Aquatic animal
[1] 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 use lungs to breathe air and are essentially holding their breath when living in water.
Some species of gastropod mollusc, such as the eastern emerald sea slug, are even capable of kleptoplastic photosynthesis via endosymbiosis with ingested yellow-green algae.
Some animals have fully aquatic life stages (typically as eggs and larvae), while as adults they become terrestrial or semi-aquatic after undergoing metamorphosis.
Such examples include amphibians such as frogs, many flying insects such as mosquitoes, mayflies, dragonflies, damselflies and caddisflies, as well as some species of cephalopod molluscs such as the algae octopus (whose larvae are completely planktonic, but adults are highly terrestrial).
Aquatic animals are a diverse polyphyletic group based purely on the natural environments they inhabit, and many morphological and behavioral similarities among them are the result of convergent evolution.
Aquatic animals are subject to pressure from overfishing/hunting, destructive fishing, water pollution, acidification, climate change and competition from invasive species.
Axolotl, a Mexican salamander that retains its larval external gills into adulthood, is the only extant amphibian that remains fully aquatic throughout the entire life cycle.
[citation needed] Most molluscs have gills, while some freshwater gastropods (e.g. Planorbidae) have evolved pallial lungs and some amphibious species (e.g. Ampullariidae) have both.
[citation needed] Aquatic animals play an important role for the environment as indicator species, as they are particularly sensitive to deterioration in water quality and climate change.
The harvesting of aquatic animals, especially finfish, shellfish and inkfish, provides direct and indirect employment to the livelihood of over 500 million people in developing countries, and both the fishing industry and aquaculture make up a major component of the primary sector of the economy.
The United Nations Food and Agriculture Organization estimates that global consumption of aquatic animals in 2022 was 185 million tonnes (live weight equivalent), an increase of 4 percent from 2020.
[16] Aquatic animal are highly perishable and several chemical and biological changes take place immediately after death; this can result in spoilage and food safety risks if good handling and preservation practices are not applied all along the supply chain.
Aquatic animal products also require special facilities such as cold storage and refrigerated transport, and rapid delivery to consumers.
