Semi-aquatic in the context of "Gill"

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 wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor (anoxic) processes taking place, especially in the soils. Wetlands form a transitional zone between waterbodies and dry lands, and are different from other terrestrial or aquatic ecosystems due to their vegetation's roots having adapted to oxygen-poor waterlogged soils. They are considered among the most biologically diverse of all ecosystems, serving as habitats to a wide range of aquatic and semi-aquatic plants and animals, with often improved water quality due to plant removal of excess nutrients such as nitrates and phosphorus.

Wetlands exist on every continent, except Antarctica. The water in wetlands is either freshwater, brackish or saltwater. The main types of wetland are defined based on the dominant plants and the source of the water. For example, marshes are wetlands dominated by emergent herbaceous vegetation such as reeds, cattails and sedges. Swamps are dominated by woody vegetation such as trees and shrubs (although reed swamps in Europe are dominated by reeds, not trees). Mangrove forest are wetlands with mangroves and halophytic woody plants that have evolved to tolerate salty water.

Freshwater ecosystems are a subset of Earth's aquatic ecosystems that include the biological communities inhabiting freshwater waterbodies such as lakes, ponds, rivers, streams, springs, bogs, and wetlands. They can be contrasted with marine ecosystems, which have a much higher salinity. Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation.

There are three basic types of freshwater ecosystems: lentic (slow moving water, including pools, ponds, and lakes), lotic (faster moving streams, for example creeks and rivers) and wetlands (semi-aquatic areas where the soil is saturated or inundated for at least part of the time). Freshwater ecosystems contain 41% of the world's known fish species.

Galliformes /ˌɡælɪˈfɔːrmiːz/, also known as gallinaceous birds or landfowl, is an order of heavy-bodied terrestrial birds that contains about 290 species. The order is divided into five families: Phasianidae (chicken and junglefowls, Old World quails, partridges, pheasants, turkeys, peafowl and grouse), Odontophoridae (New World quails), Numididae (guinea fowl), Cracidae (including chachalacas and curassows), and Megapodiidae (incubator birds like malleefowl and brushturkeys). Galliformes and the semi-aquatic order Anseriformes (waterfowl) are collectively called fowl.

Galliform birds have adapted to most environments except innermost deserts and perpetually icy regions, inhabiting every continent except Antarctica. They are important in their ecosystems as seed dispersers and insectivores/vermivores, and many are skilled ground-dwellers that escape predators by running rather than flying. They are sexually dimorphic, and males of most species are more colorful than the females, with often elaborate courtship behaviors that include strutting, fluffing of tail or head feathers, and vocal sounds. They are mainly non-migratory.

The grass snake (Natrix natrix), sometimes called the ringed snake or water snake, is a Eurasian semi-aquatic non-venomous colubrid snake. It is often found near water and feeds almost exclusively on amphibians.

Titanoboa (/ˌtaɪtənəˈboʊə/; lit. 'titanic boa') is a genus of extinct giant boid (the family that includes all boas and anacondas) snakes that lived during the middle and late Paleocene. Titanoboa was first discovered in the early 2000s by the Smithsonian Tropical Research Institute, which–along with students from the University of Florida–recovered 186 fossils of Titanoboa from the Cerrejón coal mines in the La Guajira department of northeastern Colombia. It was named and described in 2009 as Titanoboa cerrejonensis, the largest snake ever found at that time. It was originally known only from thoracic vertebrae and ribs, but later expeditions collected parts of the skull and teeth. Titanoboa is in the subfamily Boinae, being most closely related to other extant boines from Madagascar and the Pacific.

Titanoboa could grow up to 12.8 m (42 ft) or perhaps even up to 14.3 m (47 ft) long, and weigh around 730–1,135 kg (1,610–2,500 lb). The discovery of Titanoboa cerrejonensis supplanted the previous record holder, Gigantophis garstini, which is known from the Eocene of Egypt. Titanoboa evolved following the extinction of all nonavian dinosaurs, being one of the largest reptiles to evolve after the Cretaceous–Paleogene extinction event. Its vertebrae are very robust and wide, with a pentagonal shape in anterior view, as in other members of Boinae. Titanoboa is thought to have been a semi-aquatic apex predator, with a diet consisting primarily of fish.