The nematode Caenorhabditis elegans is a filter feeder that lives in various viscous habitats such as soil, the intestines of slugs, and rotting materials such as fruits and stems. Caenorhabditis elegans draws in suspensions of bacteria and separates bacteria from water using the pharyngeal pump. Although these worms often live in highly viscous habitats, it is still unclear how they survive in these environments by eating bacteria. In this study, we investigated the effects of suspension viscosity on the survival rate of malnourished worms by combining live imaging and scaling analyses. We found that survival rate decreased with increases in viscosity because the high viscosity suppressed the amount of food ingested. The same tendency was found in two feeding-defective mutants, eat-6(ad467) and eat-6(ad997). We also found that the high viscosity weakened pump function, but the velocities in the pharynx were not zero, even in the most viscous suspensions. Finally, we estimated the amount of ingested food using scaling analyses, which provided a master curve of the experimental survival rates. These results illustrate that the survival rate of C. elegans worms is strongly dependent on the ingested bacteria per unit time associated with physical environments, such as the viscosity of food suspensions and the cell density of bacteria. The pump function of the C. elegans pharynx is not completely lost even in fluids that have 105 times higher viscosity than water, which may contribute to their ability to survive around the world in highly viscous environments.
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