What two organ system do gills belong to

I know one of them is the respiratory system but I’m not to sure about the second one I think it’s the circulatory but I’m not to sure

Although each organ (see Tissues and Organs) has its specific functions, organs also function together in groups, called organ systems (see Table: Major Organ Systems). Doctors categorize disorders and their own medical specialties according to organ systems, and this book also is structured largely around the concept of organ systems.

Some examples of organ systems and their functions include

The digestive (or gastrointestinal) system (see Overview of the Digestive System), extending from the mouth to the anus, is responsible for receiving and digesting food and excreting waste. This system includes not only the stomach, small intestine, and large intestine, which move and absorb food, but associated organs such as the pancreas, liver, and gallbladder, which produce digestive enzymes, remove toxins, and store substances necessary for digestion.

The cardiovascular system (see Biology of the Heart) includes the heart (cardio) and blood vessels (vascular). The cardiovascular system is responsible for pumping and circulating the blood.

The musculoskeletal system (see Introduction to the Biology of the Musculoskeletal System) includes the bones, muscles, ligaments, tendons, and joints, which support and move the body.