1. Aquatic Lifestyle:
* Gills for Respiration: Fish breathe underwater using gills, specialized organs that extract oxygen from the water.
* Streamlined Body Shape: Their bodies are often streamlined to reduce drag while swimming.
* Fins for Movement: Fish use fins to propel themselves through the water and to maneuver.
2. Skeleton:
* Bones or Cartilage: Fish can have either bony skeletons (bony fish) or cartilaginous skeletons (cartilaginous fish like sharks and rays).
* Scales: Most fish have scales that protect their skin and help with buoyancy.
3. Reproduction:
* External Fertilization: Most fish reproduce through external fertilization, where eggs are released by the female and fertilized by the male in the water.
* Eggs or Live Young: Some fish lay eggs, while others give birth to live young.
4. Sensory Systems:
* Lateral Line: Fish have a lateral line system, a network of sensory organs that detect vibrations and pressure changes in the water.
* Electroreception: Some fish, like sharks and rays, can detect electrical fields in the water.
5. Circulatory System:
* Single Circulation: Fish have a single circulatory system, meaning the blood flows through the heart only once per complete circuit.
6. Temperature Regulation:
* Ectothermic: Fish are ectothermic, meaning they rely on external sources of heat to regulate their body temperature.
7. Diversity:
* Vast Number of Species: There are over 30,000 species of fish, making them the most diverse group of vertebrates.
Other Notable Differences:
* Lack of Limbs: Fish lack the four limbs typical of other vertebrates.
* Lack of Lungs: Fish do not have lungs and cannot breathe air directly.
* Specialized Jaws: Many fish have specialized jaws for feeding on different types of prey.
It's important to note that fish are a very diverse group and there are many exceptions to these general characteristics.