6ebbba77_Marine_habitats__principles__Living_in_the_sea
[Factor] Breathing [Living on the land] Land animals have lungs for breathing. They breathe in, then out. In the lungs, air passes through ever smaller passages. Lung tissue is thin so that gases can exchange between air and blood. It is relatively difficult for gases to pass from air to liquid and back. Infection through the air is difficult because few organisms can live in air. [Living in the sea] 
Sea animals have gills. Water passes through in one direction since reversing the current each time would cost too much energy. The gills are like lungs: the blood passing through ever smaller passages. Like lung tissue, gill tissue is very thin to allow gases to exchange easily. Gases pass easily from liquid to liquid. Infection through the water is easy. It is also teeming with parasites and viruses and decomposing (infectious) bacteria.
Marine habitats, principles
Living in the sea