The aquatic world is different from the terrestrial environment because of physical and structural differences. Organisms underwater look different than organisms on land because of the density of water, gravity, and movement underwater. On land, eggs need to be protected against gravity, but under water the eggs look like jelly. Larvae and seeds can travel enormous distances in water. In air, the Oxygen is equally distributed, but in water there are oxygen rich layers and layers with hardly any oxygen at all. To take oxygen from the water, lungs do not work. Animals “breathing“ underwater use gills. With increasing depth, both the intensity of light and the amount of colors reduce.
Plants need light to live and to produce Oxygen, meaning that plants can only live close to the surface. The maximum depth at which sufficient light could penetrate for any meaningful photosynthesis is 180 meters. On land plants dominate the ecosystem, but under water the animals are dominant with respect to the environment. The number and variation in plants under water is very limited compared to land. The food chain underwater can be longer than the food chain on land because animal plankton is eaten by smaller fish, which in turn are eaten by bigger fish.