Linking environmental salinity to respiratory phenotypes and metabolic rate in fishes: a data mining and modelling approach

Effects of structural remodelling on gill physiology

The complex relationships between the structure and function of fish gills have been of interest to comparative physiologists for many years. Morphological plasticity of the gill provides a dynamic mechanism to reversibly alter its structure in response to changes in the conditions experienced by the fish. The best known example of gill remodelling is the growth or retraction of cell masses between the lamellae, a rapid process that alters the lamellar surface area that is exposed to the water (i.e. the functional lamellar surface area). Decreases in environmental O2 availability and/or increases in metabolic O2 demand stimulate uncovering of the lamellae, presumably to increase the capacity for O2 uptake. This review addresses four questions about gill remodelling: (1) what types of reversible morphological changes occur; (2) how do these changes affect physiological function from the gill to the whole animal; (3) what factors regulate reversible gill plasticity; and (4) is remodelling phylogenetically widespread among fishes? We address these questions by surveying the current state of knowledge of gill remodelling in fishes, with a focus on identifying gaps in our understanding that future research should consider.

The effect of temperature on haemoglobin-oxygen binding affinity in regionally endothermic and ectothermic sharks

Haemoglobin (Hb)-O2 binding affinity typically decreases with increasing temperature, but several species of ectothermic and regionally endothermic fishes exhibit reduced Hb thermal sensitivity. Regionally endothermic sharks, including the common thresher shark (Alopias vulpinus) and lamnid sharks such as the shortfin mako shark (Isurus oxyrinchus), can maintain select tissues and organs warmer than ambient temperature by retaining metabolic heat with vascular heat exchangers. In the ectothermic bigeye thresher shark (Alopias superciliosus), diurnal movements above and below the thermocline subject the tissues, including the blood, to a wide range of operating temperatures. Therefore, blood-O2 transport must occur across internal temperature gradients in regionally endothermic species, and over the range of environmental temperatures encountered by the ectothermic bigeye thresher shark. While previous studies have shown temperature-independent Hb-O2 affinity in lamnid sharks, including shortfin mako, the Hb-O2 affinity of the common and bigeye thresher sharks is unknown. Therefore, we examined the effect of temperature on whole-blood Hb-O2 affinity in common thresher shark and bigeye thresher shark. For comparison, analyses were also conducted on the shortfin mako shark and two ectothermic species, blue shark (Prionace glauca) and spiny dogfish (Squalus acanthias). Blood-O2 binding affinity was temperature independent for common thresher shark and shortfin mako shark, which should prevent internal temperature gradients from negatively affecting blood-O2 transport. Blue shark and spiny dogfish blood-O2 affinity decreased with increasing temperature, as expected, but bigeye thresher shark blood exhibited both a reduced temperature dependence and a high Hb-O2 affinity, which likely prevents large changes in environment temperature and low environmental oxygen from affecting O2 uptake.

Body mass and cell size shape the tolerance of fishes to low oxygen in a temperature-dependent manner

Aerobic metabolism generates 15-20 times more energy (ATP) than anaerobic metabolism, which is crucial in maintaining energy budgets in animals, fueling metabolism, activity, growth and reproduction. For ectothermic water-breathers such as fishes, low dissolved oxygen may limit oxygen uptake and hence aerobic metabolism. Here, we assess, within a phylogenetic context, how abiotic and biotic drivers explain the variation in hypoxia tolerance observed in fishes. To do so, we assembled a database of hypoxia tolerance, measured as critical oxygen tensions (P_crit ) for 195 fish species. Overall, we found that hypoxia tolerance has a clear phylogenetic signal and is further modulated by temperature, body mass, cell size, salinity and metabolic rate. Marine fishes were more susceptible to hypoxia than freshwater fishes. This pattern is consistent with greater fluctuations in oxygen and temperature in freshwater habitats. Fishes with higher oxygen requirements (e.g. a high metabolic rate relative to body mass) also were more susceptible to hypoxia. We also found evidence that hypoxia and warming can act synergistically, as hypoxia tolerance was generally lower in warmer waters. However, we found significant interactions between temperature and the body and cell size of a fish. Constraints in oxygen uptake related to cellular surface area to volume ratios and effects of viscosity on the thickness of the boundary layers enveloping the gills could explain these thermal dependencies. The lower hypoxia tolerance in warmer waters was particularly pronounced for fishes with larger bodies and larger cell sizes. Previous studies have found a wide diversity in the direction and strength of relationships between P_crit and body mass. By including interactions with temperature, our study may help resolve these divergent findings, explaining the size dependency of hypoxia tolerance in fish.

Ecomechanics and the Rules of Life: a Critical Conduit Between the Physical and Natural Sciences

Nature provides the parameters, or boundaries, within which organisms must cope in order to survive. Therefore, ecological conditions have an unequivocal influence on the ability of organisms to perform the necessary functions for survival. Biomechanics brings together physics and biology to understand how an organism will function under a suite of conditions. Despite a relatively rich recent history linking physiology and morphology with ecology, less attention has been paid to the linkage between biomechanics and ecology. This linkage, however, could provide key insights into patterns and processes of evolution. Ecomechanics, also known as ecological biomechanics or mechanical ecology, is not necessarily new, but has received far less attention than ecophysiology or ecomorphology. Here, we briefly review the history of ecomechanics, and then identify what we believe are grand challenges for the discipline and how they can inform some of the most pressing questions in science today, such as how organisms will cope with global change.