ABSTRACT
Considering that swim-flume or chasing methods fail in the estimation of maximum metabolic rate and in the estimation of Aerobic Scope (AS) of sedentary or sluggish aquatic ectotherms, we propose a novel conceptual approach in which high metabolic rates can be obtained through stimulation of organism metabolic activity using high and low non-lethal temperatures that induce high (HMR) and low metabolic rates (LMR), This method was defined as TIMR: Temperature Induced Metabolic Rate, designed to obtain an aerobic power budget based on temperature-induced metabolic scope which may mirror thermal metabolic scope (TMS = HMR-LMR). Prior to use, the researcher should know the critical thermal maximum (CT max) and minimum (CT min) of animals, and calculate temperature TIMR max (at temperatures -5-10% below CT max) and TIMR min (at temperatures +5-10% above CT min), or choose a high and low non-lethal temperature that provoke a higher and lower metabolic rate than observed in routine conditions. Two sets of experiments were carried out. The first compared swim-flume open respirometry and the TIMR protocol using Centropomus undecimalis (snook), an endurance swimmer, acclimated at different temperatures. Results showed that independent of the method used and of the magnitude of the metabolic response, a similar relationship between maximum metabolic budget and acclimation temperature was observed, demonstrating that the TIMR method allows the identification of TMS. The second evaluated the effect of acclimation temperature in snook, semi-sedentary yellow tail (Ocyurus chrysurus), and sedentary clownfish (Amphiprion ocellaris), using TIMR and the chasing method. Both methods produced similar maximum metabolic rates in snook and yellowtail fish, but strong differences became visible in clownfish. In clownfish, the TIMR method led to a significantly higher TMS than the chasing method indicating that chasing may not fully exploit the aerobic power budget in sedentary species. Thus, the TIMR method provides an alternative way to estimate the difference between high and low metabolic activity under different acclimation conditions that, although not equivalent to AS may allow the standardized estimation of TMS that is relevant for sedentary species where measurement of AS via maximal swimming is inappropriate.
ABSTRACT
In this article, we pointed out that understanding the physiology of differential climate change effects on organisms is one of the many urgent challenges faced in ecology and evolutionary biology. We explore how physiological ecology can contribute to a holistic view of climate change impacts on organisms and ecosystems and their evolutionary responses. We suggest that theoretical and experimental efforts not only need to improve our understanding of thermal limits to organisms, but also to consider multiple stressors both on land and in the oceans. As an example, we discuss recent efforts to understand the effects of various global change drivers on aquatic ectotherms in the field that led to the development of the concept of oxygen and capacity limited thermal tolerance (OCLTT) as a framework integrating various drivers and linking organisational levels from ecosystem to organism, tissue, cell, and molecules. We suggest seven core objectives of a comprehensive research program comprising the interplay among physiological, ecological, and evolutionary approaches for both aquatic and terrestrial organisms. While studies of individual aspects are already underway in many laboratories worldwide, integration of these findings into conceptual frameworks is needed not only within one organism group such as animals but also across organism domains such as Archaea, Bacteria, and Eukarya. Indeed, development of unifying concepts is relevant for interpreting existing and future findings in a coherent way and for projecting the future ecological and evolutionary effects of climate change on functional biodiversity. We also suggest that OCLTT may in the end and from an evolutionary point of view, be able to explain the limited thermal tolerance of metazoans when compared to other organisms.
ABSTRACT
Studies of thermal tolerance in marine ectotherms are key in understanding climate effects on ecosystems; however, tolerance of their larval stages has rarely been analyzed. Larval stages are expected to be particularly sensitive. Thermal stress may affect their potential for dispersal and zoogeographical distribution. A mismatch between oxygen demand and the limited capacity of oxygen supply to tissues has been hypothesized to be the first mechanism restricting survival at thermal extremes. Therefore, thermal tolerance of stage zoea I larvae was examined in two populations of the Chilean kelp crab Taliepus dentatus, which are separated by latitude and the thermal regime. We measured temperature-dependent activity, oxygen consumption, cardiac performance, body mass and the carbon (C) and nitrogen (N) composition in order to: (1) examine thermal effects from organismal to cellular levels, and (2) compare the thermal tolerance of larvae from two environmental temperature regimes. We found that larval performance is affected at thermal extremes indicated by decreases in activity, mainly in maxilliped beat rates, followed by decreases in oxygen consumption rates. Cardiac stroke volume was almost temperature-independent. Through changes in heart rate, cardiac output supported oxygen demand within the thermal window whereas at low and high temperature extremes heart rate declined. The comparison between southern and central populations suggests the adaptation of southern larvae to a colder temperature regime, with higher cardiac outputs due to increased cardiac stroke volumes, larger body sizes but similar body composition as indicated by similar C:N ratios. This limited but clear differentiation of thermal windows between populations allows the species to widen its biogeographical range.