Bivalves

Below are examples of bivalves you are likely to run across if you spend any time at all on Oregon’s exposed shores. At the bottom of the page, I list some of my favorite field guides and identification resources that should help if you want to take things a little farther. My organization follows 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 noted in the caption. Some of the examples below, like California mussels, Mytilus californianus, live on intertidal rocks. Some, like Pacific razor clams, Siliqua patula, are abundant in intertidal sands. Several other examples don’t live on the exposed coast, but we see them there because they wash up on beaches after living out their lives in sister ecosystems. Gaper clams, Tresus, are an excellent example of that.

Mytilus trossulus, blue mussel, bay mussel
On exposed shores, Pacific blues seem to settle best, or maybe I should say stand out most, in the higher intertidal, among acorn barnacles, Balanus glandula. On my home shores, patches of blue mussels are usually small, and they last but a short time. It’s not a slam dunk these are Mytilus trossulus. Three similar mussels — they all go by blue or bay mussels — occur on our shores; and hybridize! Blue mussels appeared in my post Standing Out in Mytilus trossulus.

Mytilus californianus, California mussel
They’re so ubiquitous in the rocky intertidal I tend to ignore them. That’s a genuine mistake. I wasn’t sure I had any good photos, but here are a few that show the different sides of Mytilus californianus. You can find additional images and a few more words about M. californianus at The Curves and Colors of California Mussels.

Adula, date mussel, pea pod
These are probably Adula californiensis. Empty shells washed up on the beach are all I’ve seen.

Chlamys hastata, spiny pink scallop
Mostly subtidal, the few I’ve encountered have been shells washed up on sandy beaches.

Crassadoma gigantea, rock scallop
I haven’t seen live rock scallops in the intertidal. However, shells are not too uncommon in the drift. Kozloff says the right-hand valve is attached, so I imagine the ones shown here are the free left valve.

Magallana gigas, Pacific oyster
Introduced, I’ve only seen them on the protected shores of the San Juan and Gulf Islands.

Pododesmus macroschisma, green false-jingle
I haven’t run into live false-jingles, but it’s not too uncommon to find small empty shells (this one’s about 2.5 cm in its long dimension) washed up on the beach.

Clinocardium nuttallii, heart cockle
Empty shells and shell fragments, such as the ones shown here, are common on exposed surf-swept beaches. However, in life, heart cockles prefer bays, eelgrass beds, and other quiet intertidal and subtidal settings. Therefore, any heart cockle shells you find on exposed beaches have drifted from different ecosystems.

Tresus capax, fat gaper
Fat gapers are likely to be the biggest clam you’ll come across in the Pacific Northwest. On exposed beaches, bleached shells can be abundant, especially near bays with good clam beds. Another less commonly encountered gaper in our area is, Tresus nuttallii, but we’ve never crossed paths. Fat gapers are featured in Getting to Know Gapers, and Have I Shown You My Chondrophore Collection?

Siliqua patula, Pacific razor clam
Bleached shells are abundant on most northern Oregon beaches.

Tellins or Tellens
I have always assumed any relatively large tellin I find is Megangulus bodegensis, the Bodega tellin. There are several species of tellins occupying various habitats, but Bodegas are considered bay clams in most accounts I’ve read. Kozloff says their habitat is clean sand at the mouth of bays that open onto the exposed coast.

Macomas
Any macoma I’ve found on exposed beaches has been a a drifted shell. There are lots of species. Some are bay clams and others live on exposed sandy shores with pounding surf. I haven’t tried my hand at identifying macomas, so I’ll leave it at that. The drifted shells are usually completely bleached, and not too common on my home beaches.

Nuttallia obscurata, purple varnish clam
In northern Oregon, this introduced clam is a bay clam. Any shells you find on exposed beaches have drifted there, or humans have carried them.

Saxidomus, butter clams
Those shown here are probably S. gigantea. I’ve always thought of butter clams in northern Oregon as bay clams. However, they also occur in sheltered Salish Sea waters, where I took some of the images below. Butter clams are large, like gapers, but they lack chondrophores; you won’t confuse the two.

Leukoma staminea, Pacific littleneck clam
Sometimes thought of as bay clams, littlenecks can also be found in other sheltered settings, even on the exposed coast, as the two below were. However, unless you dig a little, you’ll probably only find drifted shells.

Mya arenaria, soft-shelled clam
In northern Oregon, soft-shells are bay clams. Any shells you find on exposed beaches have drifted there, or humans have carried them.

Penitella penita, flat-tip piddock
Shells can be abundant on beaches near sediments suitable for burrowing. I wrote a little about flat-tips in Flat-tip Piddock, Penitella penita.

Boring clams and their workings
I’m not sure what these borers are. If you know, please drop me a line. Borings, when numerous, can transform a landscape. I wrote a little about that in Boring Clams Lend an Otherworldly Appearance to a Miocene Shelf. Many a beach walker has come across a piece of shale riddled by boring clam workings. These are frequently dragged ashore in bull kelp holdfasts.


References

Gotshall, D. W. 2005. Guide to Marine Invertebrates, Alaska to Baja California 2nd Edition (Revised). Shoreline Press.

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 molluscs page


This page was updated on October 17, 2021