Final answer:
All fishes have gills, a vertebral column, single-loop blood circulation, and gill-based respiration as common characteristics. Features like an exoskeleton and nutritional deficiencies are not shared by all fish and are not used to define the group.
Step-by-step explanation:
The four characteristics present in all fishes are: gills, a vertebral column, single-loop blood circulation, and respiration. Fishes exhibit a variety of adaptations to their aquatic environments, but these four attributes are common to all fish species.
Gills are essential for oxygen absorption from water. The vertebral column is a structural characteristic of all vertebrates, including fish. Single-loop blood circulation is a circulatory system feature where blood travels from the heart to the gills and then to the rest of the body in a single circuit. Lastly, respiration in fish occurs primarily through the gills, allowing the exchange of oxygen and carbon dioxide with the water.
Characteristics such as an exoskeleton, nutritional deficiencies, and cutaneous respiration are not universally present in all fish species, and thus are not among the traits we can use to define the entire group of fish.