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!
Amphisbetia furcata, surfgrass hydroid
This hydroid seems fairly common wherever there is surfgrass.
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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 fours shown below are attached to vertical rocks. They also favor kelp stipes.
Light orange reproductive structures the shape (and size) of a rice grain 
Aglaophenia likes vertical structure and kelp stipes 
Aglaophenia is a good-sized hydroid, fronds can reach 15 cm, but that’s unusual in the intertidal 
Drifted ostrich plumes are fairly common
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Ectopleura (maybe), and if they are Ectopleura, they’re probably E. marina, pink-mouth hydroid
These are from a surf-exposed headland. 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.
Ectopleura marina exposed at low tide 
E. marina polyps, tentacles extended
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. 2009. The Beachcomber’s Guide to Seashore Life in the Pacific Northwest. Revised ed. 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
Pink-mouth Hydroid, Ectopleura marina
This page was updated on February 7, 2021
