Zebra mussels are filter feeders. Their gills strain particles from the water through a net formed by the cilia on each gill sheet (ctenidium). The particles are swept by the cilia toward the mouth. As water passes through the gills via the ostia (small openings), oxygen is exchanged across the gills into the blood to be transported throughout the mussel.
Food consists of micro-algae, micro-invertebrates, bacteria, detritus, and other organic material. Thus, zebra mussels filter both organic and inorganic particles from water. Sprung and Rose (1988) reported that maximum retention occurred with particles 5 to 35 Ám in diameter. Zebra mussels can filter out and ingest bacteria that are <1 Ám, but not as efficiently.
Once removed from water, particles are moved by ciliary action to the food groove, which is located on the ventral edge of the gill filaments. Cilia located in the food groove move particles anteriorly toward the two labial palps located on each side of the mouth. Cilia on the labial palps sort particles into edible and inedible portions and move the particles that are the appropriate size and palatability to the mouth. Rejected particles are bound in mucus and moved to the edge of the mantle. Rapid closing of the shell valves ejects these mucoid-bound particle strings, i.e., pseudofeces, out of the mussel via the inhalant siphon. Silverman et al. (1996) present a more detailed analysis of particle capture on the gill filaments.
Once food particles enter the mouth, they pass through the esophagus into the stomach where a combination of mechanical and chemical digestion breaks them into smaller particles. The stomach is a thin-walled sac, lined with a gastric shield around which the crystalline style revolves. In all bivalves, the style is a gelatinous rod-like body that contains starch-digesting enzymes and is continually being used up and renewed (Ellis 1978). The breakdown of food particles is facilitated by these enzymes and the mixing of the stomach contents by the rotation of the crystalline style against the gastric shield.
The digestive gland secretes digestive enzymes that facilitate chemical digestion. The remainder of the stomach is extensively lined with cilia, with many ridges and folds that sort the material. When the particles are broken down sufficiently, they are carried on ciliary tracts in the stomach to the digestive diverticulum for intracellular digestion. Digestive cells in the digestive diverticulum take up small food particles into food vacuoles within the cells where nutrients can be used by the cells. The intestine carries both undigested particles and waste products to the rectum, where they are stored prior to evacuation from the anus through the exhalant siphon. Morton (1971,1983) discusses feeding and digestion in bivalves in detail.
Life History and Biology Introduction
Anatomy and Physiology