Cartilaginous vs Bony Fish: Evolutionary Perspectives on Stress and Immune Adaptations

The marine environment is widely considered one of the least explored places on Earth. However, it is home to some of the oldest vertebrate lineages, dating back more than 200 million years, including cartilaginous fish (Chondrichthyes) and bony fish (Osteichthyes). These two ancient groups have developed important differences in their stress physiology, immune systems, and their ability to withstand environmental stressors. Sharks, specifically of the cartilaginous group, are thought to have one of the most distinct immune systems and are widely hypothesized to exhibit resistance to certain diseases—traits that may have contributed to their long-term survival through harsh environmental changes. Cartilaginous species possess features of adaptive immunity, such as immunoglobulins and structural differences in the major histocompatibility complex, along with other components such as physical barriers, immune cells, and elements of the innate immune system. These differ in structure and function from those of bony fish, which also display unique immune adaptations. Additionally, this thesis explores stress physiology, including the neuroendocrine stress response and its influence on immunity and reproductive health. Understanding these systems in each group offers insight into how these animals adapt and survive in an ever-changing marine environment. The present thesis aims to explore and compare the immune and stress responses of cartilaginous and bony fish to understand how evolutionary history, physiology, and ecological pressures have shaped their survival strategies.