The Standard Two Composite Class of Rangiwahia School asks :-

What do pauas breathe out?

Harry Taylor, a zoologist at the University of Canterbury and who is studying paua, responded.

Paua breathe in (inhale) sea water between the head and the edge of the shell near the end of the row of four or five holes in the shell.

The water enters a space, called the mantle cavity, which lies underneath these openings and contains a pair of feathery gills. The water then passes through the gills and is breathed out (exhaled) through the holes. Normally the water is moved slowly by microscopic hairs on the head and gills, but the paua can also jet out water by pulling the shell quickly downwards.

Paua, like other animals, take in oxygen to convert the food they eat (seaweed) into energy and release another gas called carbon dioxide. In paua, the gills transfer oxygen from sea water into the blood. Interestingly, paua blood is blue not red; the blue substance carries the oxygen to where it is needed in the body. The gills also release carbon dioxide.

So the simple answer to your question is that paua breathe out sea water which contains less oxygen and more carbon dioxide than they breathe in. However the mantle cavity also contains the openings of the gut and the kidneys so the exhaled water contains excretory products. If the animal is irritated, it releases slimy mucus through the holes to put off its attackers. The exhalent openings are also used during breeding for the release of milky clouds of eggs and sperm.