Dive Dry with Dr. Bill

120: Cerianthid "Anemones"

One of the more beautiful, yet relatively simple, animals in the Dive Park is the tube-dwelling or cerianthid anemone. They may be found on sandy or muddy bottoms in protected areas down to depths of 70 feet or more. There are several species found along the West Coast between British Columbia and San Diego. All have long, sweeping feeding tentacles and a parchment-like tube. In our waters one may find beautiful orange, cream or dark reddish-brown individuals. I am especially partial to the orange ones and often stop to "smell the anemones" while diving since they look a bit like their flower namesake. Individuals may reach lengths of up to a foot, and be up to 2.5" in diameter.

The cerianthids belong to the group of organisms known as cnidarians. Their relatives include the corals, the soft corals, and jellyfish. Although they are commonly called "anemones," they differ from the true anemones we more commonly encounter in tidepools and elsewhere. One of the major differences is that they possess an anal opening at the other end of the body from the mouth. True anemones have a single opening into the digestive tract through which food enters... and wastes exit. That makes me think back to what I used to teach my high school ecology students... "you are what you eat... minus what you excrete." I don't think any of them found a question about that on the SAT tests though.

These soft-bodied organisms remain within their tubes, extending only the tentacles past the tube opening. The tube itself may go down as much as two feet deep in the bottom sediments. If approached by a predator, they quickly withdraw into their tubes. Based on my observations they seem to react to water movement rather than shadows or other changes in light. Laboratory studies have shown that nerve impulses can travel up to 4.25 feet per second. Mighty fast for such a simple organism.

There are actually two types of tentacles. The long, outer set are the feeding tentacles. They have the stinging cells known as nematocysts that allow these animals (and their relatives) to capture their food. On the inner disk is a set of small, bristle-like tentacles. The feeding tentacles are dragged across these inner bristles to scrape food off, which is then transferred to the digestive opening.

The parchment tubes are lined with mucus. They may also be lined with detached but still active stinging nematocysts. These help protect the animal from intruders who enter the tube. They are eaten by a species of nudibranch. Occasionally the nudibranchs are accidentally pulled down into the tube with the tentacles. Usually this predator merely snips off the tips of a few tentacles to munch on, and rarely kills the anemone. The animal can grow back the portions of the tentacles that are eaten, and may live up to 10 years despite this periodic grazing! It is reported that the tentacles can fluoresce in some species, which might also serve to frighten a predator.

© 2004 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Cerianthid anemone with feeding tentacles, inner set of "brush" tentacles,
parchment tube of cerianthid, partially withdrawn cerianthid .

This document maintained by Dr. Bill Bushing.
Material © 2004 Star Thrower Educational Multimedia