Lying close to the stomach is a sac which contains a cucumber-shaped, jelly-like crystalline style. The end of this style projects into the stomach. It rotates clockwise and dissolves its enzymes in the stomach which aid in digesting the food, that is, in converting starch into sugar. The style, numerous cilia and the furrows on the stomach wall aid in churning the food.
The fecal pellets of mollusks are often very distinctive for the various genera and species. Some are cylindrical rods, others elongated strings or ribbons, and a few consist of strands wrapped up in round balls. In cross-section, some rods are characteristically bi- or trilobed.
Feeding is not done at all times, although a great part of a bivalve’s life is spent in securing food. The oyster, for instance, spends from seventeen to twenty hours of each 24-hour period in taking in water for the purpose of feeding and breathing. Individuals living in the intertidal zone and left dry by receding tides or exposed to water heavily charged with silt spend considerably less time feeding. During cold periods, when the water temperature falls below 40° F, the oyster goes into a state of hibernation, and it ceases to feed because of the lack of coordination of the ciliary motion along the surface of the gills. Under ideal conditions, the Giant Pacific Oyster (Crassostrea gigas) filters 51⁄2 quarts of water per hour at 77° F (1 quart at 34° F). In a year, the total amount would fill a 10,000-gallon tank car.
Perhaps the most startling modification of obtaining food nutrients is exhibited in the giant Tridacna clams of the Indo-Pacific reefs. These clams literally “farm” colonies of brown-colored algal plants (Zooxanthellae) in their huge, exposed mantle edges. Unlike most clams, the Tridacnas spread their valves open and expose their mantles to as much sunlight as possible for the benefit of these single-celled seaweeds. In addition, small, fleshy tubercles grow on the surface of the mantle in which are located lens-like, clear cells. Sunlight can thus penetrate down into the flesh and be diffused into areas which otherwise would not receive enough light for the algae. Surplus plant cells are engulfed by phagocyte blood cells of the clam and transported to the digestive gland for absorption as food. The giant clams also feed in the conventional gill-to-mouth manner and are therefore not entirely dependent on the algae. The algae, however, must have a clam as a host in order to survive. This peculiar symbiosis is found to a lesser extent in the Bear Paw Clam (Hippopus), the Heart Cockle (Corculum) and the nudibranch, Phestilla. This phenomenon is not to be confused with the pathologic entry of the parasitic blue-green algae in fresh-water mussels, Anodonta and Unio.
While the gills are the main organs for catching, sorting and transporting food in the majority of clams, they are limited to respiratory functions in a few groups. The smallest and most inefficient gills are found in the primitive protobranchs (Nucula, Nuculana, etc.) and in the small, highly evolved
