the Irukandji jellyfish species Alatina alata

Abstract

Movements undertaken by marine organisms occur for varying reasons and knowledge surrounding them is critically important for understanding population structures, ecology and for effective management and conservation of species. The objective of this study was to test a hypothesis that the cubozoan jellyfishAlatina alataspends a large part of its life at great depths by utilising the recently validated technique of statolith elemental chemistry. The approach was to ground-truth estimates of temperature based on a previous manipulative laboratory-based experiment, determine Sr:Ca and Ba:Ca ratios in the statoliths ofA. alataat different life-history stages and use elemental chemistry as an environmental marker to estimate their life-time movements. High Sr:Ca values in the core and edge of the statoliths were found which corresponded with the time jellyfish were in shallow waters. Ambient water temperatures estimated for the end period of the jellyfish's lives closely matched known sea surface temperatures in Hawaii, hence supporting a correlation between statolith Sr:Ca and temperature. For the middle section of the statoliths, strong evidence from both Sr:Ca and Ba:Ca ratios suggested that the jellyfish reached depths of at least 200-400 m. Ba:Sr maxima further supported this as Ba concentrations are usually higher below the thermocline. Individual Sr:Ca tracks also suggested that individuals moved over a depth range of tens to hundreds of meters through undertaking regular vertical movements. This study demonstrates that the use of elemental chemistry within cubozoan statoliths has the potential to determine vertical and horizontal movements where temperature gradients are strong.