Question: @catshark When it comes to transporting oxygen around the body, how has your body structure adapted to aid in this process while underwater?

Lesser-Spotted Catshark answered on 27 Nov 2017:

Catharks have a closed circulatory system which is powered by their two-chambered heart pumping blood though their arteries/veins. Water passes usually through the mouth and over the gills which contain large numbers of capillaries where oxygen is exchanged with carbon dioxide. In addition, catsharks have a modified gill slit called a spiracle which sits next to their eye. This acts as pump allowing the intake of water to pass over their gills independent from their mouth, which can be quite useful when you have a mouth full of fish!