Chapter 1. Breathing water in a warming worldChapter 2. What is growth? Pütter, von Bertalanffy and living systemsChapter 3. Channeling the fire of life: fish gills and their geometryChapter 4. Temperature and its biochemical impact on water-breathing animalsChapter 5. Temperature and its effects on water-breathing animalsChapter 6. Objections to the theory and what to learn from themChapter 7. Why bony fishes mature and spawn when they doChapter 8. Growth and reproduction of ChondrichthyesChapter 9. Post-spawning growth accelerationChapter 10. Gigantism in fishChapter 11. Air-breathing fish: between two worldsChapter 12. Growth, respiration, and reproduction of water-breathing arthropodsChapter 13. Molluscs and the GOLTChapter 14. Growth and reproduction in sponges, cnidarians and chaetognathsChapter 15. Limits of adaptationChapter 16. Inferring causality in metabolic scalingChapter 17. Epilog – Size and shape in a three-dimensional universeClimate change and deoxygenation are among the most urgent threats to our planet’s oceans, rivers, and lakes, and the animals that inhabit them. The impact of warming waters and declining oxygen levels includes shifts in individual growth and reproduction, as well as changes in population dynamics and species distribution. These developments demand action, but to act effectively it is important to understand the general mechanisms that drive these trends.This book presents a theoretical framework for explaining how warming waters and deoxygenation affect the growth and reproduction of fish and other water-breathing animals. Despite their diversity, these organisms share fundamental life-history traits shaped by common physiological mechanisms and constraints. We address these mechanisms within the Gill-Oxygen Limitation Theory (GOLT), which explains growth and reproduction in terms of oxygen uptake capacity. Building on decades of research, this book outlines the principles of GOLT and explores its wide-ranging applications in ecology, physiology, and reproductive biology.By presenting this unifying framework, the book provides a foundation for more effective responses to the current crisis in our planet's waters.