Tube-dwelling anemone

Tube-dwelling anemones, tube anemones or ceriantharians. scientific name: Ceriantharia, are an order of hexacorals.

Tube-dwelling anemones
Cerianthus sp.
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Subphylum:	Anthozoa
Class:	Hexacorallia
Order:	Ceriantharia
Families
See text.

Characteristics

They look very similar to the sea anemones (Actiniaria), but are not directly related to them. They are solitary, living buried in soft sediments. They live inside and can withdraw into tubes, which are composed of a fibrous material made from secreted mucus and threads of nematocyst-like organelles known as ptychocysts. Within the tubes of the tube anemones, more than one polyp is present, which is an exceptional trait because species that create tube systems usually contain only one polyp per tube.^[1]

Tube anemones 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.^[2] The tentacles of tube anemones are also venomous, similar to Cnidarians, which allows them to protect themselves from predators. The recent exploration of tube anemones and their venomous tentacles permits the exploration of the lineage of venom genes in Anthozoas.^[3]

A few species such as Anactinia pelagica are pelagic 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.^[4][5]

Fossil record

So far, the only fossil record for the group is from Eocene and Oligocene deep-water rocks in Washington State, USA.^[6]

Taxonomy

The tube anemones (Ceriantharia) were formerly classified in the taxon (subclass) Ceriantipatharia along with the black corals (Antipatharia)^[7], but they are now considered to be basal hexacorals (or, equivalently, a sister taxon to the hexacorals) and unrelated to the black corals.

The current system is shown below:

order Ceriantharia:

Suborder Spirularia

• Family Botrucnidiferidae Carlgren, 1912^[8]
  • 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^[9]
  • 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^[10]

• 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 reconstructions and morphological assessment across the three families recovered Arachnactidae as a well-supported clade, but did not recover Cerianthidae and Botrucnidiferidae as monophyletic, drawing into question the validity of the Spirularia suborder^[11]