ANIMAL KINDOM - 3
Structure-Reproduction and development-Ecology -Diversity -Size -Numbers and habitats of major phyla -Evolutionary origin -External phylogeny
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.[23] During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as shells, bones, and spicules.[24] In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.[25] Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.[26]
With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues.[27] These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
Reproduction and development
Nearly all animals make use of some form of sexual reproduction.[29] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova.[30] These fuse to form zygotes,[31] which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.[32] In most other groups, the blastula undergoes more complicated rearrangement.[33] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.[34] In most cases, a third germ layer, the mesoderm, also develops between them.[35] These germ layers then differentiate to form tissues and organs.[36]
Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits.[37][38] Animals have evolved numerous mechanisms for avoiding close inbreeding.[39]
Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.[40][41]
Ecology
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Animals are categorised into ecological groups depending on their trophic levels and how they consume organic material. Such groupings include carnivores (further divided into subcategories such as piscivores, insectivores, ovivores, etc.), herbivores (subcategorized into folivores, graminivores, frugivores, granivores, nectarivores, algivores, etc.), omnivores, fungivores, scavengers/detritivores,[42] and parasites.[43] Interactions between animals of each biome form complex food webs within that ecosystem. In carnivorous or omnivorous species, predation is a consumer–resource interaction where the predator feeds on another organism, its prey,[44] who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/competitive coevolutions.[45][46] Almost all multicellular predators are animals.[47] Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process,[48] but the adults primarily consume nectar from flowers.[49] Other animals may have very specific feeding behaviours, such as hawksbill sea turtles which mainly eat sponges.[50]
Most animals rely on biomass and bioenergy produced by plants and phytoplanktons (collectively called producers) through photosynthesis. Herbivores, as primary consumers, eat the plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher trophic levels indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidize carbohydrates, lipids, proteins and other biomolecules, which allows the animal to grow and to sustain basal metabolism and fuel other biological processes such as locomotion.[51][52][53] Some benthic animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter produced through chemosynthesis (via oxidizing inorganic compounds such as hydrogen sulfide) by archaea and bacteria.[54]
Animals evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician.[55] Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago.[56][57] Animals occupy virtually all of earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms.[58] Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F)[59] or in the most extreme cold deserts of continental Antarctica.[60]
Diversity
Size
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The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long.[61][62][63] The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes[61] and measuring up to 10.67 metres (35.0 ft) long.[61] The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 meters.[64][65] Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 μm,[66] and one of the smallest species (Myxobolus shekel) is no more than 8.5 μm when fully grown.[67]
Numbers and habitats of major phyla
The following table lists estimated numbers of described extant species for the major animal phyla,[68] along with their principal habitats (terrestrial, fresh water,[69] and marine),[70] and free-living or parasitic ways of life.[71] Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.[72] Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.[73][74][b]
| Phylum | Example | Described species | Land | Sea | Freshwater | Free-living | Parasitic |
|---|---|---|---|---|---|---|---|
| Arthropoda |  | 1,257,000[68] | Yes 1,000,000 (insects)[76] | Yes >40,000 (Malac- ostraca)[77] | Yes 94,000[69] | Yes[70] | Yes >45,000[c][71] |
| Mollusca |  | 85,000[68] 107,000[78] | Yes 35,000[78] | Yes 60,000[78] | Yes 5,000[69] 12,000[78] | Yes[70] | Yes >5,600[71] |
| Chordata |  | >70,000[68][79] | Yes 23,000[80] | Yes 13,000[80] | Yes 18,000[69] 9,000[80] | Yes | Yes 40 (catfish)[81][71] |
| Platyhelminthes |  | 29,500[68] | Yes[82] | Yes[70] | Yes 1,300[69] | Yes[70] 3,000–6,500[83] | Yes >40,000[71] 4,000–25,000[83] |
| Nematoda |  | 25,000[68] | Yes (soil)[70] | Yes 4,000[72] | Yes 2,000[69] | Yes 11,000[72] | Yes 14,000[72] |
| Annelida |  | 17,000[68] | Yes (soil)[70] | Yes[70] | Yes 1,750[69] | Yes | Yes 400[71] |
| Cnidaria |  | 16,000[68] | Yes[70] | Yes (few)[70] | Yes[70] | Yes >1,350 (Myxozoa)[71] | |
| Porifera |  | 10,800[68] | Yes[70] | 200–300[69] | Yes | Yes[84] | |
| Echinodermata |  | 7,500[68] | Yes 7,500[68] | Yes[70] | |||
| Bryozoa | .jpg) | 6,000[68] | Yes[70] | Yes 60–80[69] | Yes | ||
| Rotifera |  | 2,000[68] | Yes >400[85] | Yes 2,000[69] | Yes | Yes[86] | |
| Nemertea |  | 1,350[87][88] | Yes | Yes | Yes | ||
| Tardigrada | .jpg) | 1,335[68] | Yes[89] (moist plants) | Yes | Yes | Yes | |
Total number of described extant species as of 2013: 1,525,728[68] | |||||||
Evolutionary origin
Evidence of animals is found as long ago as the Cryogenian period. 24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it is only produced by sponges and pelagophyte algae. Its likely origin is from sponges based on molecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a Phanerozoic origin, while analyses of sponges recover a Neoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record.[90][91]
The first body fossils of animals appear in the Ediacaran, represented by forms such as Charnia and Spriggina. It had long been doubted whether these fossils truly represented animals,[92][93][94] but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature.[95] Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.[96]
Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 539 million years ago, in beds such as the Burgess shale.[97] Extant phyla in these rocks include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, along with numerous now-extinct forms such as the predatory Anomalocaris. The apparent suddenness of the event may however be an artifact of the fossil record, rather than showing that all these animals appeared simultaneously.[98][99][100][101][102] That view is supported by the discovery of Auroralumina attenboroughii, the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from Charnwood Forest, England. It is thought to be one of the earliest predators, catching small prey with its nematocysts as modern cnidarians do.[103]
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[104] Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as most probably being early sponges.[105] Trace fossils such as tracks and burrows found in the Tonian period (from 1 gya) may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms.[106] However, similar tracks are produced by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution.[107][108] Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals.[109] Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin is disputed, as they might be water-escape or other structures.[110][111]
![Dickinsonia costata from the Ediacaran biota (c. 635–542 mya) is one of the earliest animal species known.[95]](https://en.wikipedia.org/wiki/File:DickinsoniaCostata.jpg)
![Auroralumina attenboroughii, an Ediacaran predator (c. 560 mya)[103]](https://en.wikipedia.org/wiki/File:Auroralumina_attenboroughii_reconstruction.jpg)
Phylogeny
External phylogeny
Animals are monophyletic, meaning they are derived from a common ancestor. Animals are the sister group to the choanoflagellates, with which they form the Choanozoa.[112] The dates on the phylogenetic tree indicate approximately how many millions of years ago (mya) the lineages split.[113][114][115][116][117]
Ros-Rocher and colleagues (2021) trace the origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines.[118]
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Internal phylogeny
The most basal animals, the Porifera, Ctenophora, Cnidaria, and Placozoa, have body plans that lack bilateral symmetry. Their relationships are still disputed; the sister group to all other animals could be the Porifera or the Ctenophora,[119] both of which lack hox genes, which are important for body plan development.[120]
Hox genes are found in the Placozoa,[121][122] Cnidaria,[123] and Bilateria.[124][125] 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development.[126][127]
Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about the structure at the base of the tree (dashed lines).[128]
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An alternative phylogeny, from Kapli and colleagues (2021), proposes a clade Xenambulacraria for the Xenacoelamorpha + Ambulacraria; this is either within Deuterostomia, as sister to Chordata, or the Deuterostomia are recovered as paraphyletic, and Xenambulacraria is sister to the proposed clade Centroneuralia, consisting of Chordata + Protostomia.[129]
Eumetazoa, a clade which contains Ctenophora and ParaHoxozoa, has been proposed as a sister group to Porifera.[130] A competing hypothesis is the Benthozoa clade, which would consist of Porifera and ParaHoxozoa as a sister group of Ctenophora.
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