Getting to Know Gapers

Everybody loves collecting seashells on the beach and I’ve collected a few myself. Sandy beaches are great for finding beautiful shells because scouring sands keep them clean and round off jagged edges. Currents transport shells long distances from their home waters, so you never know what you’re going to find. I wrote about this in Answering the Clam Challenge: Which Shells for Which Beaches? If I’m walking along the beach, I’m thinking about the tides, watching the surf and keeping an eye out for interesting items in the driftline. The sweep of the surf reveals lots of shells, often only fragments; and learning to identify them has become a rewarding pastime.

These are all gaper clam shells. We think of gapers as bay clams but their shells can be abundant on exposed coastal beaches adjacent to river mouths. I’m guessing the shells of long gone gapers are transported to surf-swept shores by heavy flows in the estuaries. That’s probably most of the story, but some gaper shells found on open coast beaches may have an oceanic origin. It figures that Ricketts and Calvin would have something to say about this. Referring to the Pacific gaper, Tresus nuttallii, Ricketts and Calvin say:

“…although it decidedly prefers quiet bays, it may be found on the outside coast; but outer-coast specimens are small and look rather battered.”

Lots of shells end up on the beaches and with a little practice, most are easy to identify. Here are a few I’ve found.

You can easily a distinguish a gaper shell, even a fragment, from other shells on the beach by the the distinctive spoon-shaped chondrophore on the hinge. It’s the toughest part of the shell and the last to disintegrate under constant sand scouring; in gaper country chondrophores get abundant. I happen to have 23 chondrophores. I collected them haphazardly over the past few years and haven’t looked at them much, but I examined each one while writing this post. Turns out I collected 12 left valve and 11 right valve chondrophores. No valve bias here.

Fat gaper chondropores
Fat gaper chondrophores

Gapers are renowned for their variety of local common names. For the fat gaper, T. capax, alone, local names include horse clam, fat horse clam, Alaskan gaper, summer clam, otter clam, horseneck, rubberneck, blue clam, Washington clam, and northern gaper. The list for the Pacific gaper, T. nuttallii, is at least as long and mostly overlaps that of T. capax. Ricketts and Calvin are in great form when they comment on the vernacular:

“After this appalling array of popular names it is almost restful to call the animal Tresus (once familiarly known as Schizothaerus!) nuttalli…”

All the gaper images in this post are the fat gaper, Tresus capax. It’s the only gaper I seem to encounter, but as I have mentioned, there is another common gaper. The Pacific gaper, Tresus nuttallii, is the most common gaper in central and northern California. The shells of the two common gapers are slightly different in shape. Kozloff says the shell of T. nuttallii is generally more than one and a half times as long as it is high, while the shell of T. capax is generally about one and a half times as long as high. If Kozloff is correct, I can perform a test. The measurements for the eight complete gaper shells in my collection are:
Allowing that there may have been a bit of erosion along the ventral margin, looks like I was right – fat gapers. I look forward to finding the shell of the Pacific gaper; I’ll keep my eye on the driftline.


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.

Ricketts, E. W. and J. Calvin. 1968. Between Pacific Tides. 4th ed., revised by J. W. Hedgpeth. Stanford University Press.

Sept. J. D. 2009. The Beachcomber’s Guide to Seashore Life in the Pacific Northwest. Revised ed. Harbour Publishing.

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