Effect of food availability on the growth and thermal physiology of juvenile Dungeness crabs (Metacarcinus magister)

The impacts of diet on cardiac performance under changing environments

Natural and anthropogenic stressors are dramatically altering environments, impacting key animal physiological traits, including cardiac performance. Animals require energy and nutrients from their diet to support cardiac performance and plasticity; however, the nutritional landscape is changing in response to environmental perturbations. Diet quantity, quality and options vary in space and time across heterogeneous environments, over the lifetime of an organism and in response to environmental stressors. Variation in dietary energy and nutrients (e.g. lipids, amino acids, vitamins, minerals) impact the heart's structure and performance, and thus whole-animal resilience to environmental change. Notably, many animals can alter their diet in response to environmental cues, depending on the context. Yet, most studies feed animals ad libitum using a fixed diet, thus underestimating the role of food in impacting cardiac performance and resilience. By applying an ecological lens to the study of cardiac plasticity, this Commentary aims to further our understanding of cardiac function in the context of environmental change.

Diet effects on ectotherm thermal performance

The environment is changing rapidly, and considerable research is aimed at understanding the capacity of organisms to respond. Changes in environmental temperature are particularly concerning as most animals are ectothermic, with temperature considered a key factor governing their ecology, biogeography, behaviour and physiology. The ability of ectotherms to persist in an increasingly warm, variable, and unpredictable future will depend on their nutritional status. Nutritional resources (e.g. food availability, quality, options) vary across space and time and in response to environmental change, but animals also have the capacity to alter how much they eat and what they eat, which may help them improve their performance under climate change. In this review, we discuss the state of knowledge in the intersection between animal nutrition and temperature. We take a mechanistic approach to describe nutrients (i.e. broad macronutrients, specific lipids, and micronutrients) that may impact thermal performance and discuss what is currently known about their role in ectotherm thermal plasticity, thermoregulatory behaviour, diet preference, and thermal tolerance. We finish by describing how this topic can inform ectotherm biogeography, behaviour, and aquaculture research.

Artificial light at night does not alter heart rate or locomotor behaviour in Caribbean spiny lobster (Panulirus argus): insights into light pollution and physiological disturbance using biologgers

Light pollution is a rapidly growing threat to biodiversity, with many unknown or poorly understood effects on species and ecosystems spanning terrestrial and aquatic environments. Relative to other taxa, the effects of artificial light at night on aquatic invertebrates are poorly understood, despite the abundance and integral significance of invertebrates to marine and freshwater ecosystems. We affixed heart rate and acceleration biologgers to adult Caribbean spiny lobster (Panulirus argus), an ecologically, culturally and economically significant species in the western Atlantic ocean, to test the effect of artificial light at night on this species' physiology and behaviour relative to appropriate controls. The experiment was conducted in a simulated reef mesocosm in The Bahamas with incandescent lighting used to illuminate it at 1 lux, approximating light levels offshore of urban areas. In the conditions tested here, artificial light at night was found to have no effect on heart rate or locomotor activity in P. argus. We observed a dissociation between activity and heart rate at both short-term and long-term temporal scales. Lobsters were more active at night and nocturnal activity was higher in trials closer to new moon; however, heart rate did not vary with diel or lunar cycle. There was less than 8% difference between daytime and night time average heart rate despite the average percentage of time spent active almost tripling in nights versus days, to 19.5% from 7.2%, respectively. Our findings suggest P. argus may have some resilience to low levels of light pollution, which warrants further research on aspects of this species' life history, performance and fitness in the face of this potential anthropogenic disturbance.

The complete mitochondrial genome and phylogenetic analysis of Cancer magister (Decapoda, Cancridae)

The complete mitochondrial (mt) genome of Cancer magister was obtained using next-generation sequencing. The circular genome was 39,658 bp in length, consisting of 13 protein-coding genes, 26 transfer RNA genes, and 2 ribosomal RNA genes. Unfortunately, the control region was not found in mitochondrial genome. Of the 41 genes, 24 were encoded by the heavy strand, while the others were encoded by the light strand. The genome composition with A + T bias (69.90%). The phylogenetic analysis showed that C. magister and Cancer pagurus was clustered together, then grouped with A. alayseae and Gandalfus puia, which may suggest Cancroidea was close with Bythograeidae. The newly described mitochondrial genome may provide valuable data for phylogenetic analysis for Cancridae.

Effects of temperature on survival, moulting, and expression of neuropeptide and mTOR signalling genes in juvenile Dungeness crab (Metacarcinus magister)

Mechanistic target of rapamymcin (mTOR) is a highly conserved protein kinase that controls cellular protein synthesis and energy homeostasis. We hypothesize that mTOR integrates intrinsic signals (moulting hormones) and extrinsic signals (thermal stress) to regulate moulting and growth in decapod crustaceans. The effects of temperature on survival, moulting and mRNA levels of mTOR signalling genes (Mm-Rheb, Mm-mTOR, Mm-AMPKα, Mm-S6K and Mm-AKT) and neuropeptides (Mm-CHH and Mm-MIH) were quantified in juvenile Metacarcinus magister Crabs at different moult stages (12, 19 or 26 days postmoult) were transferred from ambient temperature (∼15°C) to temperatures between 5 and 30°C for up to 14 days. Survival was 97-100% from 5 to 20°C, but none survived at 25 or 30°C. Moult stage progression accelerated from 5 to 15°C, but did not accelerate further at 20°C. In eyestalk ganglia, Mm-Rheb, Mm-AMPKα and Mm-AKT mRNA levels decreased with increasing temperatures. Mm-MIH and Mm-CHH mRNA levels were lowest in the eyestalk ganglia of mid-premoult animals at 20°C. In the Y-organ, Mm-Rheb mRNA levels decreased with increasing temperature and increased during premoult, and were positively correlated with haemolymph ecdysteroid titre. In the heart, moult stage had no effect on mTOR signalling gene mRNA levels; only Mm-Rheb, Mm-S6K and Mm-mTOR mRNA levels were higher in intermoult animals at 10°C. These data suggest that temperature compensation of neuropeptide and mTOR signalling gene expression in the eyestalk ganglia and Y-organ contributes to regulate moulting in the 10 to 20°C range. The limited warm compensation in the heart may contribute to mortality at temperatures above 20°C.

Experimental setup influences the cardiovascular responses of decapod crustaceans to environmental change

The effects of different holding methods on heart rate (HR) changes in the green crab, Carcinus maenas (Linnaeus, 1758), were investigated. Green crabs were held in perforated plastic boxes (with or without a layer of sand) suspended above the bottom of the tank or strapped to a weighted plastic grate. The HR of green crabs classified as unrestrained (plastic box with or without sand) dropped more rapidly compared with restrained (hanging from band, strapped to grate) green crabs. Within 1 h, unrestrained green crabs exhibited periods of cardiac pausing accounting for between 8% and 14% of the hourly time. In contrast, restrained green crabs rarely exhibited cardiac pausing. When the green crabs were subjected to a temperature increase (10–30 °C), the HR of unrestrained green crabs reached higher levels than that of the restrained animals. The four restraining methods were also used to investigate cardiac responses to hypoxia. During progressive hypoxia (100%–20% oxygen), the HR of unrestrained green crabs declined to lower levels than that of the restrained animals. The restraining methods appeared to be more stressful for the green crabs that maintained elevated HRs and were less able to respond to environmental change compared with green crabs that could move freely within a small chamber. This suggests that even subtle changes in experimental design may alter physiological responses.

Application of Miniature Heart Rate Data Loggers for Use in Large Free-Moving Decapod Crustaceans: Method Development and Validation

Cardiovascular responses of decapod crustaceans to environmental challenges have received extensive attention. However, nearly all of these studies have been restricted to lab-based experiments; here we describe a methodology that will enable measurement of heart rate (HR) in free-moving decapods in the field. Data storage tag heart rate and temperature loggers (DST micro-HRT; Star-Oddi) were used to record electrocardiograms (ECG) and HR in large decapod crustaceans. These loggers were originally designed for use in vertebrates and must be surgically implanted in the body cavity near the heart in order to function. We adapted these loggers for external use in large decapod crustaceans. The method involved abrading the carapace directly above the heart and placing the electrodes of the logger directly on top of the dermal tissue. The logger was then secured in place with periphery wax. This method negated some of the more intricate operations used for vertebrates. The rapid setup time of approximately 5 min suggested that animals could be easily instrumented in the field and without the use of anesthetic. The logger was calibrated by simultaneously measuring the HR changes of a West Indian spider crab Mithrax spinosissimus with a pulsed-Doppler flowmeter. The data gathered with the two methods showed a tight correlation during an increase in temperature. The loggers were also successfully implanted in a variety of other large species of aquatic and terrestrial decapods. The data obtained showed that the method works in a broad range of species, under different experimental conditions. In each case, the loggers comprised less than 1% of the body mass and would be suitable for use in animals >300 g. All animals survived the attachment procedures and were feeding and active after removal of the loggers. Nearly all previous cardiac measurements on decapods have been carried out in controlled laboratory settings. The use of these loggers will make significant advances in measuring HR in unrestrained, undisturbed animals in their natural environment during extended periods of time and has the potential to lead to novel findings.