Life’s a beach for the false sandcastle worm

Eyes Under Puget Sound - Critter of the Month

Top end of worm with spiky golden hairs and curly pink tentacles, on a black background.
Have you spent countless hours lounging around this summer, playing on sandy beaches? Well, this month’s critter has you beat. With its beachy name and sandy dwelling, the false sandcastle worm is the quintessential beach bum.  


Brick and mortar

The false sandcastle worm belongs to the family Sabellariidae, a group of marine annelids known for their tube-building abilities. Lots of worms glue sand or mud together to make dwellings, but the sabellariid tube is in a class of its own. We are talking "benthic brick house" — hard as a rock and very difficult to break apart. The secret is in the “mortar” — a mucous-protein cement that is secreted from a U-shaped gland behind the worm’s head, called (very aptly) the building organ.

Classification text reads Kingdom Animalia, Phylum Annelida, Class Polychaeta, Family Sabellariidae, Genus/Species Neosabellaria cementarium

Don’t worry - you won’t unearth these worms while constructing your own castles in the sand. The false sandcastle worm prefers to work its architectural magic below the low water line, in intertidal to subtidal depths from Alaska to Southern California.


Tiny tube living

The false sandcastle worm spends its entire life in its 4 mm-wide tube, and it has some amazing adaptations to make tiny tube life a bit easier. Thick golden hairs called paleae on the worm’s head interlock to form an operculum, or door, that plugs the tube at the top. Not only does this keep predators at bay, but at low tide, it can seal a fresh batch of sea water inside. A pair of branchiae, or gills, on each body segment pulls oxygen from the trapped water until the tube is re-submerged.

A pink spiky worm with a body curved in a C-shape and a long brown tube sticking out of its rear end, on a black background.

A whole N. cementarium specimen from Bellingham Bay with tube removed.

The operculum is also used as a scrub brush, periodically scouring the opening of the tube to keep it sparkling clean. Speaking of cleaning… did you notice the long appendage sticking out of the worm pictured to the left? Well, that is its rear end, and it is filled with poop. With such a lengthy behind, waste gets expelled much closer to the opening of the tube, keeping things neat and tidy down below.


True or false?

So… what makes our Puget Sound species “false,” anyway? When this critter was described in 1906, the name “sandcastle worm” was already taken — by its much-more-famous cousin, Phragmatopoma californica from Southern California. You tell these two brilliant builders apart simply by color. Unlike Neosabellaria cementarium’s golden crown, P. californica’s tentacles and operculum are royal purple.

A spiky white and yellow worm with purple tentacles on a black background.

A live specimen of Phragmatopoma californica collected from San Pedro, Los Angeles. Photo by Josh Silberg, Hakai Institute.



These two species can also be distinguished by the grandness of the structures they create. P. californica sticks its tubes together in amazing reefs that, when exposed at low tide, make parts of the California coast look like the surface of the moon. With all the tube openings pointing in the same direction, the formations resemble the honeycomb structure of bees, giving rise to the alternate family name, “honeycomb worms.” Whatever you call them, these worm reefs actually function like real reefs, providing habitat for organisms like sponges, bryozoans, and tunicates. Many species of worms use the empty tubes and crevices for shelter. And of course, predators like fish and hermit crabs love feasting on the sandcastle worms’ tentacles when they are extended during feeding.


I’ll settle for you

You might be wondering how a tiny worm larva floating around in the water column finds its future castle-mates. All baby sabellariids need sand to induce metamorphosis into their adult form, and to tell them where it is suitable to settle. Reef-building species like P. californica respond best to sand that’s already been made into tubes by adults of their own species. Fatty acids in the mucous cement of the tubes give off a unique chemical signature that the larvae follow like a road map to their sandy settling ground.


Going it alone

The false sandcastle worm, on the other hand, is more of a loner. Throughout most of its range, it prefers to shack up by itself rather than adding on to a giant worm fortress. In laboratory experiments, the larvae showed no preference between the tubes of their brethren and plain old sand, suggesting that this species does not aggregate in response to a chemical cue. They generally colonize rocks or even the shells of bivalves (like the spiny pink scallop) in small numbers, instead of forming reefs. 

Speckled brown rock with yellow arrows indicating sandy worm tubes stuck to it. Background is black with scale bar reading 5 millimeters.

This small Puget Sound pebble from Bellingham Bay has several Neosabellaria cementarium tubes attached to it.


Worms of a feather

There are exceptions to this rule, however. Scientists diving near Coos Bay, Oregon in the late 1990s discovered an extensive worm reef composed largely of N. cementarium tubes — something that had not been observed in the area before. There is a similar aggregation in Northern Puget Sound. Perhaps reef-building occurs where sandy substrate is limited, or when conditions are just too perfect to pass up.

Amazingly, there’s one thing the worms are never desperate enough to do. In parts of California where the ranges of the “true” and “false” sandcastle worms overlap, they don’t intermingle… ever. I guess a worm’s home really is its castle and no other’s!


Critter of the Month

Dany is a benthic taxonomist, a scientist who identifies and counts the sediment-dwelling organisms in our samples as part of our Marine Sediment Monitoring Program.We track the numbers and types of species we see to detect changes over time and understand the health of Puget Sound.

A smiling female scientist in yellow foul weather gear, red bandana and life jacket holds up a large piece of red seaweed on a sunny day.
Dany shares her 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.