Eyes Under Puget Sound: Critter of the Month — Tube-Dwelling Anemone

Wildflower season is upon us here in the Pacific Northwest, and this month’s critter is a reminder that the marine environment has “flowers” of its own! Meet the tube-dwelling anemone, a delicate blossom at the bottom of Puget Sound.

Kingdom, Animalia; Phylum, Cnidaria; Class, Anthozoa; Order, Spirularia; Family, Cerianthidae; Genus, Pachycerianthus; Species:P. fimbriatus

Blooms of many colors 

With their wispy crown of tentacles, tube-dwelling anemones might easily be confused with tube worms, but they actually belong to the phylum Cnidaria, which also includes corals and jellyfish.

These tube-dwellers are not true anemones; they are in a special group called cerianthids, and Pachycerianthus fimbriatus is the only one found in Puget Sound. They range in color from white to purple to orange, and often grow in large groups that look a bit like flowers in a meadow.

Rough tubular base on sand with flowing tentacles

The tube-dwelling anemone Pachycerianthus fimbriatus, photographed at the Seattle Aquarium

P. fimbriatus can be found from southern Alaska to Baja California, in depths of up to 268 meters (about 800 feet). In our sampling, we encounter them most frequently in Bellingham Bay, Sinclair Inlet, and the South Sound — places where they can easily burrow into the soft mucky sediment.

Down the tubes

P. fimbriatus has a soft, vulnerable body, so it burrows into the mud and creates its own protective dwelling around it. Its building materials are not sand or mud as you might expect, but a special type of thread secreted by the animal, called a ptychocyst (pronounced “TIE-co-sist”).

Three anemones with tentacles of pink and white

Different-colored individuals of Pachycerianthus fimbriatus at the Monterey Bay Aquarium; photo courtesy of Dave Cowles at inverts.wallawalla.edu

All cnidarians have nematocysts (stinging cells) but ptychocysts are unique to tube-dwelling anemones. The ptychocyst threads are woven together to form a thick, slippery casing that projects above the sediment’s surface, and may extend up to 1 meter (3 feet) in the mud below.

The anemone can retract its entire body into the tube, which comes in handy when it encounters its main predator — the giant nudibranch, Dendronotus iris. The nudibranch grazes on the anemone’s tentacles, and also cleverly lays its eggs on the outside of the anemone’s tube, so its young are born right on top of their meal. All this snacking doesn’t necessarily mean death for the anemone — it has tentacles to spare, and can live up to 10 years.

Pinkish slug with bright orange appendages climbs the “tree” of the anemone. The egg mass looks like bubbles or a clump of white lace.

P. fimbriatus with its sea slug predator, Dendronotus iris, climbing its tube. Note the slug’s coiled white egg mass in the background. Photo courtesy of John Yasaki, Oakland, CA.

Lord of the rings

P. fimbriatus can be easily identified by its two distinct rings of tentacles. The outer ring is long (used for reaching out to catch the small crustaceans it likes to eat) and the inner ring is short, used for transferring food to the mouth.

In some species of tube anemones, the tentacles are also able to absorb ultraviolet rays and glow with a fluorescent light.

Straight white translucent spines emerge from a center extrude small wavy purple tentacles surrounded by a pinkish flower-shaped ring.

P. fimbriatus, close-up view of tentacle rings from above. Photo courtesy of Mike Munroe, www.mikejmunroe.com.

Race against slime

Cross-section of gray clump with labels: center is “dissected tube.” Inside, a thick lumpy pouch—“body,” pinkish paintbrush head—"tentacles"

A preserved P. fimbriatus specimen with its tube cut open to reveal the animal inside.

Although a living tube-dwelling anemone is a pretty sight underwater - with its colorful tentacles waving in the current — nothing makes us marine scientists groan more than seeing one in a freshly collected sediment sample.

When we catch P. fimbriatus in our benthic grab, the fine threads of its tube become a tangled mass, turning into a black slimy goo ball that entangles everything it touches. We try to separate these animals from the rest of the sample as soon as possible to avoid creating a giant mess!

Critter of the Month

Our benthic taxonomists, Dany and Angela, are scientists who identify and count the benthic (sediment-dwelling) organisms in our samples as part of our Marine Sediment Monitoring Program. We track the numbers and types of species we see in order to understand the health of Puget Sound and detect changes over time.

Dany and Angela share their discoveries by bringing us a Benthic Critter of the Month. These posts will give you a peek into the life of Puget Sound’s least-known inhabitants. We’ll share details on identification, habitat, life history, and the role each critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.