Tube-dwelling anemones or ceriantharians look very similar to

sea anemone Sea anemones ( ) are a group of predation, predatory marine invertebrates constituting the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemone ...

s but belong to an entirely different

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

anthozoa Anthozoa is one of the three subphyla of Cnidaria, along with Medusozoa and Endocnidozoa. It includes Sessility (motility), sessile marine invertebrates and invertebrates of brackish water, such as sea anemones, Scleractinia, stony corals, soft c ...

ns. They are solitary, living buried in soft sediments. Tube anemones live inside and can withdraw into tubes, which are composed of a fibrous material made from secreted mucus and threads of

nematocyst A cnidocyte (also known as a cnidoblast) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...

-like

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s known as ptychocysts. Within the tubes of these ceriantharians, more than one polyp is present, which is an exceptional trait because species that create tube systems usually contain only one polyp per tube. Ceriantharians were formerly classified in the taxon Ceriantipatharia along with the black corals but have since been moved to their own order, Ceriantharia. Ceriantharians have a crown of tentacles that are composed of two whorls of distinctly different-sized tentacles. The outer whorl consists of large tentacles that extend outwards. These tentacles taper to points and are mostly used in food capture and defense. The smaller inner tentacles are held more erect than the larger lateral tentacles and are used for food manipulation and ingestion. The tentacles of Ceriantharians are also venomous, similar to Cnidarians, which allows them to protect themselves from predators. The recent exploration of Ceriantharians and their venomous tentacles permits the exploration of the lineage of venom genes in Anthozoas. A few species such as '' Anactinia pelagica'' are

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

and are not attached to the bottom; instead, they have a gas chamber within the pedal disc, allowing them to float upside down near the surface of the water.

Taxonomy

;Suborder Spirularia * Family

Botrucnidiferidae Botrucnidiferidae is a family of cnidarians belonging to the order Spirularia. Genera Genera: ** Genus ''Angianthula'' Leloup, 1964 ** Genus ''Atractanthula'' Leloup, 1964 ** Genus ''Botruanthus'' McMurrich, 1910 ** Genus ''Botrucnidiata'' Lel ...

Carlgren, 1912 ** Genus '' Angianthula'' Leloup, 1964 ** Genus '' Atractanthula'' Leloup, 1964 ** Genus '' Botruanthus'' McMurrich, 1910 ** Genus '' Botrucnidiata'' Leloup, 1932 ** Genus '' Botrucnidifer'' Carlgren, 1912 ** Genus '' Calpanthula'' van Beneden, 1897 ** Genus '' Cerianthula'' Beneden, 1898 ** Genus '' Gymnanthula'' Leloup, 1964 ** Genus '' Hensenanthula'' van Beneden, 1897 ** Genus '' Ovanthula'' van Beneden, 1897 ** Genus '' Sphaeranthula'' Leloup, 1955 * Family Cerianthidae Milne-Edwards & Haime, 1852 ** Genus '' Anthoactis'' Leloup, 1932 ** Genus '' Apiactis'' van Beneden, 1897 ** Genus '' Bursanthus'' Leloup, 1968 ** Genus '' Ceriantheomorphe'' Carlgren, 1931 ** Genus '' Ceriantheopsis'' Carlgren, 1912 ** Genus '' Cerianthus'' Delle Chiaje, 1830 ** Genus '' Engodactylactis'' Leloup, 1942 ** Genus '' Isodactylactis'' Carlgren, 1924 ** Genus '' Nautanthus'' Leloup, 1964 ** Genus '' Pachycerianthus'' Roule, 1904 ** Genus '' Paradactylactis'' Carlgren, 1924 ** Genus '' Parovactis'' Leloup, 1964 ** Genus '' Peponactis'' van Beneden, 1897 ** Genus '' Plesiodactylactis'' Leloup, 1942 ** Genus '' Sacculactis'' Leloup, 1964 ** Genus '' Solasteractis'' van Beneden, 1897 ** Genus '' Synarachnactis'' Carlgren, 1924 ** Genus '' Syndactylactis'' Carlgren, 1924 ** Genus '' Trichactis'' Leloup, 1964 ;Suborder Penicillaria * Family Arachnactidae McMurrich, 1910 ** Genus '' Anactinia'' Annandale, 1909 ** Genus '' Arachnactis'' Sars, 1846 ** Genus '' Arachnanthus'' Carlgren, 1912 ** Genus '' Dactylactis'' van Beneden, 1897 ** Genus '' Isapiactis'' Carlgren, 1924 ** Genus '' Isarachnactis'' Carlgren, 1924 ** Genus '' Isarachnanthus'' Carlgren, 1924 ** Genus '' Isovactis'' ** Genus '' Ovactis'' ** Genus '' Paranactinia'' A 2020 integrative study incorporating

molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

reconstructions and morphological assessment across the three families recovered Arachnactidae as a well-supported

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

, but did not recover Cerianthidae and

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

, drawing into question the validity of the Spirularia suborder

References

External links

Photos of Tube Anemones

Tube-dwelling anemone toxins have pharmacological potential, mapping study shows
on: Eurekalert!, 29 October 2020. Source: Fundação de Amparo à Pesquisa do Estado de São Paulo {{Taxonbar, from=Q245695 Anthozoa Animal orders