Biology subjects

McKenzie, D. J.

Publications and source records attributed to McKenzie, D. J..

4 recordsLinked to original sources

bioRxiv2021

Cardiac and behavioural responses to hypoxia and warming in free-swimming gilthead seabream Sparus aurata

Cardiac and behavioural responses to hypoxia and warming were investigated in free-swimming gilthead seabream Sparus aurata equipped with biologging tags in the peritoneal cavity. After suitable recovery in a holding tank, heart rate (fH) and the variance of tri-axial body acceleration (VARm) were logged during exposure to stepwise progressive hypoxia or warming, comparing when either swimming in a tank or confined to individual respirometer chambers. When undisturbed under control conditions (normoxia, 21 {degrees}C), mean fH was significantly lower in tank than respirometers. In progressive hypoxia (100 - 15% oxygen saturation), mean fH in the tank was significantly lower than respirometers at oxygen levels until 40%, with significant bradycardia in both holding conditions below this. Mean VARm was low and invariant in hypoxia. Warming (21 to 31 {degrees}C) caused progressive tachycardia with no differences in fH between holding conditions. Mean VARm was, however, significantly higher in the tank during warming, with a positive relationship between VARm and fH across all temperatures. Therefore, spontaneous activity contributed to raising fH of fish in the tank during warming. Mean fH in respirometers had a highly significant linear relationship with mean rates of oxygen uptake, considering data from hypoxia and warming together. The high fH of confined S. aurata indicates that static respirometry techniques may bias estimates of metabolic traits in some fish species. Biologging on free-swimming fish revealed novel information about cardiac responses to environmental stressors, which may be closer to responses exhibited by fish in their natural environment. SUMMARY STATEMENTImplantable biologgers were used to provide the first measurements of cardiac responses to hypoxia and warming in a free-swimming fish, revealing that confinement in respirometer chambers raises heart rate, with consequences for estimates of metabolic rates.

physiology↗

bioRxiv2021

Evidence that standard metabolic rate and risk-taking to breathe air are linked to boldness, activity level and exploratory behaviour in a catfish.

We used an air-breathing catfish, Clarias gariepinus, to investigate the hypothesis that individual variation in metabolic rate, and the propensity to take risks to obtain a resource (oxygen from air), would be correlated with behavioural tendencies such as boldness, activity level and exploratory behaviour. The standard metabolic rate (SMR) of 58 juvenile catfish was positively correlated with their rates of aerial respiration in daylight when surfacing was inherently risky. SMR was positively correlated with boldness measured in two contexts, namely the time-lag to resume air-breathing in a potentially dangerous environment (T-res, measured in a respirometer), and the timelag to enter the centre of a novel environment (T-centre, measured in an open field test (OFT)). These two measures of boldness were very highly correlated. Thus, these data support the hypothesis that high SMR and an associated tendency to take risks to acquire resources are linked to increased boldness in animals. Individual SMR was positively correlated with the proportion of time the fish spent moving in the OFT, but was negatively correlated with movement speed. The data confirmed previous observations that these catfish may exhibit a bimodal distribution of T-res phenotypes, whereby individuals either resumed air-breathing relatively rapidly (< 85 min, bold n = 26) or more slowly (> 115 min, shy n = 31) after a startle stimulus. Bold T-res phenotypes had significantly higher SMR than shy; breathed more air during the day, and showed greater boldness but less activity and exploration in the OFT. No parallel bold/shy dichotomy was observed, however, in any measure of boldness in the OFT. Therefore, the data support propositions regarding how SMR and risk-taking should relate to boldness, but provide mixed results about how SMR relates to activity and exploration, and whether bold/shy is a dichotomy or spectrum.

animal behavior and cognition↗

bioRxiv2020

Intraspecific variation in thermal tolerance differs between tropical and temperate fishes

How ectothermic animals will cope with global warming, especially more frequent and intense heatwaves, is a critical determinant of the ecological impacts of climate change. There has been extensive study of upper thermal tolerance limits among fish species but how intraspecific variation in tolerance may be affected by habitat characteristics and evolutionary history has not been considered. Intraspecific variation is a primary determinant of species vulnerability to climate change, with implications for global patterns of impacts of ongoing warming. Using published critical thermal maximum (CTmax) data on 203 marine and freshwater fish species, we found that intraspecific vsariation in upper thermal tolerance varies according to a species latitude and evolutionary history. Notably, freshwater tropical species have lower variation in tolerance than temperate species in the northern hemisphere, which implies increased vulnerability to impacts of thermal stress. The extent of variation in CTmax among fish species has a strong phylogenetic signal, which may indicate a constraint on evolvability to rising temperatures in tropical fishes. That is, in addition to living closer to their upper thermal limits, tropical species may have higher sensitivity and lower adaptability to global warming compared to temperate counterparts. This is evidence that tropical fish communities, worldwide, are especially vulnerable to ongoing climate change.

physiology↗

bioRxiv2020

Tolerance of an acute warming challenge declines with body mass in Nile tilapia: evidence of a link to capacity for oxygen uptake

It is proposed that larger individuals within fish species may be more sensitive to global warming, due to 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 21-313g), capacity to provide oxygen for aerobic activities (aerobic scope) was independent of mass at acclimation temperature (26{degrees}C). Tolerance of acute warming, however, declined significantly with mass when evaluated as critical temperature for fatigue from aerobic swimming (CTswim). The CTswim protocol challenges a fish to meet the oxygen demands of constant intense aerobic exercise while their demands for basal metabolism are accelerated by incremental warming, culminating in fatigue. CTswim elicited pronounced increases in oxygen uptake but maximum rates achieved prior to fatigue declined very significantly with mass. Mass-related variation in CTswim and maximum oxygen uptake rates were positively correlated, which may indicate a causal relationship. When faced with acute thermal stress, larger fishes within populations may become constrained in their ability to swim at lower temperatures than smaller con-specifics. This could affect survival and fitness of larger fish in a world with more frequent and extreme heatwaves, with consequences for population productivity.

ecology↗