The term for an animal that can only survive in saltwater is "haline." Haline organisms, such as certain species of fish and marine invertebrates, are adapted to life in oceanic environments where salinity levels are high. These animals have developed specialized physiological mechanisms to tolerate and regulate the high salt concentrations found in seawater. Many of them have evolved specific osmotic regulation techniques that allow them to maintain their internal balance of fluids and electrolytes. This adaptation is crucial for their survival and reproduction in marine ecosystems.