What do we call an ectothermic vertebrate that lives in the water and has fins?

An ectothermic vertebrate that lives in water and has fins is classified as a fish. Fish are characterized by their cold-blooded nature, meaning their body temperature is regulated by the surrounding water temperature. They possess gills for breathing oxygen from water and fins for swimming, which are key features that distinguish them from other types of animals.

In contrast, amphibians are generally not fully aquatic (many undergo a life cycle that includes a land stage) and do not have fins. Aquatic mammals, like whales and dolphins, are warm-blooded and have different adaptations for living in water, including lungs for breathing air. Crustaceans, which include organisms like crabs and shrimp, are invertebrates and do not belong to the vertebrate category, as they have exoskeletons instead of internal skeletons.

Understanding the characteristics of these groups helps clarify why fish uniquely fit the description of an ectothermic vertebrate living in water with fins.