The solitary pink-mouth hydroid keeps it together (at a distance)

Eyes Under Puget Sound - Critter of the Month

Pink hydroid polyp with flower-like tentacles on black background
As we prepare to re-enter the world with cautious optimism, many questions remain about the extent to which we can return to our old way of life. Can we pick up where we left off with parties, gatherings, and crowds or are our ideas about proximity to others forever changed? This month’s critter demonstrates that it might just be possible to embrace togetherness — at a safe distance.

Social circle

Hydroids are the lesser-known cousins of corals, jellyfish, and anemones. Like corals, hydroids typically form colonies, with many polyps physically connected and functioning as one organism.

Text reads, Kingdom Animalia, Phylum Cnidaria, Class Hydrozoa, Order Anthoathecata, Family Tubulariidae, Genus/Species Ectopleura marina
The solitary pink hydroid, Ectopleura marina, isn’t one of these colonial species — each individual lives separately. Don’t let the name fool you, however — they aren’t total loners. You can find these pretty polyps from British Columbia to California, living in well-spaced clusters with their closest family and friends.

Tropical state of mind

Beautiful and delicate, E. marina looks more like a tropical palm tree waving in the spring breeze than an animal — that is, if palm trees came in pink and lived underwater! Their habitats of choice are rocks, floats, and other hard surfaces in the low intertidal and subtidal zones where the current is fast-flowing, especially on exposed or semi-protected rocky shores. In Puget Sound, they love the Nisqually Reach area, where they can stick onto small pebbles and the hard tubes of other organisms.  

Two side-by-side photos showing several circular polyps with wispy white tentacles extended, in front of rocky reef backgrounds

LEFT: Ectopleura marina. Photo by Ryan Murphy, Race Rocks Ecological Reserve. RIGHT: E. marina polyps with feeding tentacles extended. Photo courtesy of Jackie Hildering,

Mouthing off

Tough, chitinous filaments at the base of E. marina’s slender, three-inch stalk keep it anchored in place so it can focus on what it does best — feeding. The pom-pom of tentacles at the top of the stalk are arranged in two rings: an inner ring of short oral tentacles around the mouth and a longer outer ring of feeding tentacles that reach out to snag prey. Like all cnidarians, their feeding tentacles contain stinging cells called nematocysts that stun small crustaceans, chaetognaths, marine worms, and any planktonic larvae that happen to be drifting by. 

A white and orange sea slug creeps up a hydroid stalk towards the pink polyp, surrounded by other hydroid stalks on a dark green background.

Opalescent nudibranch on the attack! Photo courtesy of Jackie Hildering,

You might think stinging cells would also deter predators, but guess again. E. marina is a favorite food of the voracious opalescent nudibranch. Instead of shying away from the hydroid’s secret weapon, the nudibranch chows down on the tentacles and incorporates the nematocysts into its own tissues.

Baby pink

The little blobs between E. marina’s tentacle rings are reproductive structures called gonophores. Each hydroid polyp is a single sex, and the gonophores produce either eggs or sperm. Sperm are released into the water, but fertilized eggs stay in the gonophores and develop into larvae called actinulae. After the tiny, tentacled actinulae hatch out, their first act of business is finding a place to settle. Generally, this means posting up close to their parents while leaving plenty of space to maintain their solitary and independent lifestyle.

Top view of pink polyp showing grape-like reproductive structures at the base of the long white tentacles.

Top view of Ectopleura marina polyp. Photo by A. Harding, courtesy of Race Rocks Ecological Reserve

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 with long hair and a black polka dot blouse sits in front of a computer and microscope.
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.