Responses to Environmental Stressors in Developing Animals: Costs and Benefits of Phenotypic Plasticity

Abstract

The phenotype of an animal results from interaction between genetic information and environmental influences. Embryos and larvae of most animals are freely exposed to the environment, so environmental influences may start affecting development as soon as egg deposition. A response to the environment requires information about the environment, which then triggers modifications in cell and organ function. Even in the earliest developmental stages, receptors may be functional, and a modification of receptor activity may induce physiological responses or modifications in cell proliferation. In embryos, operation of these control loops, which in adults typically are humoral or neuronal loops, is hampered by the somewhat delayed development of the nervous system. In addition, so-called critical windows may severely restrict the time of responsiveness to certain signals or stressors during development. Nevertheless, modifications in heart rate, ventilation, or metabolic activity demonstrate the existence of physiological plasticity in earliest developmental stages. In addition, differences between individuals or populations in cell number and organ size reveal a remarkable plasticity in structural development. Flexibility and plasticity certainly are beneficial because aerobic metabolism, for example, can be adjusted to changes in oxygen availability, ensuring an optimal outcome especially in an environment with variable oxygen tensions, like aquatic environments. The timing of developmental milestones like hatching can be adjusted to optimal environmental conditions in terms of oxygen availability, temperature, or humidity or in response to biotic cues from conspecifics or predators. On the other hand, any response takes time and additional energy and thus may slow down development and growth rate and thus increase the risk of predation in vulnerable larval stages. In species with thermal sex determination, the environmental influence may result in an unbalanced sex ratio of a population, which obviously may be very disadvantageous for the propagation of a population. Organ size and function may be adjusted to the current situation encountered during development and cannot be reversed if environmental conditions return to previous settings. In this case reduced fitness may be the consequence. Our discussion of these options reveals that a certain degree of phenotypic plasticity is essential and required for a species to prosper in a variable environment, and certainly many ecosystems are changing, or predicted to change, in a dramatic fashion. This flexibility derived from phenotypic plasticity does, however, come with a cost and may occasionally result in reduced fitness and disadvantageous phenotypes in the less plastic adult animals.