What is a Fish?

Fish are by far the most diverse of all the vertebrate groups. Discover the shared characteristics that make a fish, a fish. Biology - Animal Kingdom - Learning Points. Fish are aquatic vertebrates. Jellyfish, starfish and other marine invertebrates are not fish at all. Fish are cold-blooded - their temperature is controlled by their external environment. Fish share other important adaptations, such as gills, paired fins, scales and external reproduction. A Twig Curriculum Film. Delivering key learning points. Get straight to the facts in just three minutes – directly linked to core learning, Twig's curriculum films will fit neatly into your lesson plans.