Phylum

In biology, a phylum (/ˈfaɪləm/; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent.[1][2][3] Depending on definitions, the animal kingdom Animalia or Metazoa contains approximately 35 phyla; the plant kingdom Plantae contains about 14, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

The hierarchy of biological classification's eight major taxonomic ranks. A kingdom contains one or more phyla. Intermediate minor rankings are not shown.

General description

The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon (φῦλον, "race, stock"), related to phyle (φυλή, "tribe, clan").[4][5] Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as a group ("a self-contained unity"). "Wohl aber ist eine solche reale und vollkommen abgeschlossene Einheit die Summe aller Species, welche aus einer und derselben gemeinschaftlichen Stammform allmählig sich entwickelt haben, wie z. B. alle Wirbelthiere. Diese Summe nennen wir Stamm (Phylon)." which translates as: However, perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., race] (Phylon). In plant taxonomy, August W. Eichler (1883) classified plants into five groups named divisions, a term that remains in use today for groups of plants, algae and fungi.[1][6] The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier.[7]

Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan.[8] At its most basic, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[9] Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were.

Definition based on genetic relation

The most important objective measure in the above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group.[9] Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine the relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids, so the phyla were merged (the bearded worms are now an annelid family).[10] On the other hand, the highly parasitic phylum Mesozoa was divided into two phyla (Orthonectida and Rhombozoa) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes.[11]

This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of cladistics, a method in which groups are placed on a "family tree" without any formal ranking of group size.[9]

Definition based on body plan

A definition of a phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by a set of characters shared by all its living representatives.

This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on a subjective decision about which groups of organisms should be considered as phyla.

The approach is useful because it makes it easy to classify extinct organisms as "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities.[9] However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group.[9] Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.[12]

A classification using this definition may be strongly affected by the chance survival of rare groups, which can make a phylum much more diverse than it would be otherwise.[13]

Known phyla

Animals

Main article: Animal

Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species,[14] some on extrapolations to numbers of undescribed species. 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.[15]

	Protostome	Bilateria
	Deuterostome
	Basal/disputed
	Others

Phylum	Meaning	Common name	Distinguishing characteristic	Species described
Acanthocephala	Thorny head	Thorny-headed worms[16]^:278	Reversible spiny proboscis that bears many rows of hooked spines	1,420
Annelida	Little ring [16]^:306	Segmented worms	Multiple circular segment	17,000 + extant
Arthropoda	Jointed foot	Insects, crustaceans	Segmented bodies and jointed limbs, with Chitin exoskeleton	1,250,000+ extant;[14] 20,000+ extinct
Brachiopoda	Arm foot[16]^:336	Lampshells[16]^:336	Lophophore and pedicle	300-500 extant; 12,000+ extinct
Bryozoa	Moss animals	Moss animals, sea mats, ectoprocts[16]^:332	Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia	6,000 extant[14]
Chaetognatha	Longhair jaw	Arrow worms[16]^:342	Chitinous spines either side of head, fins	approx. 100 extant
Chordata	With a cord	Chordates	Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail	approx. 55,000+[14]
Cnidaria	Stinging nettle	Cnidarians	Nematocysts (stinging cells)	approx. 16,000[14]
Ctenophora	Comb bearer	Comb jellies[16]^:256	Eight "comb rows" of fused cilia	approx. 100-150 extant
Cycliophora	Wheel carrying	Symbion	Circular mouth surrounded by small cilia, sac-like bodies	3+
Echinodermata	Spiny skin	Echinoderms[16]^:348	Fivefold radial symmetry in living forms, mesodermal calcified spines	approx. 7,500 extant;[14] approx. 13,000 extinct
Entoprocta	Inside anus[16]^:292	Goblet worms	Anus inside ring of cilia	approx. 150
Gastrotricha	Hairy stomach[16]^:288	Gastrotrich worms	Two terminal adhesive tubes	approx. 690
Gnathostomulida	Jaw orifice	Jaw worms[16]^:260		approx. 100
Hemichordata	Half cord[16]^:344	Acorn worms, hemichordates	Stomochord in collar, pharyngeal slits	approx. 130 extant
Kinorhyncha	Motion snout	Mud dragons	Eleven segments, each with a dorsal plate	approx. 150
Loricifera	Corset bearer	Brush heads	Umbrella-like scales at each end	approx. 122
Micrognathozoa	Tiny jaw animals	Limnognathia	Accordion-like extensible thorax	1
Mollusca	Soft[16]^:320	Mollusks / molluscs	Muscular foot and mantle round shell	85,000+ extant;[14] 80,000+ extinct[17]
Nematoda	Thread like	Round worms, thread worms[16]^:274	Round cross section, keratin cuticle	25,000[14]
Nematomorpha	Thread form[16]^:276	Horsehair worms, gordian worms[16]^:276		approx. 320
Nemertea	A sea nymph[16]^:270	Ribbon worms, rhynchocoela[16]^:270		approx. 1,200
Onychophora	Claw bearer	Velvet worms[16]^:328	Legs tipped by chitinous claws	approx. 200 extant
Orthonectida	Straight swimming[16]^:268	Orthonectids[16]^:268	Single layer of ciliated cells surrounding a mass of sex cells	approx. 26
Phoronida	Zeus's mistress	Horseshoe worms	U-shaped gut	11
Placozoa	Plate animals	Trichoplaxes[16]^:242	Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between	3
Platyhelminthes	Flat worm[16]^:262	Flatworms[16]^:262		approx. 29,500[14]
Porifera [lower-alpha 1]	Pore bearer	Sponges[16]^:246	Perforated interior wall	10,800 extant[14]
Priapulida	Little Priapus	Penis worms		approx. 20
Rhombozoa	Lozenge animal	Rhombozoans[16]^:264	Single anteroposterior axial cell surrounded by ciliated cells	100+
Rotifera	Wheel bearer	Rotifers[16]^:282	Anterior crown of cilia	approx. 2,000[14]
Sipuncula	Small tube	Peanut worms	Mouth surrounded by invertible tentacles	144-320
Tardigrada	Slow step	Water bears, Moss piglets	Four segmented body and head	1,000
Xenacoelomorpha	Strange form without gut	Acoels, xenoturbellids	Bilaterian, but lacking typical bilaterian structures such as gut cavities, anuses, and circulatory systems[18]	400+
Total: 34				1,525,000[14]

Plants

Main article: Plant

The kingdom Plantae is defined in various ways by different biologists (see Current definitions of Plantae). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta, to form the clade Viridiplantae. The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida,[19] a group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions.

The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Pteridophyta,[20] while others place them both in Pteridophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes),[21] or for conifers alone as below.

Since the first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses.[22]

	Land plants	Viridiplantae
	Green algae
	Other algae (Biliphyta)[19]

Division	Meaning	Common name	Distinguishing characteristics	Species described
Anthocerotophyta[23]	Anthoceros-like plants	Hornworts	Horn-shaped sporophytes, no vascular system	100-300+
Bryophyta[24]	Bryum-like plants, moss plants	Mosses	Persistent unbranched sporophytes, no vascular system	approx. 12,000
Charophyta	Chara-like plants	Charophytes		approx. 1,000
Chlorophyta	(Yellow-)green plants[16]^:200	Chlorophytes		approx. 7,000
Cycadophyta[25]	Cycas-like plants, palm-like plants	Cycads	Seeds, crown of compound leaves	approx. 100-200
Ginkgophyta[26]	Ginkgo-like plants	Ginkgo, maidenhair tree	Seeds not protected by fruit (single living species)	only 1 extant; 50+ extinct
Glaucophyta	Blue-green plants	Glaucophytes		15
Gnetophyta[27]	Gnetum-like plants	Gnetophytes	Seeds and woody vascular system with vessels	approx. 70
Lycopodiophyta,[21] Lycophyta[28]	Lycopodium-like plants Wolf plants	Clubmosses & spikemosses	Microphyll leaves, vascular system	1,290 extant
Magnoliophyta	Magnolia-like plants	Flowering plants, angiosperms	Flowers and fruit, vascular system with vessels	300,000
Marchantiophyta,[29] Hepatophyta[24]	Marchantia-like plants Liver plants	Liverworts	Ephemeral unbranched sporophytes, no vascular system	approx. 9,000
Pinophyta,[21] Coniferophyta[30]	Pinus-like plants Cone-bearing plant	Conifers	Cones containing seeds and wood composed of tracheids	629 extant
Rhodophyta	Rose plants	Red algae	Use phycobiliproteins as accessory pigments.	approx. 7,000
Total: 13

Fungi

Main article: Fungi

Division	Meaning	Common name	Distinguishing characteristics
Ascomycota	Bladder fungus[16]^:396	Ascomycetes,[16]^:396 sac fungi	Tend to have fruiting bodies (ascocarp).[31] Filamentous, producing hyphae separated by septa. Can reproduce asexually.[32]
Basidiomycota	Small base fungus[16]^:402	Basidiomycetes[16]^:402	Bracket fungi, toadstools, smuts and rust. Sexual reproduction.[33]
Blastocladiomycota	Offshoot branch fungus[34]	Blastoclads
Chytridiomycota	Little cooking pot fungus[35]	Chytrids	Predominantly Aquatic saprotrophic or parasitic. Have a posterior flagellum. Tend to be single celled but can also be multicellular.[36][37][38]
Glomeromycota	Ball of yarn fungus[16]^:394	Glomeromycetes, AM fungi[16]^:394	Mainly arbuscular mycorrhizae present, terrestrial with a small presence on wetlands. Reproduction is asexual but requires plant roots.[33]
Microsporidia	Small seeds[39]	Microsporans[16]^:390
Neocallimastigomycota	New beautiful whip fungus[40]	Neocallimastigomycetes	Predominantly located in digestive tract of herbivorus animals. Anaerobic, terrestrial and aquatic.[41]
Zygomycota	Pair fungus[16]^:392	Zygomycetes[16]^:392	Most are saprobes and reproduce sexually and asexually.[41]
Total: 8

Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain,[42] and it is considered a protozoan by the International Society of Protistologists[43] (see Protista, below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor),[44] which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish the Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina, Kickxellomycotina, Mucoromycotina, and Zoopagomycotina.[42]

Protista

Main article: Protista taxonomy

Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi.[16]^:120 Protista is a polyphyletic taxon[45] (it includes groups not directly related to one another), which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system.[46]

Protist taxonomy has long been unstable,[47] with different approaches and definitions resulting in many competing classification schemes. The phyla listed here are used for Chromista and Protozoa by the Catalogue of Life,[48] adapted from the system used by the International Society of Protistologists.[43]

	Chromista
	Protozoa

Phylum/Division	Meaning	Common name	Distinguishing characteristics	Example
Amoebozoa	Amorphous animal	Amoebas		Amoeba
Bigyra	Two ring
Cercozoa
Choanozoa	Funnel animal
Ciliophora	Cilia bearer	Ciliates		Paramecium
Cryptista
Euglenozoa	True eye animal			Euglena
Foraminifera	Hole bearers	Forams	Complex shells with one or more chambers	Forams
Haptophyta
Loukozoa	Groove animal
Metamonada				Giardia
Microsporidia	Small spore
Myzozoa	Suckling animal
Mycetozoa		Slime molds
Ochrophyta	Yellow plant	Diatoms		Diatoms
Oomycota	Egg fungus[16]^:184	Oomycetes
Percolozoa
Radiozoa	Ray animal	Radiolarians
Sarcomastigophora
Sulcozoa
Total: 20

The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae,[48] but other systems consider these phyla part of Protista.[49]

Bacteria

Main article: Bacterial phyla

Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN)[50]

1. Acidobacteria, phenotypically diverse and mostly uncultured
2. Actinobacteria, High-G+C Gram positive species
3. Aquificae, only 14 thermophilic genera, deep branching
4. Armatimonadetes
5. Bacteroidetes
6. Caldiserica, formerly candidate division OP5, Caldisericum exile is the sole representative
7. Chlamydiae, only 6 genera
8. Chlorobi, only 7 genera, green sulphur bacteria
9. Chloroflexi, green non-sulphur bacteria
10. Chrysiogenetes, only 3 genera (Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
11. Cyanobacteria, also known as the blue-green algae
12. Deferribacteres
13. Deinococcus-Thermus, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
14. Dictyoglomi
15. Elusimicrobia, formerly candidate division Thermite Group 1
16. Fibrobacteres
17. Firmicutes, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic)
18. Fusobacteria
19. Gemmatimonadetes
20. Lentisphaerae, formerly clade VadinBE97
21. Nitrospira
22. Planctomycetes
23. Proteobacteria, the most known phyla, containing species such as Escherichia coli or Pseudomonas aeruginosa
24. Spirochaetes, species include Borrelia burgdorferi, which causes Lyme disease
25. Synergistetes
26. Tenericutes, alternatively class Mollicutes in phylum Firmicutes (notable genus: Mycoplasma)
27. Thermodesulfobacteria
28. Thermotogae, deep branching
29. Verrucomicrobia

Archaea

Main article: Archaea

Currently there are five phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN).[50]

1. Crenarchaeota, second most common archaeal phylum
2. Euryarchaeota, most common archaeal phylum
3. Korarchaeota
4. Nanoarchaeota, ultra-small symbiotes, single known species
5. Thaumarchaeota

See also

• Cladistics
• Phylogenetics
• Systematics
• Taxonomy

Notes

1. Paraphyletic

References

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External links

• Are phyla "real"? Is there really a well-defined "number of animal phyla" extant and in the fossil record?
• Major Phyla Of Animals