Food transport in the C . elegans pharynx Leon

many soil nematodes, Caenorhabditis elegans feeds on bacteria suspended in liquid. Bacteria are transported from the mouth to the intestine by a neuromuscular tube called the pharynx. The pharynx functions as a pump, generating pressure to force food into the intestine, necessary because the interior of the nematode is at higher pressure than the surrounding environment (Harris and Crofton, 1957). The pharynx, however, is more than just a pump; it also concentrates food. Bacteria are trapped and transported posteriorly along the pharyngeal lumen, but most of the liquid in which they are suspended is expelled through the mouth, rather than being pumped into the intestine. This concentration of food particles is presumably important for efficient digestion and absorption. The volume of the intestinal lumen is limited, and a small ingested volume allows food to reside in the intestine longer. [Ingested materials spend an astonishingly short time in the intestine: tracers such as mineral oil or latex beads pass through the intestine of a rapidly feeding adult in 3–10·min (L. Avery, unpublished data)]. The smaller ingested food volume would also concentrate nutrients, allowing them to be more efficiently absorbed, and reduce the dilution of any digestive enzymes. Concentration of particles by the pharynx does not depend on their specific chemical properties, since 0.9·µm carboxylated latex beads are also efficiently concentrated (L. Avery, unpublished data). C. elegans is, therefore, a filter feeder. It takes in liquid with suspended food particles, traps the particles, and expels the liquid. However, unlike many filter feeders, which separate food from medium by passing the suspension through a mesh that traps the food particles, there is no obvious filter in the pharynx through which particles and liquid are separated. A striking example of differential motion of particles and fluid is posteriorward transport of particles within the anterior half of the pharynx, the corpus (Fig.·1). During active feeding, bacteria are found at various points along the length of the relaxed corpus. Corpus muscles contract when a feeding motion (a pump) begins, and fluid rushes in at the mouth, sweeping food particles posteriorly. The contraction is followed by relaxation, which closes the lumen and expels the fluid. Unlike the fluid, the bacteria do not return to their original positions; when the motion is finished, they are seen to be located more posteriorly than they were before it started. This posteriorward transport is of course essential for the function of …