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The Beach Assessment project recruits citizen volunteers to help assess the size and composition of the clam populations on several of King County's beaches. The volunteers collect live clams, identify, weigh and measure them, and then return them unharmed to the beach. The following identification key was developed for the volunteers and other interested citizens to identify commonly found clams without harming the clams or their environment.

Marine beaches are sometimes closed to shellfish harvesting (clams, oysters, geoducks, and mussels) because of dangerous levels of naturally occurring toxins in the water. Shellfish acquire these toxins from phytoplankton, their food, and can pass it on to humans who eat the affected shellfish. There are many different forms of toxins, and the effects on humans can range from gastro-intestinal problems to death.

Please find out if your beach of interest is closed to shellfish harvesting before digging or eating any clams from Puget Sound. This information is located on the Department of Health Marine Biotoxin web site or by calling 1-800-562-5632.

The most commonly encountered clams on King County's marine beaches are the manila clam, native littleneck, butter clam, cockle, horse clam, eastern softshell and macoma clams. To identify these clams, you need only know whether the leathery hinge which holds the shells together (the ligament) is visible from the outside of the shells, whether the shells are mirror images of each other, the dominant type of ornamentation on the shells ( rings or ribs), and whether the shell is rounded on both ends or is pointed on one end.

Most commonly found intertidal clams




First, look for the ligament, a leathery hinge which holds the shells together. It is visible on five of the clams but is invisible on two -- the horse clam and the softshell clam (Figure 2).


Figure 2


To tell the horse and softshell clams apart, hold the clams on their ends and see whether the outlines of both shells are the same. If they are, then it is a horse clam (Figure 3 - right). If one shell seems to overlap onto the other (Figure 3 - left), then it is an eastern softshell clam. Also, look at the end of the neck. If there are leathery plates or flaps present (Figure 4), then it is a horse clam.

horse clam
Figure 3
Figure 4

Now, back to the other five clams (Figure 5). Do the shells have obvious ornamentation or are the shells mostly smooth but showing faint rings?

If the dominant ornamentation consists of ribs (Figure 6), it is probably a cockle.

Figure 5
Figure 6

If both rings and ribs are about equally dominant giving the shell a checkered appearance (Figure 7), then the clam is either a manila clam or a native littleneck.

Figure 7

Those two can be differentiated by looking for a depression next to the umbos (points where the shells started growing). If the area is flat or depressed (Figure 7 - top), then it is a manila clam. If a ridge formed by the edges of the two shells runs through that area, then the clam is a native littleneck (Figure 7 - bottom).

If there is little ornamentation on the shells, then look to see whether the ends of the shells are rounded to nearly the same extent (butter clam) or if one end is somewhat pointed giving the shell outline a triangular appearance (sand clam, a macoma) (Figure 8, left). The chalky grayish-white butter clam shell very nearly describes an oval (Figure 8, right).

sand macoma
butter clam
Figure 8
Figure 8

The macomas comprise a fairly large group of clams in this area but these four species are those most likely to be found in the intertidal zone of Puget Sound beaches: inconspicuous macoma (Macoma inconspicua), sand clam (Macoma secta), bent-nose clam (Macoma nasuta) and the polluted macoma (Macoma irus). The Macomas are interesting in that they tend to lay flat in the substrate unlike most of the other intertidal clams which are usually on their edges. Also, unlike most other clams, their siphons are separate -- not fused together. Most clams passively filter the water to obtain their food while the macomas use their long (up to 18) inhalant siphon like a vacuum cleaner hose which they move around to suck up detritus from the surface of the substrate.

macoma clams Of the four, only the sand clam grows large enough -- up to 4 -- to be sought out for food. The other three only reach a maximum length between 1 to 2. Another distinguishing characteristic is that one edge of the sand clam is nearly straight as though the end had been cut off. When the bent nose clam (M. nasuta) is laid on a flat surface or is looked at from the edge, the curve in the shell which gives the clam its name is very obvious.

While all of the macoma shells are at least vaguely triangular in outline, the inconspicuous macoma (Macoma inconspicua) is the least triangular but it can be differentiated by the eroded appearance of the shell and pink color exposed in the eroded areas. The inside of the shell is usually deep pink to salmon colored.

The polluted macoma (Macoma irus) is definitely wedge shaped, cream colored and has a band of brown periostracum (papery coating) along the margin of the shell. This one is more often found in the same habitat as the littlenecks and butter clams.

There are other reasons for wanting to be able to identify the clams which you dig up on the beach.

  • Some clams have size and weight limits placed on them by the Department of Fish and Wildlife or some other management agency. Some clams are more palatable than others.
  • Cooking procedures vary with species so you need to know which clams you have before you can choose the best way to cook them.

The Washington State Department of Fish and Wildlife sets size and bag limits for clams. These are the 1996-97 limits, so please contact the Department for current limits and regulations.

  • Horse clams - Daily limit = first seven dug Geoducs - Daily limit = first three dug Manila, littleneck, butter, cockles, softshells, macomas and other native clam species (except horse clams and geoducs) - Daily limit = 10 lbs. in the shell or 40 clams, whichever comes first.
  • Manila, littleneck and butter clams must be at least 1 1/2" (inches) across the longest distance of the shell. No size limit on the other clams.

Each person digging for clams must:

  • Have a shellfish/seaweed license and wear it on his/her outer clothing Dig their own limit and keep this limit separate from others' clams
  • Fill in the holes before leaving the beach

Here is an abbreviated edition of the clam identification key. You can print this out, take it along on your next visit to a marine beach, and identify the clams you find.