Energy homeostasis as an integrative tool for assessing limits of environmental stress tolerance in aquatic invertebrates.

Energy balance is a fundamental requirement of stress adaptation and tolerance. We explore the links between metabolism, energy balance and stress tolerance using aquatic invertebrates as an example and demonstrate that using key parameters of energy balance (aerobic scope for growth, reproduction and activity; tissue energy status; metabolic rate depression; and compensatory onset of anaerobiosis) can assist in integrating the effects of multiple stressors and their interactions and in predicting the whole-organism and population-level consequences of environmental stress. We argue that limitations of both the amount of available energy and the rates of its acquisition and metabolic conversions result in trade-offs between basal maintenance of a stressed organism and energy costs of fitness-related functions such as reproduction, development and growth and can set limit to the tolerance of a broad range of environmental stressors. The degree of stress-induced disturbance of energy balance delineates transition from moderate stress compatible with population persistence (pejus range) to extreme stress where only time-limited existence is possible (pessimum range). It also determines the predominant adaptive strategy of metabolic responses (energy compensation vs. conservation) that allows an organism to survive the disturbance. We propose that energy-related biomarkers can be used to determine the conditions when these metabolic transitions occur and thus predict ecological consequences of stress exposures. Bioenergetic considerations can also provide common denominator for integrating stress responses and predicting tolerance limits under the environmentally realistic scenarios when multiple and often variable stressors act simultaneously on an organism. Determination of bioenergetic sustainability at the organism's level (or lack thereof) has practical implications. It can help identify the habitats and/or conditions where a population can survive (even if at the cost of reduced reproduction and growth) and those that are incapable of supporting viable populations. Such an approach will assist in explaining and predicting the species' distribution limits in the face of the environmental change and informing the conservation efforts and resource management practices.

Aging reduces reproductive success in mussels Mytilus edulis.

The present study was aimed to determine whether reproductive success constantly increases with age in a relatively short-lived invertebrate with continuous growth - the bivalve mollusc Mytilus edulis or there is an age-related decline such as observed in species with finite growth (mammals, insects, nematodes, etc.). We studied the reproductive output and viability of the offspring during early embryogenesis in females of different sizes and ages, and used allometric approaches to correct for the effects of the body size and to discern pure age-specific effects on these reproductive traits. We have also determined contributions of females of different age and size classes to the total larval pool of a population. Both gonadosomatic index and individual fecundity significantly decreased in the course of aging if the size of the animals was accounted for. The proportion of normally developing embryos declined from almost 100% to 60% in females of 2-10-year-old. We suggest that animals with infinite growth and "slow aging", such as molluscs, undergo senescence, the physiological manifestations of which can be masked by a more pronounced effect of continuously increasing size.

Oxidative stress and expression of chaperones in aging mollusks.

The mechanisms of aging are not well understood in animals with continuous growth such as fish, reptiles, amphibians and numerous invertebrates, including mollusks. We studied the effects of age on oxidative stress, cellular defense mechanisms (including two major antioxidant enzymes, superoxide dismutase (SOD) and catalase), and molecular chaperones in two mollusks--eastern oysters Crassostrea virginica and hard clams Mercenaria mercenaria. In order to detect the age-related changes in these parameters, correction for the effects of size was performed where appropriate to account for growth-related dilution. Fluorescent age pigments accumulated with age in both species. Protein carbonyls did not change with age or size indicating that they are not a good marker of aging in mollusks possibly due to the fast turnover and degradation of oxidized proteins in growing tissues. SOD did not show a compensatory increase with aging in either species, while catalase significantly decreased with age. Mitochondrial heat shock protein (HSP60) decreased with age in mollusks suggesting an age-related decline in mitochondrial chaperone protection. In contrast, changes in cytosolic chaperones were species-specific. HSP70 increased and HSP90 declined with age in clams, whereas in oysters HSP70 expression did not change, and HSP90 increased with aging.