Aquatic respiration in the context of "Gills"

An aquatic animal is any animal, whether vertebrate or invertebrate, that lives in a body of water for all or most of its lifetime. Aquatic animals generally conduct aquatic respiration by extracting dissolved oxygen in water via specialised respiratory organs called gills, through the skin or across enteral mucosae, although some are secondarily aquatic animals (e.g. marine reptiles and marine mammals) evolved from terrestrial ancestors that re-adapted to aquatic environments, 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.

Almost all aquatic animals reproduce in water, either oviparously or viviparously, and many species routinely migrate between different water bodies during their life cycle. 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).

A gill (/ɡɪl/ ) is a specialized respiratory organ that many aquatic animals use for aquatic gas exchange, i.e. to extract dissolved oxygen from water and to excrete carbon dioxide. Branchia (pl.: branchiae, from Ancient Greek βράγχια) is the zoologists' academic name for gills.

The gills of some semi-aquatic clades (e.g. crabs, terrestrial hermit crabs and amphibious fishes), have also adapted to allow air respiration (breathing) on land provided the gills are kept moist. In some terrestrial chelicerates species (tetrapulmonates such as spiders and scorpions), their ancestral gills have evolved into fully air-breathing book lungs. In some planktivorous bony fish species (e.g. silver and bighead carps), the gills are also used as a filter feeding organ via comb-like projections called gill rakers.

Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least partly aquatic. These animals are called "secondarily aquatic" because although all tetrapods descended from freshwater lobe finned fish (see evolution of tetrapods), their more recent ancestors are terrestrial vertebrates that evolved on land for hundreds of millions of years, and their clades only re-adapted to aquatic environment much later.

Unlike primarily aquatic vertebrates (i.e. fish), secondarily aquatic tetrapods (especially aquatic amniotes), while having appendages such as flippers, dorsal fin and tail fins (flukes) that resemble fish fins due to convergent evolution, still have physiology based on their terrestrial ancestry, most notably their air-breathing respiration via lungs (instead of aquatic respiration via gills) and excretion of nitrogenous waste as urea or uric acid (instead of ammonia like most fish). Nearly all extant aquatic tetrapods are secondarily aquatic, with only larval amphibians (tadpoles) being primarily aquatic with gills, and only some species of paedomorphic mole salamanders (most notably the fully aquatic axolotl) retain the gill-based physiology into adulthood.

An octopus (pl.: octopuses or octopodes) is a soft-bodied, eight-limbed mollusc of the order Octopoda (/ɒkˈtɒpədə/, ok-TOP-ə-də). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the centre point of the eight limbs. An octopus can radically deform its shape, enabling it to squeeze through small gaps. They trail their appendages behind them as they swim. The siphon is used for respiration and locomotion (by water jet propulsion). Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse invertebrates.

Octopuses inhabit various ocean habitats, including coral reefs, pelagic waters, and the seabed; some live in the intertidal zone and others at abyssal depths. Most species grow quickly, mature early, and are short-lived. In most species, the male uses a specially-adapted arm to deliver sperm directly into the female's mantle cavity, after which he becomes senescent and dies, while the female deposits fertilised eggs in a den and cares for them until they hatch, after which she also dies. They are predators and hunt crustaceans, bivalves, gastropods and fish. Strategies to defend themselves against their own predators include expelling ink, camouflage, and threat displays, the ability to jet quickly through the water and hide, and deceit. All octopuses are venomous, but only the blue-ringed octopuses are known to be deadly to humans.

Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In contrast, bony fishes have a single outer bony gill covering called an operculum.

Most sharks and rays have five pairs of gill slits, but a few species have 6 or 7 pairs. Shark gill slits lie in a row behind the head. The anterior edge of a gill slit is motile, moving outward to allow water to exit, but closing to prevent reverse flow. A modified slit, called a spiracle, lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active pelagic sharks. While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.

Fish gills are organs that allow fish to breathe underwater. Most fish exchange gases like oxygen and carbon dioxide using gills on both sides of the pharynx (throat). Gills possess tissues resembling short threads, referred to as gill filaments or lamellae. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Other than respiration, these filaments have other functions including the exchange of ions, water, acids, and ammonia.

Fish respire by pulling oxygen-rich water through their mouths and pumping it over their gills. Within the gill filaments, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings, but the most common group of fish alive, the bony fish, have a single gill opening on each side. This opening is hidden beneath a protective bony cover called the operculum.