Acila castrensis (Hinds, 1843)

Common name(s): Divaricate nutclam, Camp nutclam, Tent nutshell, Sculptured nutshell

Synonyms: Nucula castrensis Shell sculpture
Subclass Protobranchia (Paleotaxodonta) 
Acila castrensis, 12 mm long (left to right), picked up from the sediment of Burrows Bay at about 15 m depth by otter trawl July 2017. The hinge is at the top and the anterior end is to the left.
(Photo by:  Dave Cowles, July 2017)

Description:  Nut shells (Family Nuculidae) are small Protobranch clams with no siphons, an approximately oval shape with the anterior end somewhat squared off, and a pearly interior (photo). They have taxodont dentition on the shell (photo). Acila castrensis has a length approximately the same as the width, and both greater than the height. The outside of the valves are covered with a honey-colored, olive, or dark brown periostracum. The exterior sculpture of the shell is distinctive (see above): A series of radial ridges diverge ventrally from one another along the midline of the shell. Each ridge consists of a series of raised bumps, making a chevron-like pattern looking somewhat like a row of tents. Length to 2 cm, usually 15 mm or less.

How to Distinguish from Similar Species:  The small size and divaricate sculpture pattern on the shell is distinctive. Of clams with taxodont dentition, Ennucula tenuis is elongated and has no radial sculpturing. 

Geographical Range:  East Pacific ocean, from Bering Sea, Alaska to Baja California, Mexico and in the Gulf of California.

Depth Range:  Subtidal, 5-400 m

Habitat:  Muddy-sand bottoms

Biology/Natural History:  Little is written about the biology of this clam. Protobranch clams do not filter feed with their gills as do other clams. Instead, they have much smaller, simple gills that are used mainly for respiration. For feeding, they extend two pairs of elongated labial palps to pick up edible particles from the sediment by ciliary action. The innermost pair (flanking the mouth) are called labial palps, while the two outermost palps are longer and called palp proboscides. It is the palp proboscides which are extended outside the shell, while the labial palps do later sorting of particles. Order Nuculida is mainly found in the deep sea.

Stasek (1961) studied the complex ciliary patterns on the labial palps and concluded that the clams can capture food from the palp proboscides, from the labial palps themselves (via particles in the mantle cavity sticking to them), or from particles which stick to the gills and are passed on to the palps.

Unlike most bivalves, the blood of this species contains hemocyanin (Morse, 1986, Mangum et al., 1987).

The names of this clam refer to the distinctive chevron-like pattern of diverging ridges on the shell, which looks like an orderly camp of rows of tents.



Dichotomous Keys:
  Carlton, 2007
  Kozloff, 1987, 1996

General References:
  Abbott, 1986
  Harbo, 1997
  Lamb and Hanby, 2005

Scientific Articles:

Checa, Antonio G., Julyan H.E. Cartwright, and Marc-Georg Willinger, 2011.  Mineral bridges in nacre. Journal of Structural Biology 176: pp 330-339

Kellogg, James L., 1915. Ciliary mechanisms of lamellibranchs with descriptions of anatomy. Journal of Morphology 26:4 pp 625-701.  doi: 10.1002/jmor.1050260403

Mangum, C.P., J.L. Scott, K.I. Miller, K.E. Van Holde, and M.P. Morse, 1987. Bivalve hemocyanin: structural, functional, and phylogenetic relationships. The Biological Bulletin 173:11 pp 205-221. doi: 10.2307/1541873

Morse, M. Patricia, 1986. Hemocyanin respiratory pigment in bivalve mollusks. Science  231 pp 1302-

Stasek, Charles R., 1961. The ciliation and function of the labial palps of Acila castrensis (Protobranchia, Nuculidae), with an evaluation of the role of the protobranch organs of feeding in the evolution of the bivalvia. Journal of Zoology 137:4 pp 511-538. doi: 10.1111/j.1469-7998.1961.tb06087.x

Zardus, John D. and M. Patricia Morse, 1998. Embryogenesis, morphology and ultrastructure of the pericalymma larva of  Acila castrensis (Bivalvia: Protobranchia: Nuculoida). Invertebrate Biology 177:3 pp. 221-244

Web sites:

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

A group of Acila castrensis trawled from Burrows Bay sediment, July 2017

This opened individual shows the internal anatomy. The anterior end of the shell is to the right. The bright yellow membrane in the foreground is the mantle. The lighter yellow structure to the right appears to be the palps (probably the palp proboscides). The cream-colored structure to the left of the palps is likely the foot. It appears that this individual was a female with orange eggs, as can be seen at the top left of the visceral mass. Stacked photo by Dave Cowles, July 2017

The inside of the shell is pearly. In the upper valve the two large adductor muscle scars can be seen. The taxodont dentition keeps the valves aligned. The hinge ligament on this species is internal and attached via a spoonlike chondrophore. Golden remnants of the ligament still attached in the chondrophore can be seen here. The anterior end of the shell is to the right. Stacked photo by Dave Cowles, July 2017

Hinge teeth
A closeup of the hinge teeth.  Anterior end of the shell is to the right. Photo by Dave Cowles, July 2017

Hinge teeth
Another view of the hinge teeth, stacked for greater depth-of-field of the taxodont dentition. The hinge ligament has been removed from the chondrophore of the top valve but it remains in the chondrophore of the valve at the bottom of the image. Anterior end of the shell is to the right. Stacked image by Dave Cowles, July 2017

Authors and Editors of Page:
Dave Cowles (2017):  Created original page
CSS coding for page developed by Jonathan Cowles

Salish Sea Invertebrates web site provided courtesy of Walla Walla University