Metabolic rates and spontaneous swimming activity of two krill species (Euphausiacea) under different temperature regimes in the St. Lawrence Estuary, Canada

Abstract

Two dominant krill species Meganyctiphanes norvegica (Sars, 1857) and Thysanoessa raschii (Sars, 1864) coexist in the subarctic waters of lower St. Lawrence Estuary, Canada. Both species perform diel vertical migrations representing often large displacements of ~100–150 m through several temperature regimes. We studied the impact of temperature, a fundamental factor controlling the metabolism of ectothermic species, on the metabolic rate and swimming activity of the two species. Annular respirometers were used to quantify simultaneously oxygen consumption (ṀO₂ g^–1 wet mass) and the spontaneous swimming activity of individual krill over a period of 24 h at six temperatures, by intermittent-flow respirometry. Both species significantly increased their low routine and maximal metabolic rates from 0 °C to 15 °C, suggesting high thermal plasticity. The spontaneous swimming activity of M. norvegica was reduced to almost zero at 0 °C, whereas T. raschii swam 1.0 cm s^–1 at this temperature. Based on swimming performance, M. norvegica might avoid the cold intermediate layer (CIL, < 1 °C) in the estuary, which coincides with actual daytime distribution below the CIL in the warmer deep-water layer. Despite the rare occurrence of 15 °C in the estuary, both species still showed high metabolic and swimming performance at that temperature. High and differential thermal plasticity of both krill species might have important ecological consequences for their distribution patterns in their natural environment, as energy requirements differ in the two species.