This edited scientific volume examines appendicularians, also known as larvacean tunicates, and their role in marine ecosystems under changing environmental conditions. It brings together research on how these gelatinous zooplankton influence plankton communities, carbon cycling, food webs, and biogeochemical processes, with attention to climate variability and broader global change. The contributions combine ecological observations, physiological studies, and regional case analyses to clarify the importance of appendicularians in coastal and open-ocean systems. Organized around taxonomy, distribution, feeding, population dynamics, and ecosystem effects, the book serves as a specialized reference for marine biologists, oceanographers, ecologists, and advanced students. It highlights why blooms, grazing activity, and particle-processing behavior can shape ecosystem structure and material fluxes in the sea.