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Aquatic animals generally conduct gas exchange in water by extracting dissolved oxygen via specialised respiratory organs called gills, through the skin or across enteral mucosae, although some are evolved from terrestrial ancestors that re-adapted to aquatic environments (e.g. marine reptiles and marine mammals), in which case they actually ...
Secondary aquatic adaptations tend to develop in early speciation as the animal ventures into water in order to find available food. As successive generations spend more time in the water, natural selection causes the acquisition of more adaptations. Animals of later generations may spend most their life in the water, coming ashore for mating.
Organisms that live freely at the ocean surface, termed neuston, include keystone organisms like the golden seaweed Sargassum that makes up the Sargasso Sea, floating barnacles, marine snails, nudibranchs, and cnidarians. Many ecologically and economically important fish species live as or rely upon neuston.
Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water but need to return to land for important activities such as mating, breeding and molting.
Marine iguana: marine iguanas live only on the Galápagos Islands and are not fully adapted to marine life. Although they feed exclusively on marine plants and spend a good deal of their time in the water, they do nest on land and need to bask in the sun to reach their ideal body temperature; they are thus also subject to terrestrial predators ...
The amount of dissolved oxygen in a water body is frequently the key substance in determining the extent and kinds of organic life in the water body. Fish need dissolved oxygen to survive, although their tolerance to low oxygen varies among species; in extreme cases of low oxygen, some fish even resort to air gulping. [25]
[20]: 587 This lack of ossification is an adaptation to save energy when food is scarce. [26] Most of the animals that live in the bathyal zone are invertebrates such as sea sponges, cephalopods, and echinoderms. With the exception of very deep areas of the ocean, the bathyal zone usually reaches the benthic zone on the seafloor. [24]
Coastal wetlands also reduce pollution from human waste, [41] [42] remove excess nutrients from the water column, [43] trap pollutants, [44] and sequester carbon. [45] Further, near-shore wetlands act as both essential nursery habitats and feeding grounds for game fish , supporting a diverse group of economically important species.