Orbiniida is an order of small

polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

worms in the phylum Annelida. It is the earliest diverging

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

Sedentaria Sedentaria is a diverse clade of annelid worms. It is traditionally treated as a subclass of the paraphyletic class Polychaeta, but it is also a monophyletic group uniting several polychaetes and the monophyletic class Clitellata. It is the sister ...

. Along with

Protodriliformia Protodriliformia is a clade of small marine life, marine polychaetes, comprised by the groups of meiofaunal interstitial worms Protodrilida and Polygordiidae, formerly considered "archiannelida, archiannelids". It is the most basal clade of Errant ...

(in

Errantia Errantia is a diverse group of marine polychaete worms in the phylum Annelida. Traditionally a subclass of the paraphyletic class "Polychaeta", it is currently regarded as a monophyletic group within the larger Pleistoannelida, composed of Erran ...

), this order is composed of

meiofauna Meiobenthos, also called meiofauna, are small benthic invertebrates that live in both marine and fresh water environments. The term ''meiofauna'' loosely defines a group of organisms by their size, larger than microfauna but smaller than macrofau ...

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

worms formerly known as " archiannelids". These worms inhabit the

interstitial ecosystem, the space between

sand grain Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...

Classification

First mentioned in 1977, Orbiniida was an order of the

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...

infraclass "

Scolecida Scolecida is an Infraclass (biology), infraclass of polychaete worms. Scolecids are mostly unselective Detritivore, deposit feeders on marine detritus. Characteristics Scolecids have Parapodium, parapodia with rami that are all alike. The pros ...

" of the class "

Polychaeta Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are m ...

". It was composed of the families

Orbiniidae Orbiniidae is a family of polychaete worms. Orbiniids are mostly unselective deposit feeders on marine detritus. They can be found from the neritic zone to abyssal depths. The family was revised in 1957 by Hartman and some further revisions w ...

Paraonidae Paraonidae is a family of polychaetes belonging to the order Cirratulida Cirratulida is an order of polychaetes belonging to the class Polychaeta. Families: * Cirratulidae Cirratulidae is a family of marine polychaete worms. Members of the f ...

and

Questidae Orbiniidae is a family of polychaete worms. Orbiniids are mostly unselective deposit feeders on marine detritus. They can be found from the neritic zone to abyssal depths. The family was revised in 1957 by Hartman and some further revisions w ...

, all of which share in common the lack of antennae and palps, and the presence of an eversible

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

and biramous

parapodia In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; plural: parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed ...

with simple

chaeta A chaeta or cheta (from Greek χαίτη “crest, mane, flowing hair"; plural: chaetae) is a chitinous bristle or seta found in annelid worms, (although the term is also frequently used to describe similar structures in other invertebrates suc ...

e. However, through

phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

it was found that

and

are more closely related to

Clitellata The Clitellata are a class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, the ...

and

Cirratulida Cirratulida is an order of polychaetes belonging to the class Polychaeta. Families: * Cirratulidae Cirratulidae is a family of marine polychaete worms. Members of the family are found worldwide, mostly living in mud or rock crevices. Most are ...

respectively. Because of this, the taxonomic composition of Orbiniida was changed to exclude these two families and include several interstitial groups previously belonging to the

Archiannelida Haplodrili, or Archiannelida, is an order of primitive polychaete worms. Zoologist Ray Lankester gave it the name haplodrili, while zoologist Berthold Hatschek later named it Archiannelida. Once considered to be a class under Annelida, and even ...

Parergodrilidae Parergodrilidae is an enigmatic family of polychaetes with only two genera, one living on the coast, the other terrestrial. They share much in common with the clitellates, but molecular data place them with Questa and Orbiniidae Orbiniidae i ...

Nerillidae The Nerillidae are a family of invertebrates containing these genera: * '' Meganerilla'' * '' Mesonerilla'' * '' Nerilla'' * '' Nerillidium'' * '' Nerillidopsis'' * '' Paranerilla'' * '' Psammoriedlia'' * '' Thalassochaetus'' * '' Troglochaetus'' ...

Diurodrilidae ''Diurodrilus'' is a genus of tiny marine animals that has traditionally been assigned to the annelid worms, although this affinity is not certain. With a maximum length of 0.45 mm, it has an unusual morphology with many traits not found in ...

, and '' Apharyngtus'', and possibly Dinophilidae too.

Evolutionary history

Phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

of annelids has found Orbiniida to be the earliest diverging clade of

. At the same time, the other half of " archiannelid" worms,

, was found to be the earliest diverging clade of

. The

convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen * "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that united the four Wei ...

between these two groups occurred through

progenesis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compared ...

and

miniaturization Miniaturization ( Br.Eng.: ''Miniaturisation'') is the trend to manufacture ever smaller mechanical, optical and electronic products and devices. Examples include miniaturization of mobile phones, computers and vehicle engine downsizing. In ele ...

, as a way to adapt to the marine interstitial ecosystem between sand grains (i.e. interstitium). This means that the

larval A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The l ...

or juvenile stages of a larger pleistoannelid ancestor temporarily inhabiting the interstitium stopped their somatic growth, became

sexually mature Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definitio ...

and stayed in the interstitium permanently. Although Dinophilidae possibly belongs to Orbiniida, a more recent analysis recovered several times the clade Dinophiliformia, containing the genera in Dinophilidae, as sister group to

. This would imply a third separate lineage of interstitial, miniaturized worms.

References

{{taxonbar, worms=884 Annelid orders Polychaetes