The food of most clams is phytoplankton and other organic particles that are filtered from the water on fine-mesh gill surfaces.  Most clams bury themselves in sediment and communicate to the sediment-water interface by siphons.  Water enters the clam via the inhalent siphon and leaves via the exhalent siphon, driven by ciliary beating on the ctenidial filaments.  The exhalent water carries away urinary excretions, undigested wastes as feces, gametes (in season), and carbon dioxide from metabolism. The gills or ctenidia are arranged on either side of the body in the form of of 2 demibranchs (shown in the diagram below is one demibranch, representing one-half of a total of 8 gill faces). Water enters the filaments over all 8 faces, and has its load of food particles sieved out on the filaments.

The water passes through the filaments, then moves up and posteriorad, and exits from the exhalent siphon.  The closely aligned gill filaments form a sieve to trap food particles.  Cilia move the particles via ventral food grooves located at the bottom of each demibranch (there are other grooves above the gills) to paired palps located on either side of the mouth.  The palps sort the particles according to size and palatability, and bind up inedible matter in mucus and reject it.  Periodically, the clam claps its shell valves together and forcibly expels this material, known as pseudofeces, from the inhalent siphon.  Edible particles move along ciliated grooves into the mouth. Topics dealt with here include pumping & suspension-feeding, deposit-feeding, and the special case of shipworms.

NOTE  filter- or suspension-feeding follows a similar pattern in clams, oysters, scallops, and mussels

NOTE lit. “false feces” – however, this material not actually feces because it has not passed through the clam’s digestive tract

Analyses of stomach contents of horse clams Tresus capax collected from Esquimalt, British Columbia reveals an assortment of diatoms, possible remains of many flagellates, and detritus.  The author compares what species are eaten with what are available in the plankton, and notes that larger diatom species or ones effectively made larger by possession of long spines such as Coscinodiscus, Chaetoceras, and others, and even some larger flagellates, are not found in the stomach.  These may be rejected by the sorting mechanism of the labial palps.  Reid 1969 Veliger 11: 378.

Siphon spurting of a horse clam Tresus sp.
caused by the touch of a shovel. The spurt is water
displaced from the siphon as it is suddenly retracted

Pumping in bivalves is not as energetically costly as one might suppose.  Measurements on 8 species of British Columbia bivalves show that the metabolic cost of pumping accounts for less than 1% of the total oxygen uptake.  Bernard & Noakes 1990 Can J Fish Aquat Sci 47: 1302.

NOTE the 8 species are Solemya reidi, Yoldia thraciaeformis, Chlamys hastata, Mytilus trossulus, Crassostrea gigas, Clinocardium nuttallii, Saxidomus gigantea, and Mya truncata

Not all clams feed on phytoplankton drifting by in the water column.  Most or all of the dozen or so species of west-coast Macoma have split siphons and several of these use their inhalent siphons to feed on deposits near to their burrows. At least one species, Macoma secta, uniquely eats bacterial films from sand grains that it sucks in. The stomach is adapted to this diet by the enlargement of the gastric shield. Pockets in the gastric shield receive mucus-bound masses of sand grains together with their epiflora, detrital particles, and small diatoms, and these are ground against the shield, partly aided by the rotating crystalline style. All species of Macoma appear to have enlarged labial palps, which may be involved in filtering of suspended food as well as in sorting particles for edibility. Reid &Reid 1969 Can J Zool 47: 649. Photo of M. secta courtesy R. Perry and Univ Calif Los Angeles OceanGLOBE.

NOTE the authors describe feeding and gut morphology in 8 species of Macoma, only one of which is dealt with here

Macoma secta 1X

Other Macoma species also with split siphons, such as M. nasuta, are able facultatively to feed on suspended matter or on deposits depending upon habitat. Reid & Reid 1969 Can J Zool 47: 649; Rae 1979 Veliger 21: 384.

Two views of the "bent-nose" clam Macoma nasuta.
The function of the asymmetry is not known 1X

The sediments sucked in indiscriminately by Macoma spp. are rich in organic matter falling from the water column, and in bacteria and other micro-organisms feeding on this material.  The material is filtered on the gill surfaces as described for suspension-feeding clams, then sorted once at the level of the labial palps, and then again in the stomach.  Here, surface films on the sand grains are removed, aided by digestive activity of resident symbiotic bacteria, and the grains are shunted to the intestine for release in feces.  Studies at Friday Harbor Laboratories, Washington on M. nasuta indicate that up to 97% of the material sucked in is ejected initially as pseudofeces.  Many small animals are consumed and some of these apparently survive their transit through the gut to emerge alive in the feces. Hylleberg & Gallucci 1975 Mar Biol 32: 167.

View of a generalised deposit-feeding Macoma stomach, opened from the left side to see the main ductings. Food- and sand-bearing mucus strands enter the stomach from the esophagus (top-most blue area), are ground against the gastric shield (aided by rotation of the crystalline style), and edible matter is sorted and shunted into the digestive gland (opening in center of diagram). Sand particles still bearing food matter may be recycled. The appendix is an ancillary storage area for sand
grains. Eventually, inedible matter is moved via rejection tracts and secondary intestinal grooves into the intestine.
Here it is coated in mucus and released into the exhalent water flow as feces. Reid & Reid 1969 Can J Zool 47: 649.

Shipworms Bankia setacea use the serrated edges of their shells to drill burrows into wood.  Some of the scrapings are consumed and, because shipworms do not produce their own cellulase enzymes, are digested within a special pouch in their intestine filled with cellulose-digesting symbiotic bacteria.  Breakdown of cellulose provides glucose for energy.  The clams presumably balance their nutritional needs by filter-feeding from the plankton in the usual way.  Given the information in the "NOTES" and Research Study 2 below, further research on nutrition of shipworms is needed. Crosby & Reid 1971 Can J Zool, Lond 49: 617. Photograph courtesy Faculty of Forestry, University of British Columbia.

NOTE a early study on west-coast shipworms Bankia setacea provides evidence for at least some digestion of wood, but the source of this digestion, whether enymatic or bacterial, is not investigated.  Miller & Boynton 1924 Science 63: 524. Similarly, a report on digestion of wood in shipworms in Atlantic coast Teredo identifies cellulase-type enzymes and notes the absence of symbiotic bacteria or protists. Greenfield & Lane 1953 J Biol Chem 204: 669. 

NOTE shipworms additionally have bacterial symbionts in their gills that provide their hosts with essential amino acids.  Trytek & Allen 1980 Comp Biochem Physiol 67A: 419.

How does a larva or juvenile shipworm Teredo obtain an inoculating bacterium to start a culture in its pouch?  Apparently, the bacterial symbionts are present within the ovaries of the parent and incorporate themselves directly into the egg.  This means that a settling larva is fully equipped for digestion of the first wood scrapings eaten after metamorphosis.  Sipe et al. 2000 Applied Envir Microbiol 66: 1685.

Burrows of shipworms Bankia setacea, unlike those of related
Teredo spp., are lined with calcium carbonate. The tiny burrows
in the photo are made by wood-boring isopods Limnoria sp. 1X