Putative neural and endocrine control of thermal acclimation in fish

Fishes can acclimate to a range of temperatures. However, the signalling factors controlling thermal acclimation are not well understood. Here, in two experiments, we examined the putative roles of plasma-borne factors (e.g. hormones) and skin thermoreception in the acclimation process. In experiment 1, 16°C-acclimated Atlantic cod (<i>Gadus morhua</i>) were subjected to a transfusion treatment by injecting plasma from 8°C (cold), 16°C (control) or 21°C (warm) acclimated cod, 10 times over four days. Plasma was collected from donor cod that were 24 h into their acclimation. In experiment 2, 16°C-acclimated goldsinny wrasse (<i>Ctenolabrus rupestris</i>) were exposed to an immersion treatment consisting of 10 s immersions in an 8°C (cold), 16°C (control) or 24°C (warm) water bath, repeated five times daily for five days. These brief immersions allowed for changes to skin temperature but not deeper tissues. Following these treatments, we measured the critical thermal maximum (CT_max) of all fish and the standard metabolic rate (SMR) in cod. Neither the immersions nor transfusions affected fish CT_max. However, the SMR was elevated in cod receiving plasma from cold-acclimated donors, suggesting that circulating molecules transferred from donors had initiated metabolic compensation in recipients. Thyroid hormone plasma levels were not different amongst acclimated donors and thus appear not to have been involved in the metabolic compensation. Our experiments found no evidence that brief, repeated cutaneous exposures to temperature changes can trigger acclimation, but do demonstrate a potential role of haematological endocrine control in metabolic acclimation, although further experiments will be required to investigate this process.