When is an anemone not an anemone?

The pretty creatures pictured here look like anemones, but they are not true anemones. They are Cerianthids, commonly referred to as 'tube anemones', which are taxonomically quite distinct from true anemones.

Cerianthids and true anemones do belong to the same phylum, Cnidaria, and the same class, Anthozoa, but tube anemones belong to the subclass Ceriantipatharia, a taxon that also includes the so-called 'black corals' (Antipatharia).

One of the visible features that distinguishes Cerianthid tube anemones from true anemones is the morphology of their tentacles. The macro photo below shows that Cerianthids have shorter tentacles in their centers, and longer tentacles around the margin. The color of the shorter tentacles usually is different from that of the longer tentacles, making them look a lot like flowers (at least to me).

Cerianthids dwell inside a rubbery tube (thus the name tube anemone) which is built from mucus secreted by the animal. The tube is embedded in mud or packed sand. When not feeding, or when disturbed, the animal retracts inside its tube for protection.

These creatures can be difficult to photograph for several reasons. Most Cerianthids are relatively small; their crowns of tentacles are perhaps 5 cm (2 in) across, so it's necessary to get very close to them in order to photograph them. If the photographer accidentally touches one of the tentacles, piff! the critter retracts. And although Cerianthids happily feed in gentle currents, any nearby turbulence -- like that created by the photographer as he or she moves about -- causes the critter to quickly go into hiding.

These are deep-dwelling creatures -- all of the examples in this post were photographed at depths greater than 40 meters (130 ft). They are accustomed to low levels of ambient light at those depths, so Cerianthids do not take kindly to blasts of artificial light from a camera strobe. At best, one or two shots of an individual is all that a photographer can hope for before all that is left to photograph is the tube!

All of the Cerianthid tube anemones pictured in this post were photographed off the west coast of Hawaii's Big Island. This post was adapted from an article I wrote last year on my blog, The Right Blue, where you can find further details and more images of Cerianthids.