Differential temperature preferences exhibited in the juvenile Antarctic notothenioids Trematomus bernacchii and Trematomus pennellii

Abstract High-latitude Antarctic fishes experience the world’s coldest ocean temperatures, which have remained consistent for millions of years. Living at sub-zero temperatures limits biochemical processes and has driven the evolution of unique physiological mechanisms that allow survival, but at increased energetic costs. We sought to identify whether juvenile Antarctic fishes would behaviorally thermoregulate to slightly warmer temperatures than presently available in this extreme environment, which may offer an energetic reprieve to the high costs of protein malformation, protein denaturation, and antifreeze glycoprotein production at colder temperatures. Two species of juvenile Antarctic rockcod, Trematomus bernacchii ( n = 29) and T. pennellii ( n = 30), were collected from McMurdo Sound and exposed to an annular temperature gradient over two trials permitting each fish to select a temperature preference twice. We calculated temperature preference using three approaches: the average temperature selected, the most commonly selected temperature, and the temperature selected for the longest continuous time. These measures were generally comparable across trials. We found both species exhibited mean temperature preferences significantly above the ambient temperature of the Ross Sea (− 1.9 °C, p ≤ 0.0001). T. bernacchii exhibited temperature preferences near 0 °C (− 0.89–0.49 °C), while T. pennellii preferred significantly ( p < 0.01) warmer temperatures approaching 1 °C (0.92–1.44 °C). Both species preferred lower temperatures and exhibited less movement during the second trial ( p ≤ 0.02). The behavioral results of this study, which demonstrate thermal preference at or above freezing, supports the hypothesis that sub-zero temperatures are associated with energetic costs that could be partially alleviated under conditions of slight warming. Furthermore, the different species-specific temperature preference of two closely related fishes with overlapping niches could indicate T. pennellii are more resilient than T. bernacchii to a warming ocean.