What is the respiratory organ of fish?

Gills are specialized respiratory structures found in aquatic animals such as fish, amphibians, crustaceans and molluscs.

Gills are found in aquatic invertebrates and allow oxygen to be extracted from the water. They do not have lungs.

Tracheae are air tubes that carry oxygen-rich air directly to cells. 

Spiders have book lungs, which are specialized respiratory structures with a large surface area for gas exchange.