8 - Reproduction and Development

Abstract

1. Introduction 2. Regulation of Reproduction 2.1. Patterns and Environmental Regulation of Reproduction 2.2. Endocrine Control of Reproduction 3. Effects of Stress on Reproduction 3.1. Effects on Reproductive Performance 3.2. Effects of Stress on the Reproductive Endocrine System 3.3. Thermal Stress: A Special Case? 3.4. Effects of Hypoxia 3.5. Stimulatory Effects of Stress on Reproduction 4. Mechanisms of Stress Action 4.1. The Role of Cortisol: In Vivo Protocols 4.2. The Role of Cortisol: In Vitro Protocols 4.3. Effects of Other Stress Factors 5. Stress Effects on Reproduction in Natural Environments 6. Future Directions Stress has a consistent inhibitory effect on reproductive performance in fish of both sexes, but in a smaller subset of conditions can have stimulatory effects. Inhibitory effects include the suppression of ovarian and testicular development, inhibition of ovulation and spawning, and the production of smaller eggs and larvae. Long-term effects on progeny remain largely undescribed. Endocrine effects include the suppression of hypothalamic, pituitary, and gonadal steroid hormones, with the effects on the production of gonadal androgens and estrogens generally being most profound. Understanding the mechanisms by which stress interferes with reproduction is complicated by the fact that stress-modulated hormones can have systemic effects as well as direct effects on the reproductive endocrine system, and experimental paradigms often don’t allow distinction between the two. With that caveat, there is evidence for inhibitory effects on reproduction from all levels in the stress endocrine axis but strongest evidence is available for the role of corticosteroids, noting that the dominance of the literature by studies on the effects of cortisol is partly a reflection of the relative ease of measurement of steroid hormones. Proposed mechanisms of action include systemic metabolic effects, genomic glucocorticoid receptor-mediated effects and direct action through nongenomic processes that may include substrate competition for steroid-converting enzymes and binding proteins. The majority of stress studies have involved laboratory assessment of captive or cultured fish populations and there is much less information on the effects of stress among free-living, wild fishes. It is clear that reproductive processes can be maintained over a wide range of corticosteroid concentrations, but there is also increasing evidence that social control of reproduction may be mediated by stress processes. There remains scope for improved understanding of stress-reproduction interactions at all levels of reproductive function.