Fish are a diverse group of aquatic vertebrates that are characterized by several key features:
Cold-Blooded Vertebrates: Fish are cold-blooded, or ectothermic, meaning their body temperature is regulated by the surrounding environment.
Aquatic Habitat: While most fish live in water, some species can survive in brackish water (a mix of saltwater and freshwater) or even damp environments for short periods.
Gills for Breathing: Fish breathe through gills, which extract oxygen from water. They have specialized structures called gill filaments that allow them to extract oxygen and excrete carbon dioxide.
Fins for Swimming: Fish have fins that help them navigate and propel themselves through the water. These fins, along with their streamlined body shape, are adaptations for efficient swimming.
Scales and Skin: Fish are covered in scales, which provide protection and reduce friction as they move through the water. Some species also have a slimy mucus layer on their skin for further protection.
Reproduction: Most fish reproduce by laying eggs, although there are exceptions, such as some species of sharks and rays, which give birth to live young.
It's important to note that these characteristics define what a fish is biologically. While fish primarily live in aquatic environments, some species have adaptations that allow them to survive in various conditions for short periods. These adaptations do not change their classification as fish.