Tina N. Molodtsova, Shirshov Institute of Oceanology RAS
Video courtesy of NOAA Ocean Exploration, 2021 North Atlantic Stepping Stones: New England and Corner Rise Seamounts. Download largest version (mp4, 62 MB).
Cerianthid or tube anemones, belonging to the subclass Ceriantharia, are rather distant relatives of sea anemones. All tube anemones have a quite uniform appearance, with an elongated body and two crowns of non-branched tentacles. Shorter tentacles surround the mouth and longer tentacles are arranged at the edge of the tube anemone’s oral disc.
Although it is not obvious at first glance, tube anemones are principally bilaterally symmetrical, which is unusual for members of the phylum Cnidaria, which also includes animals such as corals and jellyfish. Tube anemones have only one multiplication zone where tentacles and septa originate one by one. Morphologically and genetically, tube anemones are so distant from all other anemones and corals that they are believed to form a separate subclass of Anthozoa.
Adult tube anemones are benthic (bottom dwelling) and usually inhabit long, fibrous, mucous-covered tubes buried deep in seafloor sediment. Only a few adult ceriantharians have ever been reported as not having a tube. The fibrous tubes gave rise to their common name of “tube anemone.” The tubes are built from discharged tubules of a special type of stinging cells – ptychocysts – found exclusively in animals in the subclass Ceriantharia. The framework built of ptychocyst tubules is further encrusted by mucus and substrate particles.
When alarmed, tube anemones retract almost immediately into their tubes. Given that the tubes of some ceriantharians can be up to 0.5 meters (1.6 feet) deep, they can be difficult to collect with traditional deep-sea tools such as trawls, dredges, or even the manipulator arms of remotely operated vehicles.
Ceriantharians are known from the intertidal zone to abyssal depths of all ocean basins, in brackish waters and in full oceanic salinity. Most known species of tube anemones were described from shallow waters, and very little is known about Ceriantharia in the deep ocean.
Published July 14, 2021
