RESUMEN
It has been proposed that larger individuals within fish species may be more sensitive to global warming, as a result of limitations in their capacity to provide oxygen for aerobic metabolic activities. This could affect size distributions of populations in a warmer world but evidence is lacking. In Nile tilapia Oreochromis niloticus (n=18, mass range 21-313â g), capacity to provide oxygen for aerobic activities (aerobic scope) was independent of mass at an acclimation temperature of 26°C. Tolerance of acute warming, however, declined significantly with mass when evaluated as the critical temperature for fatigue from aerobic swimming (CTSmax). The CTSmax protocol challenges a fish to meet the oxygen demands of constant aerobic exercise while their demands for basal metabolism are accelerated by incremental warming, culminating in fatigue. CTSmax elicited pronounced increases in oxygen uptake in the tilapia but the maximum rates achieved prior to fatigue declined very significantly with mass. Mass-related variation in CTSmax and maximum oxygen uptake rates were positively correlated, which may indicate a causal relationship. When fish populations are faced with acute thermal stress, larger individuals may become constrained in their ability to perform aerobic activities at lower temperatures than smaller conspecifics. This could affect survival and fitness of larger fish in a future world with more frequent and extreme heatwaves, with consequences for population productivity.
Asunto(s)
Cíclidos , Consumo de Oxígeno , Aclimatación , Animales , Fatiga , Oxígeno , TemperaturaRESUMEN
Social rank in a structured society has been linked to basal levels of glucocorticoids in various species, with dominant individuals generally presenting lower levels than subordinates. The biotic and abiotic factors influencing glucocorticoids levels across social ranks are still, however, unclear in fishes. We investigated the influences of group size, fish size, sex, age, and reproduction type, plus water salinity and temperature, on the basal levels of cortisol, the major stress hormone in fishes. A phylogenetically controlled meta-analysis was performed on data from 72 studies over 22 species of fishes. As expected, dominants generally exhibited lower levels of cortisol than subordinates. More importantly, the strength of the correlation between cortisol and rank was modulated by three main factors, group size, environmental temperature, and fish size. Differences in basal cortisol between dominants and subordinates were significantly greater in small groups (dyadic contexts) when compared to larger groups. Differences between dominants and subordinates were also greater in temperate regions when compared to the tropics, and in species with larger body size. These results provide valuable insights into the links among hierarchy, stress and metabolism in fishes.
Asunto(s)
Jerarquia Social , Hidrocortisona , Animales , Tamaño Corporal , Peces , Glucocorticoides , Hidrocortisona/análisis , TemperaturaRESUMEN
We review the use of complex physiological traits, of tolerance and performance, as biomarkers of the toxicological effects of contaminants in subtropical and tropical freshwater fishes. Such traits are growing in relevance due to climate change, as exposure to contaminants may influence the capacity of fishes to tolerate and perform in an increasingly stressful environment. We review the evidence that the critical oxygen level, a measure of hypoxia tolerance, provides a valuable biomarker of impacts of diverse classes of contaminants. When coupled with measures of cardiorespiratory variables, it can provide insight into mechanisms of toxicity. The critical thermal maximum, a simple measure of tolerance of acute warming, also provides a valuable biomarker despite a lack of understanding of its mechanistic basis. Its relative ease of application renders it useful in the rapid evaluation of multiple species, and in understanding how the severity of contaminant impacts depends upon prevailing environmental temperature. The critical swimming speed is a measure of exercise performance that is widely used as a biomarker in temperate species but very few studies have been performed on subtropical or tropical fishes. Overall, the review serves to highlight a critical lack of knowledge for subtropical and tropical freshwater fishes. There is a real need to expand the knowledge base and to use physiological biomarkers in support of decision making to manage tropical freshwater fish populations and their habitats, which sustain rich biodiversity but are under relentless anthropogenic pressure.
Asunto(s)
Efectos Antropogénicos , Peces , Animales , Biomarcadores , Cambio Climático , Agua DulceRESUMEN
We investigated whether fatigue from sustained aerobic swimming provides a sub-lethal endpoint to define tolerance of acute warming in fishes, as an alternative to loss of equilibrium (LOE) during a critical thermal maximum (CTmax) protocol. Two species were studied, Nile tilapia (Oreochromis niloticus) and pacu (Piaractus mesopotamicus). Each fish underwent an incremental swim test to determine gait transition speed (UGT), where it first engaged the unsteady anaerobic swimming mode that preceded fatigue. After suitable recovery, each fish was exercised at 85% of their own UGT and warmed 1°C every 30â min, to identify the temperature at which they fatigued, denoted as CTswim Fish were also submitted to a standard CTmax, warming at the same rate as CTswim, under static conditions until LOE. All individuals fatigued in CTswim, at a mean temperature approximately 2°C lower than their CTmax Therefore, if exposed to acute warming in the wild, the ability to perform aerobic metabolic work would be constrained at temperatures significantly below those that directly threatened survival. The collapse in performance at CTswim was preceded by a gait transition qualitatively indistinguishable from that during the incremental swim test. This suggests that fatigue in CTswim was linked to an inability to meet the tissue oxygen demands of exercise plus warming. This is consistent with the oxygen and capacity limited thermal tolerance (OCLTT) hypothesis, regarding the mechanism underlying tolerance of warming in fishes. Overall, fatigue at CTswim provides an ecologically relevant sub-lethal threshold that is more sensitive to extreme events than LOE at CTmax.
Asunto(s)
Cíclidos , Peces , Aclimatación , Animales , Humanos , Oxígeno , Natación , TemperaturaRESUMEN
Hypoxia and mercury contamination often co-occur in tropical freshwater ecosystems, but the interactive effects of these two stressors on fish populations are poorly known. The effects of mercury (Hg) on recorded changes in the detailed form of the electrocardiogram (ECG) during exposure to progressive hypoxia were investigated in two Neotropical freshwater fish species, matrinxã, Brycon amazonicus and traíra, Hoplias malabaricus. Matrinxã were exposed to a sublethal concentration of 0.1 mg L-1 of HgCl2 in water for 96 h. Traíra were exposed to dietary doses of Hg by being fed over a period of 30 days with juvenile matrinxãs previously exposed to HgCl2, resulting in a dose of 0.45 mg of total Hg per fish, each 96 h. Both species showed a bradycardia in progressive hypoxia. Hg exposure impaired cardiac electrical excitability, leading to first-degree atrioventricular block, plus profound extension of the ventricular action potential (AP) plateau. Moreover, there was the development of cardiac arrhythmias and anomalies such as occasional absence of QRS complexes, extra systoles, negative Q-, R- and S-waves (QRS complex), and T wave inversion, especially in hypoxia below O2 partial pressures (PO2) of 5.3 kPa. Sub-chronic dietary Hg exposure induced intense bradycardia in normoxia in traira, plus lengthening of ventricular AP duration coupled with prolonged QRS intervals. This indicates slower ventricular AP conduction during ventricular depolarization. Overall, the data indicate that both acute waterborne and sub-chronic dietary exposure (trophic level transfer), at sublethal concentrations of mercury, cause damage in electrical stability and rhythm of the heartbeat, leading to myocardial dysfunction, which is further intensified during hypoxia. These changes could lead to impaired cardiac output, with consequences for swimming ability, foraging capacity, and hence growth and/or reproductive performance.
Asunto(s)
Peces/fisiología , Mercurio/toxicidad , Contaminantes Químicos del Agua/toxicidad , Animales , Characiformes , Ecosistema , Electrocardiografía , Eutrofización , Agua Dulce , HipoxiaRESUMEN
The contribution of air breathing to aerobic metabolic scope and exercise performance was investigated in a teleost with bimodal respiration, the banded knifefish, submitted to a critical swimming speed (U(crit)) protocol at 30°C. Seven individuals (mean ± s.e.m. mass 89±7 g, total length 230±4 mm) achieved a U(crit) of 2.1±1 body lengths (BL) s(-1) and an active metabolic rate (AMR) of 350±21 mg kg(-1) h(-1), with 38±6% derived from air breathing. All of the knifefish exhibited a significant increase in air-breathing frequency (f(AB)) with swimming speed. If denied access to air in normoxia, these individuals achieved a U(crit) of 2.0±0.2 BL s(-1) and an AMR of 368±24 mg kg(-1) h(-1) by gill ventilation alone. In normoxia, therefore, the contribution of air breathing to scope and exercise was entirely facultative. In aquatic hypoxia (P(O(2))=4 kPa) with access to normoxic air, the knifefish achieved a U(crit) of 2.0±0.1 BL s(-1) and an AMR of 338±29 mg kg(-1) h(-1), similar to aquatic normoxia, but with 55±5% of AMR derived from air breathing. Indeed, f(AB) was higher than in normoxia at all swimming speeds, with a profound exponential increase during exercise. If the knifefish were denied access to air in hypoxia, U(crit) declined to 1.2±0.1 BL s(-1) and AMR declined to 199±29 mg kg(-1) h(-1). Therefore, air breathing allowed the knifefish to avoid limitations to aerobic scope and exercise performance in aquatic hypoxia.