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gaper clam neck
Gaper clam seen feeding underwater
Gaper Clams


About gaper clams

Gaper clams Tresus capax are found in several Oregon estuaries.They are known by a variety of names including blue, Empire, horse and horseneck clams.They are Oregon's largest common clam. Geoducks can grow much larger (as much as 10 pounds!) but are rarely found south of Puget Sound in Washington. There are two species of gaper clams in Oregon. Tresus capax is by far the most common, Tresus nuttalli is found in most estuaries that have gapers but are rarely harvested.

Species profile

gaper neck

What's on their necks?

Gaper clams have large siphons that protrude above the substrate surface when feeding. Protective leathery plates are found just below the siphon tips. It is common for algae to grow on their neck as well.


gaper pea crab crawling on siphon of gaper clam

Gaper pea crabs

Clam harvesters often notice a pair of small crabs living in the mantle of their gaper clams. These crabs are gaper pea crab, Pinnixa faba. Once settled, mature females grow too large to leave the clam and spend the rest of their lives in the clam. Males, which are markedly smaller, move from clam to clam throughout their lives.

A male gaper pea crab (right) seen crawling around the siphon of a gaper clam.

gaper shell

gaper chondrophoresGaper chondrophores

At the hinge of a clam, a cavity can be found that is known as a chondrophore. In gaper clams, this cavity is very pronounced, and is the thickest part of a gaper shell. As a shell is deteriorated through the years, the chondrophore is the last part of a gaper still recognizable by species. Over time, chondrophores get polished by sand and surf and are found by beach combers. Chondrophores are used in the prefered method to age gaper clams. Annulli (annual lines) can easily be seen on the surface of these cavities.

softshell and gaper comparison

Why are they called "Gapers"?

Gaper clams have very large siphons. Having such large siphons enable gapers to live very deep in the substrate, thereby avoiding many clam predators. As seen in the picture to the left, a softshell (upper clam) is compared to a gaper (lower clam) which have evolved to not close completely, thus leaving a gape to accommodate the siphon.

geoduck, fat gaper, pacific gaper

11 gaper necks underwaterSimilar clams

The more common "fat" gaper clam and the less common Pacific gaper are sometimes mistaken for Geoduck clams.

The photo on the left shows the differences in shell shape between them.


Eleven gaper clams seen in close proximity underwater (right)








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