Here are some key differences between puffer fish and sea urchins:
1. Body structure: Puffer fish have a bony skeleton and a streamlined body, while sea urchins have a hard, spiny exoskeleton and a round or oval body.
2. Feeding: Puffer fish are predators that feed on small animals like crustaceans, mollusks, and other fish, while sea urchins are grazers or filter feeders that feed on algae and small organisms in the water.
3. Reproduction: Puffer fish reproduce sexually, and females lay eggs that are fertilized by males, while sea urchins also reproduce sexually, but they release sperm and eggs into the water for external fertilization.
4. Habitat: Puffer fish are found in a variety of habitats, including freshwater rivers and lakes, as well as brackish and marine waters, while sea urchins are primarily marine animals and are found in oceans around the world.
Overall, puffer fish and sea urchins are very different organisms, and they are not closely related.