Question

why fishes have 2 chambered heart​

Answer 1

Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle (figure a). ... (b) Amphibians have two circulatory routes: one for oxygenation of the blood through the lungs and skin, and the other to take oxygen to the rest of the body.

Answer 2

Because blood leaves the gills and immediately circulates to the rest of the body, the heart does not require additional chambers beyond the first two. Fish and other animals with 2-chambered hearts, therefore, have simpler circulatory systems than animals with lungs and subsequently, 3- and 4-chambered hearts.