Thermal Adaptation - Behavioural Thermoregulation

An ectotherm is an organism with internal temperature that is similar to its external environment. Fish, reptiles and invertebrates are most prominent examples of ectotherms. The issue with ectothermy is that the increasing environment increases internal temperature, affecting physiology such as metabolism, circulation and reproduction (Speed et al. 2012). A change in body temperature by 1°C alters the rate of many physiological processes by 6–10% (Johnston & Bennett, 1996). For ectotherms to maintain optimal or preferred body temperature, they rely significantly on behavioural adaptations. This blog post will discuss the behavioural thermoregulation of ectothermic marine and freshwater fishes, and the trade-offs involved with this process.

   

The movement of fish species is dependent on many environmental variables. These include resource availability (prey), shelter from predation, oxygen availability, water currents, tidal movements etc. (Ward et al. 2010 & Speed et al. 2012). It is important when studying behavioural thermoregulation to ensure movement is directly caused by a need to alter environmental temperature, rather than a combination of other variables. This is difficult to achieve with fish species, considering that movement to warmer areas of water generally increases risk of predation. This presents a trade-off situation; growth, reproductive or survival related benefits from thermal refuge use must outweigh the risks and cost of refuge use for the strategy to make biological sense (Westhoff et al. 2014).

A 2012 study was conducted on the behavioural thermoregulation of reef sharks in areas of the Ningaloo Reef in Western Australia. It was found that on average, body temperatures of female blacktip reef sharks (Carcharhinus melanopterus) were consistently warmer than average (±SE) water temperature by 1.3 ± 0.57°C, as a result of behavioural thermoregulation (Speed et al. 2012). In 1978 a study was conducted on the sand goby (Pomatoschistus minutus) in Norway, an example of an extremely eurythermal fish (adapted to survive between 5 and 22°C.) Common for almost all the fish tested was a period of about 10 min immediately after transfer to the gradient trough when they hovered along the sides (Hesthagen, 1979). This period of time is inferred to be for the fish to determine its thermal preferendum, when provided with a range of different temperatures to select.

Behavioural thermoregulation is vital to the optimal growth and survival of aquatic and marine ectotherms. Studies of fish such as sharks (large scale predator) through to sand goby have shown that the majority of fish have a thermal preferendum, where growth and survival rates are at their best. However, the energy expended and risks involved with achieving thermal preferendum are varied greatly among species. The common ecological situation of risk vs reward is evident in behavioural thermoregulation.

References:

Hesthagen, I. H, 1979, ‘Temperature selection and avoidance in the sand goby, Pomatoschistus minutus (Pallas), collected at different seasons’,  Environmental Biology of Fish, Vol. 4, No. 4, pg 369-377

Johnston, I. A. & Bennett, A. F, 1996, ‘Animals and Temperature : Phenotypic and Evolutionary Adaptation’. Cambridge: Cambridge University Press.

Speed, C. W. Meekan, M. G. Field, I. C. McMahon, C. R. Bradshaw, C. J. A, 2012, ‘Heat-seeking sharks: support for behavioural thermoregulation in reef sharks’, Marine Ecology Progress Series, Vol. 463: pg 231–244

Ward, A. J. W. Hensor, E. M. A. Webster, M. M. Hart, P. J. B, 2010, ‘Behavioural thermoregulation in two freshwater fish species’, Journal of Fish Biology (2010) Vol. 76, pg 2287–2298  

Westhoff, J. T. Paukert, C. Ettinger-Dietzel, S. Dodd, H. Siepker, M, 2014, ‘Behavioural thermoregulation and bioenergetics of riverine smallmouth bass associated with ambient cold-period thermal refuge’, Ecology of Freshwater Fish 2016: Vol. 25, pg 72–85