Automated Non-invasive Video-Microscopy of Oyster Spat Heart Rate during Acute Temperature Change: Impact of Acclimation Temperature

Intraspecific variations in oyster (Magallana gigas) ploidy does not affect physiological responses to microplastic pollution

Recent advances in genetic manipulation such as triploid breeding and artificial selection, have rapidly emerged as valuable hatchery methodologies for enhancing seafood stocks. The Pacific oyster Magallana gigas is a leading aquaculture species worldwide and key ecosystem engineer that has received particular attention in this field of science. In light of the growing recognition of the ecological effects of intraspecific variation, oyster polyploids provide a valuable opportunity to assess whether intraspecific diversity affects physiological responses to environmental stressors. While the responses of diploid and triploid oysters to climate change have been extensively investigated, research on their sensitivity to environmental pollution remains scarce. Here, we assess whether genotypic (i.e., ploidy) variation within Magallana gigas affects physiological responses to microplastic pollution. We show that diploid and triploid M. gigas have similar clearance rates and ingest similar amounts of microplastics under laboratory-controlled condition. In addition, they exhibited similar heart rates after prolonged exposure to microplastic leachates. Our findings suggest that intraspecific variations within M. gigas ploidy does not affect oyster responses to microplastic pollution. However, regardless of ploidy, our work highlights significant adverse effects of microplastic leachates on the heart rate of M. gigas and provides evidence of microplastic ingestion in the laboratory.

Ioxynil and diethylstilbestrol impair cardiac performance and shell growth in the mussel Mytilus coruscus

The herbicide ioxynil (IOX) and the synthetic estrogen diethylstilbestrol (DES) are environmentally relevant contaminants that act as endocrine disruptors (EDCs) and have recently been shown to be cardiovascular disruptors in vertebrates. Mussels, Mytilus coruscus, were exposed to low doses of IOX (0.37, 0.037 and 0.0037 mg/L) and DES (0.27, 0.027 and 0.0027 mg/L) via the water and the effect monitored by generating whole animal transcriptomes and measuring cardiac performance and shell growth. One day after IOX (0.37 and 0.037 mg/L) and DES (0.27 and 0.027 mg/L) exposure heart rate frequency was decreased in both groups and 0.27 mg/L DES significantly reduced heart rate frequency with increasing time of exposure (P < 0.05) and no acclimatization occurred. The functional effects were coupled to significant differential expression of genes of the serotonergic synapse pathway and cardiac-related genes at 0.027 mg/L DES, which suggests that impaired heart function may be due to interference with neuroendocrine regulation and direct cardiac effect genes. Multiple genes related to detoxifying xenobiotic substances were up regulated and genes related to immune function were down regulated in the DES group (vs. control), indicating that detoxification processes were enhanced, and the immune response was depressed. In contrast, IOX had a minor disrupting effect at a molecular level. Of note was a significant suppression (P < 0.05) by DES of shell growth in juveniles and lower doses (< 0.0027 mg/L) had a more severe effect. The shell growth depression in 0.0027 mg/L DES-treated juveniles was not accompanied by abundant differential gene expression, suggesting that the effect of 0.0027 mg/L DES on shell growth may be direct. The results obtained in the present study reveal for the first time that IOX and DES may act as neuroendocrine disrupters with a broad spectrum of effects on cardiac performance and shell growth, and that DES exposure had a much more pronounced effect than IOX in a marine bivalve.

Moving average and standard deviation thresholding (MAST): a novel algorithm for accurate R-wave detection in the murine electrocardiogram

Advances in implantable radio-telemetry or diverse biologging devices capable of acquiring high-resolution ambulatory electrocardiogram (ECG) or heart rate recordings facilitate comparative physiological investigations by enabling detailed analysis of cardiopulmonary phenotypes and responses in vivo. Two priorities guiding the meaningful adoption of such technologies are: (1) automation, to streamline and standardize large dataset analysis, and (2) flexibility in quality-control. The latter is especially relevant when considering the tendency of some fully automated software solutions to significantly underestimate heart rate when raw signals contain high-amplitude noise. We present herein moving average and standard deviation thresholding (MAST), a novel, open-access algorithm developed to perform automated, accurate, and noise-robust single-channel R-wave detection from ECG obtained in chronically instrumented mice. MAST additionally and automatically excludes and annotates segments where R-wave detection is not possible due to artefact levels exceeding signal levels. Customizable settings (e.g. window width of moving average) allow for MAST to be scaled for use in non-murine species. Two expert reviewers compared MAST's performance (true/false positive and false negative detections) with that of a commercial ECG analysis program. Both approaches were applied blindly to the same random selection of 270 3-min ECG recordings from a dataset containing varying amounts of signal artefact. MAST exhibited roughly one quarter the error rate of the commercial software and accurately detected R-waves with greater consistency and virtually no false positives (sensitivity, Se: 98.48% ± 4.32% vs. 94.59% ± 17.52%, positive predictivity, +P: 99.99% ± 0.06% vs. 99.57% ± 3.91%, P < 0.001 and P = 0.0274 respectively, Wilcoxon signed rank; values are mean ± SD). Our novel, open-access approach for automated single-channel R-wave detection enables investigators to study murine heart rate indices with greater accuracy and less effort. It also provides a foundational code for translation to other mammals, ectothermic vertebrates, and birds.

Editorial: Untangling the oxygen transport cascade: a tribute to Peter Frappell (Frapps)

This collection of research articles was put together in honour of respiratory physiologist Professor Peter Frappell's (Frapps's) academic achievements. It encompasses various topics relating to the oxygen transport cascade, which was central to Frapps' career as a comparative physiologist. This issue highlights the diversity and outreach of his influence on the field and his pioneering spirit; promoting novel perspectives, methodologies and research techniques. This issue also demonstrates how Frapps' knowledge and scientific findings answered some of the fundamental questions within the field of respiratory physiology while creating and fostering a rather unique work atmosphere in the laboratories he led. We thank Frapps for the contributions he has made and the friendships he has nurtured over his career. Cheers, Frapps - we love you mate!

Mussel acclimatization to high, variable temperatures is lost slowly upon transfer to benign conditions

Climate change is increasing the temperature variability animals face, and thermal acclimatization allows animals to adjust adaptively to this variability. Although the rate of heat acclimatization has received some study, little is known about how long these adaptive changes remain without continuing exposure to heat stress. This study explored the rate at which field acclimatization states are lost when temperature variability is minimized during constant submersion. California mussels (Mytilus californianus) with different acclimatization states were collected from high- and low-zone sites (∼12 versus ∼5°C daily temperature ranges, respectively) and then kept submerged at 15°C for 8 weeks. Each week, the cardiac thermal performance of mussels was measured as a metric of acclimatization state: critical (Tcrit) and flatline (Tflat) temperatures were recorded. Over 8 weeks of constant submersion, the mean Tcrit of high-zone mussels decreased by 1.07°C from baseline, but low-zone mussels' mean Tcrit was unchanged. High- and low-zone mussels' mean maximum heart rate (HR) and resting HR decreased ∼12 and 35%, respectively. Tflat was unchanged in both groups. These data suggest that Tcrit and HR are more physiologically plastic in response to the narrowing of an animal's daily temperature range than Tflat is, and that an animal's prior acclimatization state (high versus low) influences the acclimatory capacity of Tcrit Approximately 2 months were required for the cardiac thermal performance of the high-zone mussels to reach that of the low-zone mussels, suggesting that acclimatization to high and variable temperatures may persist long enough to enable these animals to cope with intermittent bouts of heat stress.

Thermotolerance Divergence Revealed by the Physiological and Molecular Responses in Two Oyster Subspecies of Crassostrea gigas in China

Investigating the physiological mechanisms of closely related species that exhibit distinct geographic distributions and thermal niches is essential for understanding their thermal tolerance capacities and local adaptations in view of climate warming. The variations in upper thermal limits (LT50) under acute heat shock and cardiac activity, standard metabolic rate (SMR), anaerobic metabolite production and molecular responses (expression of molecular chaperones and glycolysis metabolism genes) under increasing temperatures in two oyster subspecies were studied. The populations of two oyster subspecies, Crassostrea gigas gigas and C. gigas angulata, exhibit different latitudinal distributions along the northern and southern coastlines of China, respectively, which experience different environmental conditions. The LT50 was significantly higher, by ∼1°C, in the southern than in the northern oysters. In both subspecies, temperature increases had powerful effects on heart rate, SMR and gene expression. The southern oysters had the highest Arrhenius breakpoint temperatures for heart rate (31.4 ± 0.17°C) and SMR (33.09°C), whereas the heart rate (28.86 ± 0.3°C) and SMR (29.22°C) of the northern oysters were lower. The same patterns were observed for the Q 10 coefficients. More thermal sensitivity was observed in the northern oysters than in their southern counterparts, as the heat-shock proteins (HSPs) in the northern oysters were expressed first and had a higher induction at a lower temperature than those of southern oysters. Furthermore, different expression patterns of energetic metabolism genes (HK, PK, and PEPCK) were observed. In the northern oysters, increasing anaerobic glycolysis genes (PEPCK) and end products (succinate) were found at 36-43°C, indicating a transition from aerobic to anaerobic metabolism and a lower aerobic scope compared with the southern oysters. These two subspecies experience different environmental conditions, and their physiological performances suggested species-specific thermal tolerance windows in which the southern oysters, with mild physiological flexibility, had a higher potential capability to withstand heat stress. Overall, our results indicate that comparing and unifying physiological and molecular mechanisms can provide a framework for understanding the likely effects of global warming on marine ectotherms in intertidal regions.

Hypoxic acclimation leads to metabolic compensation after reoxygenation in Atlantic salmon yolk-sac alevins

Hypoxia is common in aquatic environments and has substantial effects on development, metabolism and survival of aquatic organisms. To understand the physiological effects of hypoxia and its dependence on temperature, metabolic rate ( [Formula: see text] ) and cardiorespiratory function were studied in response to acute hypoxia (21→5kPa) at different measurement temperatures (T_a; 4, 8 and 12°C) in Salmo salar alevins that were incubated under normoxic conditions (P_O2=21kPa) or following hypoxic acclimation (P_O2=10kPa) as well as two different temperatures (4°C or 8°C). Hypoxic acclimation lead to a developmental delay manifested through slower yolk absorption. The general response to acute hypoxia was metabolic depression (~60%). Hypoxia acclimated alevins had higher [Formula: see text] s when measured in normoxia than alevins acclimated to normoxia. [Formula: see text] s were elevated to the same degree (~30% per 4°C change) irrespective of T_a. Under severe, acute hypoxia (~5kPa) and irrespective of T_a or acclimation, [Formula: see text] s were similar between most groups. This suggests that despite different acclimation regimes, O₂ transport was limited to the same degree. While cardiorespiratory function (heart-, ventilation rate) was unchanged in response to acute hypoxia after normoxic acclimation, hypoxic acclimation led to cardiorespiratory changes predominantly in severe hypoxia, indicating earlier onset and plasticity of cardiorespiratory control mechanisms. Although [Formula: see text] in normoxia was higher after hypoxic acclimation, at the respective acclimation P_O2, [Formula: see text] was similar in normoxia and hypoxia acclimated alevins. This is indicative of metabolic compensation to an intrinsic [Formula: see text] at the acclimation condition in hypoxia-acclimated alevins after re-exposure to normoxia.