Hydroids

Below are examples of hydroids you might find in the rocky intertidal or washed up on the beaches. These are just the ones that are big enough for me to easily find and photograph, so it’s not a big list. As for identification, I can get close with the readily available field guides and online resources (see References, at the bottom of the page). There’s nothing new here except my my photographic take on common hydroids, and a natural history note if I can think of something that hasn’t already been over-said.

I use the organization followed by Lamb and Hanby (2005). Common names are my choice. I refer to WoRMS for scientific names, and I learn a ton by browsing the natural history riches in Kozloff (1993). All photos are from northern Oregon unless otherwise noted in the caption.

Let’s explore some hydroids!


This hydroid seems fairly common wherever there is surfgrass, so maybe it’s Amphisbetia furcata, the surfgrass hydroid, or something very close. It’s got the look.

Closeup, peering through the phyllospadix

And this one, hosted by Laminaria sinclairii, has a very similar look.

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Aglaophenia, ostrich plume
Most of the ostrich plumes I see are in the low intertidal. Fronds have a feather-like look. Three of the four shown below are attached to vertical rocks. They also favor kelp stipes.

In the foreground an exposed vertical wall occupied by sea cabbage and Aglaophenia. A tidepool and a distant rock wall share the scene.
Aglaophenia on an exposed wall
A medium closeup view of a patch of Aglaophenia exposed on a rock at low tide. An colonial orange tunicate encrusting the rock at the upper left.
Aglaophenia likes vertical structure and kelp stipes
A medium closeup of several fronds exposed by low tide.
Aglaophenia is a good-sized hydroid, fronds can reach 15 cm, but that’s unusual in the intertidal
A round baseball-sized mass of Aglaophenia on damp sand near a mussel shell.
Drifted ostrich plumes are fairly common
Closeup sufficient to show the reproductive structure in the context of several complete fronds exposed by low tide.
Light orange reproductive structures the shape (and size) of a rice grain
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Abietinaria, coarse sea fir

A few branches of a drifted specimen, on beach sand
Drift coarse sea fir hydroid, probably Abietinaria
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These beauties are from a surf-exposed headland. They’re probably Ectopleura, and if they are Ectopleura, they’re probably E. marina, pink-mouth hydroid. Their characteristics (no branching from the base) and habitat make me think they are E. marina rather than E. crocea, which forms clumps and tends to be a species of quieter waters. But there are alternatives to Ectopleura, and these may be lacking an essential feature. If you know what they are, please let me know.

Exposed at low tide
Polyps, tentacles extended
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References

Harbo, R. M. 2011. Whelks to Whales: Coastal Marine Life of the Pacific Northwest. 2nd ed. Harbour Publishing Co.

Kozloff, E. N. 1993. Seashore Life of the Northern Pacific Coast. 3rd ed. University of Washington Press.

Lamb, A. and B. P. Hanby. 2005. Marine Life of the Pacific Northwest. Harbour Publishing.

Sept. J. D. 2019. The New Beachcomber’s Guide to the Pacific Northwest. Harbour Publishing.


Online Resources

Biodiversity of the Central Coast’s Cnidarians and Ctenophores page

iNaturalist’s pages on hydroids are full of good browsing material. Here are some I looked at while preparing this page:

Surfgrass hydroid, Amphisbetia furcata

Ostrich-Plume Hydroids, Aglaophenia

Coarse Sea Fir Hydroids, Abietinaria

Pink-mouth Hydroid, Ectopleura marina


This page was updated on April 12, 2022