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.
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