Mya (Arenomya) arenaria Linnaeus, 1758

Common name(s): Softshell clam, Eastern softshell clam, Mud clam, Soft clam, Long-necked clam, Long clam, Steamer clam, Nanny nose, Sand gaper

Subclass Heterodonta
Order Myoida
Suborder Myina
Mya arenaria from the beach at Freeland, Holmes Harbor, Whidbey Island.
(Photo by: Dave Cowles, July 2007)
Description:   As with other members of Family Myidae, this species has a shelflike or spoonlike chondrophore projecting from the hinge plate of the left valve, has two adductor muscles of similar size, and no radial ribsMya arenaria has a well-developed, deep pallial sinus (photo).  The chondrophore (on the left valve) is well-developed, projecting as far or nearly as far as it is wide (photo).  The umbones are only slightly anterior to the middle (photo).  The hinge ligament is internal, attached to the chondrophore (photo).  The posterior end of the shell is less smoothly rounded than is the anterior but it is not truncate.  The valves may gape slightly at each end, especially the posterior end (photo).  The shell is brittle and chalky white or gray outside with some light yellow or brownish periostracum and uneven concentric rings.  The siphons are light brown with darker tips (photo).  Length to 17 cm, most often 10 cm or less in our area.

How to Distinguish from Similar Species: Mya truncata and Platyodon cancellatus have a truncate posterior end and rarely exceed 7 cm.  This species may be found along with horse clams such as Tresus capax and may look like a small individual of that species or especially like Tresus nuttallii, but the horse clams such Tresus capax have a chondrophore in BOTH valves and a large gape between the valves at the posterior end (photo).

Geographical Range:  Norton Sound, AK to Elkhorn Slough, Monterey, CA; Japan, Kamchatka, North Atlantic, North and Baltic Seas

Depth Range:  Intertidal (optimum depth in San Francisco Bay is 30 cm above zero tide line)

Habitat:  In mud or sandy mud, sometimes mixed with gravel.  Common in estuaries and other areas of reduced salinity (to as low as 23% seawater).

Biology/Natural History:  This clam has been introduced from the Atlantic, being first seen in San Francisco Bay in 1874.  It slowly spread north, and reached Alaska in the 1950's.  By the 1920's it seems to have largely displaced the native clams in San Francisco Bay.  This clam has long siphons, and can be 20-35 cm below the surface.  It can live anaerobically for several days, and dissolves the shell to buffer acidity in these conditions.  In San Francisco Bay its optimal intertidal depth is 30 cm above zero tide line.  The siphons appear as slits at the surface of the mud, and emit a spurt of water as they contract if one steps near them.  This clam burrows only slowly, without using the foot much.  Burrowing is by closing the valves and forcefully ejecting water.  Predators include skates, rays, and sharks.  Predators in Europe include oystercatchers and curlews, from which the clams have a refuge in depth if over 15 cm deep.  May contain pea crab symbionts.  Mature at about 2-4.5 cm, and spawn in spring or summer.  This species is highly esteemed for food.



Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Brusca and Brusca, 1978
  Carefoot, 1977
  Fitch, 1953
  Harbo, 1997
  Harbo, 1999
  Kozloff, 1993
  Morris, 1966
  Morris et al., 1980
  Niesen, 1994
  Niesen, 1997
  O'Clair and O'Clair, 1998
  Rice, 1973
  Sept, 1999

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

Hinge anterior view
In this view of the hinge mechanism from the anterior end of the shell, the large chondrophore projecting from the hinge plate of the left valve can be seen.
The left chondrophore attaches by the hinge ligament (dark brown in this photo) to a socket in the right valve, dorsal to the chondrophore.

The large chondrophore projecting from the left hinge can be seen here.  This view is looking toward the hinge plate from inside the clam.  The hinge ligament attaches to the other side of the chondrophore. This view shows the left chondrophore from the hinge plate side.  This is the side the hinge ligament attaches to.  Notice the projection from the anteroventral margin of the chondrophore, which probably gives stronger attachment to the ligament 
In this view of the right hinge plate from perpendicular to the shell, the socket in the right valve to which the hinge ligament from the left chondrophore attaches can be seen.  Note that the socket angles back from the plate Tipping the right valve up so that the socket can be clearly seen shows the size of the socket.

The inside view of the clam shows the deep pallial sinus, the large anterior and posterior adductor muscle scars,  and the arrangement of the chondrophore (left valve) and socket (right valve).
Anterior is to the left in this view.  The upper valve is the left valve, the lower is the right valve.

This view of the umbo, which is slightly anterior to the middle of the shell, also shows that both ends, especially the posterior end (to the right) often gape at least slightly.  No hinge ligament can be seen because it is internal and attached to the chondrophore.  Note the chalky texture and brittleness of the shell.

The siphons in this species are brown with darker tips.  These siphons are not fully extended.

Authors and Editors of Page:
Dave Cowles (2007):  Created original page
CSS coding for page developed by Jonathan Cowles (2007)