Invertebrate in the context of Parthenogenic

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).

The Holocene or Anthropocene extinction is an ongoing extinction event caused by human activity during the current geological epoch, impacting diverse families of plants and animals, including mammals, birds, reptiles, amphibians, fish, and invertebrates, as well as both terrestrial and marine species. It is sometimes also called the sixth mass extinction (or seventh if counting the Capitanian and End Permian extinctions separately).

Current extinction rates are estimated at 100 to 1,000 times higher than natural background extinction rates and are accelerating. The Holocene extinction was preceded by the Late Pleistocene megafauna extinctions (lasting from 50,000 to 10,000 years ago), in which many large mammals – including 81% of megaherbivores – went extinct, a decline attributed at least in part to human (anthropogenic) activities. There continue to be strong debates about the relative importance of anthropogenic factors and climate change, but a recent review concluded that there is little evidence for a major role of climate change and "strong" evidence for human activities as the principal driver. Examples from regions such as New Zealand, Madagascar, and Hawaii have shown how human colonization and habitat destruction have led to significant biodiversity losses.

Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (/ˈmɒləsks/). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The number of additional fossil species is estimated between 60,000 and 100,000, and the proportion of undescribed species is very high. Many taxa remain poorly studied.

Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat, as numerous groups are freshwater and even terrestrial species. The phylum is typically divided into 7 or 8 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known extant invertebrate species. The gastropods (snails, slugs and abalone) are by far the most diverse class and account for 80% of the total classified molluscan species.

Shellfish, in colloquial and fisheries usage, are exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. In addition, a few species of land crabs are eaten, for example Cardisoma guanhumi in the Caribbean. Shellfish are among the most common food allergens.

Due to narrowing in the meaning of the English word fish over the centuries, shellfish no longer fall under what is usually considered fish. Most shellfish are low on the food chain and eat a diet composed primarily of phytoplankton and zooplankton. Many varieties of shellfish, and crustaceans in particular, are actually closely related to insects and arachnids; crustaceans make up one of the main subphyla of the phylum Arthropoda. Molluscs include cephalopods (squids, octopuses, cuttlefish) and bivalves (clams, oysters), as well as gastropods (aquatic species such as whelks and winkles; land species such as snails and slugs).

Barnacles are arthropods of the subclass Cirripedia in the subphylum Crustacea. They are related to crabs and lobsters, with similar nauplius larvae. Barnacles are exclusively marine invertebrates; many species live in shallow and tidal waters. Some 2,100 species have been described.

Barnacle adults are sessile; most are suspension feeders with hard calcareous shells, but the Rhizocephala are specialized parasites of other crustaceans, with reduced bodies. Barnacles have existed since at least the mid-Carboniferous, some 325 million years ago.

The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds.

The structure of the seabed of the global ocean is governed by plate tectonics. Most of the ocean is very deep, where the seabed is known as the abyssal plain. Seafloor spreading creates mid-ocean ridges along the center line of major ocean basins, where the seabed is slightly shallower than the surrounding abyssal plain. From the abyssal plain, the seabed slopes upward toward the continents and becomes, in order from deep to shallow, the continental rise, slope, and shelf. The depth within the seabed itself, such as the depth down through a sediment core, is known as the "depth below seafloor". The ecological environment of the seabed and the deepest waters are collectively known, as a habitat for creatures, as the "benthos".

The wildlife of Antarctica are extremophiles, having adapted to the dryness, low temperatures, and high exposure common in Antarctica. The extreme weather of the interior contrasts to the relatively mild conditions on the Antarctic Peninsula and the subantarctic islands, which have warmer temperatures and more liquid water. Much of the ocean around the mainland is covered by sea ice. The oceans themselves are a more stable environment for life, both in the water column and on the seabed.

There is relatively little diversity in Antarctica compared to much of the rest of the world. Terrestrial life is concentrated in areas near the coast. Flying birds nest on the milder shores of the Peninsula and the subantarctic islands. Eight species of penguins inhabit Antarctica and its offshore islands. They share these areas with seven pinniped species. The Southern Ocean around Antarctica is home to 10 cetaceans, many of them migratory. There are very few terrestrial invertebrates on the mainland, although the species that do live there have high population densities. High densities of invertebrates also live in the ocean, with Antarctic krill forming dense and widespread swarms during the summer. Benthic animal communities also exist around the continent.

There are several plausible pathways that could lead to plant and animal species extinction from climate change. Every species has evolved to exist within a certain ecological niche, but climate change leads to changes of temperature and average weather patterns. These changes can push climatic conditions outside of the species' niche, and ultimately render it extinct. Normally, species faced with changing conditions can either adapt in place through microevolution or move to another habitat with suitable conditions. However, the speed of recent climate change is very fast. Due to this rapid change, for example cold-blooded animals (a category which includes amphibians, reptiles and all invertebrates) may struggle to find a suitable habitat within 50 km of their current location at the end of this century (for a mid-range scenario of future global warming).

Climate change also increases both the frequency and intensity of extreme weather events, which can directly wipe out regional populations of species. Those species occupying coastal and low-lying island habitats can also become extinct by sea level rise. This has already happened with Bramble Cay melomys in Australia. Finally, climate change has been linked with the increased prevalence and global spread of certain diseases affecting wildlife. This includes Batrachochytrium dendrobatidis, a fungus that is one of the main drivers of the worldwide decline in amphibian populations.

Domestication is a multi-generational mutualistic relationship in which an animal species, such as humans or leafcutter ants, takes over control and care of another species, such as sheep or fungi, to obtain from them a steady supply of resources, such as meat, milk, or labor. The process is gradual and geographically diffuse, based on trial and error. Domestication affected genes for behavior in animals, making them less aggressive. In plants, domestication affected genes for morphology, such as increasing seed size and stopping the shattering of cereal seedheads. Such changes both make domesticated organisms easier to handle and reduce their ability to survive in the wild.

The first animal to be domesticated by humans was the dog, as a commensal, at least 15,000 years ago. Other animals, including goats, sheep, and cows, were domesticated around 11,000 years ago. Among birds, the chicken was first domesticated in East Asia, seemingly for cockfighting, some 7,000 years ago. The horse came under domestication around 5,500 years ago in central Asia as a working animal. Among invertebrates, the silkworm and the western honey bee were domesticated over 5,000 years ago for silk and honey, respectively.

The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for special senses such as vision, hearing, and olfaction. Being the most specialized organ, it is responsible for receiving information from the sensory nervous system, processing that information (thought, cognition, and intelligence) and the coordination of motor control (muscle activity and endocrine system).

While invertebrate brains arise from paired segmental ganglia (each of which is only responsible for the respective body segment) of the ventral nerve cord, vertebrate brains develop axially from the midline dorsal nerve cord as a vesicular enlargement at the rostral end of the neural tube, with centralized control over all body segments. All vertebrate brains can be embryonically divided into three parts: the forebrain (prosencephalon, subdivided into telencephalon and diencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon, subdivided into metencephalon and myelencephalon). The spinal cord, which directly interacts with somatic functions below the head, can be considered a caudal extension of the myelencephalon enclosed inside the vertebral column. Together, the brain and spinal cord constitute the central nervous system in all vertebrates.

A crocodile (family Crocodylidae) or true crocodile is a large, semiaquatic reptile that lives throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include all extant members of the order Crocodilia, which includes the alligators and caimans (both members of the family Alligatoridae), the gharial and false gharial (both members of the family Gavialidae) as well as other extinct taxa.

Crocodile size, morphology, behaviour and ecology differ among species. However, they have many similarities in these areas as well. All crocodiles are semiaquatic and tend to congregate in freshwater habitats such as rivers, lakes, wetlands and sometimes in brackish water and saltwater. They are carnivorous animals, feeding mostly on vertebrates such as fish, reptiles, birds and mammals, and sometimes on invertebrates such as molluscs and crustaceans, depending on species and age. All crocodiles are tropical species that, unlike alligators, are very sensitive to cold. Many species are at the risk of extinction, some being classified as critically endangered.

Among animals which produce eggs, the yolk (/ˈjoʊk/; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, usually through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell.

As stored food, yolks are often rich in vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in regulating the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization.

Sentience is the ability to experience feelings and sensations. It may not necessarily imply higher cognitive functions such as awareness, reasoning, or complex thought processes. Some theorists define sentience exclusively as the capacity for valenced (positive or negative) mental experiences, such as pain and pleasure.

Sentience is an important concept in ethics, as the ability to experience happiness or suffering often forms a basis for determining which entities deserve moral consideration, particularly in utilitarianism.

Marine invertebrates are invertebrate animals that live in marine habitats, and make up most of the macroscopic life in the oceans. It is a polyphyletic blanket term that contains all marine animals except the marine vertebrates, including the non-vertebrate members of the phylum Chordata such as lancelets, sea squirts and salps. As the name suggests, marine invertebrates lack any mineralized axial endoskeleton, i.e. the vertebral column, and some have evolved a rigid shell, test or exoskeleton for protection and/or locomotion, while others rely on internal fluid pressure to support their bodies. Marine invertebrates have a large variety of body plans, and have been categorized into over 30 phyla.

A nest is a structure built by certain animals to hold their eggs or young. Although nests are most closely associated with birds, members of all classes of vertebrates and some invertebrates construct nests. They may be composed of organic material such as twigs, grass, and leaves, or may be a simple depression in the ground, or a hole in a rock, tree, or building. Human-made materials, such as string, plastic, cloth, or paper, may also be used. Nests can be found in all types of habitat.

Nest building is driven by a biological urge known as the nesting instinct in birds and mammals. Generally each species has a distinctive style of nest. Nest complexity is roughly correlated with the level of parental care by adults. Nest building is considered a key adaptive advantage among birds, and they exhibit the most variation in their nests ranging from simple holes in the ground to elaborate communal nests hosting hundreds of individuals. Nests of prairie dogs and several social insects can host millions of individuals.

Animal cognition encompasses the mental capacities of non-human animals, including insect cognition. The study of animal conditioning and learning used in this field was developed from comparative psychology. It has also been strongly influenced by research in ethology, behavioral ecology, and evolutionary psychology; the alternative name cognitive ethology is sometimes used. Many behaviors associated with the term animal intelligence are also subsumed within animal cognition.

Researchers have examined animal cognition in mammals (especially primates, cetaceans, elephants, bears, dogs, cats, pigs, horses, cattle, raccoons and rodents), birds (including parrots, fowl, corvids and pigeons), reptiles (lizards, crocodilians, snakes, and turtles), fish and invertebrates (including cephalopods, spiders and insects).

Insects (from Latin insectum) are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes, and a pair of antennae. Insects are the most diverse group of animals, with more than a million described species; they represent more than half of all animal species.

The insect nervous system consists of a brain and a ventral nerve cord. Most insects reproduce by laying eggs. Insects breathe air through a system of paired openings along their sides, connected to small tubes that take air directly to the tissues. The blood therefore does not carry oxygen; it is only partly contained in vessels, and some circulates in an open hemocoel. Insect vision is mainly through their compound eyes, with additional small ocelli. Many insects can hear, using tympanal organs, which may be on the legs or other parts of the body. Their sense of smell is via receptors, usually on the antennae and the mouthparts.