Yoldia myalis (Couthouy, 1838)

Common name(s):  Oval yoldia

Synonyms:  Yoldia cascoensis
Phylum Mollusca
Class Bivalvia
Subclass Palaeotaxodonta
Order Nuculoida
Family Yoldiidae
Yoldia myalis from 100 m depth trawl in San Juan Channel, WA
(Photo by: Dave Cowles, July 2007)
Description:   Clams in Order Nuculoida have valves similar to one another.  The hinge plate has taxodont dentition (photo).  The shell is aragonitic, longer than high and has no radial ribs.  The periostracum is smooth.  They are protobranchs, so their gills are simple platelike leaflets suspended in the mantle cavity.  They have large palps near the mouth which they use to gather food.  Members of Family Yoldiidae have an external hinge ligament, the interior of the valves is not pearly, they have siphons and a pallial sinus, they have a chondrophore, the shell gapes at the posterior end, and they have taxodont dentitionYoldia myalis has a rather flat (not swollen) shell,  concentric growth lines but no other concentric sculpture, the umbone is in the middle of the shell, and the length of the shell is less than twice the height.  The anterior end is rounded, while the posterior is bluntly pointed.  Brown or dark green periostracum.  The interior is yellowish-white.

How to Distinguish from Similar Species: Yoldia amygdalea has umbones posterior to the middle and the length is twice the heightYoldia seminuda has concentric lines crossing the growth lines. Yoldia thraciaeformis has a swollen shell.

Geographical Range:   Arctic Ocean to Puget Sound, North Atlantic

Depth Range:


Biology/Natural History:

Yoldia is one of a group of "primitive" "Protobranch" bivalves which are different structurally from other bivalves.  Most protobranchs live with the anterior end down in sediment.  They have two small, bipectinate gills on the posterior end.  Water is circulated from the anterior end toward the posterior end and across the gills, rather than in at the posterior ventral end, across the gills, and out at the posterior dorsal end as in most clams.  The protobranchs usually have long extensions of the mouth called labial palps, which they extend into the sediment and pick up particles for feeding.  Some Protobranchs, including Yoldia, also have a small flattened "sole" on their foot.  The sole has left and right halves which can be folded together to collapse the foot into a narrow profile.  The foot is then inserted into the sediment, the sole is unfolded to its wide configuration, and the foot is retracted to draw the clam down into the sediment.

Members of Subclass Palaeotaxodonta have equal valves with taxodont dentition and protobranch (bipectinate) gills.



Dichotomous Keys:
  Kozloff 1987, 1996

General References:
   Morris, 1966

Scientific Articles:

Web sites:

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


Members of Order Nuculoida have taxodont dentitionYoldia myalis has about 12 teeth on each side of the umbo.

Taxodont dentition
The taxodont hinge teeth project strongly from the hinge plate.  The chondrophore can be seen in the center.

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