In most rhabditid nematodes, pharyngeal contractions open the lumen and draws food in while relaxation closes the lumen and traps and transports the food down towards the posterior part of the pharynx. The contraction and relaxation cycles in M. andrassyana is almost identical to that observed in C. elegans (Avery & Shtonda, 2003). A slight delay in the relaxation of the anterior isthmus makes it possible to receive and accumulate the bacteria that have passed down from the anterior corpus. Like in C. elegans (Seymour et al., 1983; Avery & Shtonda, 2003) bacteria tend to accumulate at the base of the stoma before entering the pharyngeal lumen at the end of the contraction cycle. Regurgitation of intestinal contents into the basal bulb was not observed as in several rhabditid nematodes (Fürst von Lieven, 2003) but the possibility of this happening may not be ruled out.
M. andrassyana showed the general tendency of decline in the pharyngeal pulsations with age when studied singly. Similar observations have also been made in C. elegans (Bolanowski et al., 1981; Kenyon et al., 1993; Huang et al., 2004). Besides age, availability of food also appeared to influence the pharyngeal pumping as within the same age group pulsations declined significantly in the absence of bacteria. Croll & Smith (1978) and Horvitz et al. (1982) suggested that bacteria stimulated pharyngeal pulsation, a phenomenon that may also be operating in M. andrassyana. When nematodes were studied in groups, it becomes clear that numbers also influence the rate of pulsations in the presence of food or without food. In two day old nematodes the decline was significant between the three groups. In the absence of food although pulsations were significantly less than those with food, the differences between the three groups varied significantly. In the eight day old nematodes pulsations differed significantly only between two group. The rate of decline in the absence of food came down to just 4% in the fifteen nematode group while it was maintained above 20% in the other two groups. This near similarity in the rate of pulsations in the absence of food in the fifteen nematode group may represent a basal rate, just sufficient for survival. In young 2 day old nematodes, the significant decline in pharyngeal pumping in the presence of bacteria perhaps indicates that secretions of the nematodes together with excrements and other metabolic waste products may also be playing a role in the pulsation rate via chemo-sensory pathway. If contact with one another is also considered as a probable cause, it may represent a thigmotactic response affecting the several pathway of pharyngeal pumping.
Correlated with pharyngeal pulsation and synchronous with its decline in ageing individuals is the gradual degenerative changes in the structure of the pharynx. The swelling and vacuolation of the corpus adjacent the metastegostom not only deforms the stegostom but in all probability also affects the process of ingestion and may be one cause of bacteria clogging the stoma. The distortion of the luminal lining, a slight shrinkage/narrowing of the corpus wall and the vacuolation in the basal region of the corpus could affect the transport of food down the pharynx. All these changes in the anterior pharynx may not affect pulsations per sec but could certainly influence ingestion via metastegostomal disorganization and transport because of luminal distortion and vacuolation of corpus tissue and could result in accumulation of bacteria in the pharynx. Chow et al. (2006) suggested that bacterial plugging was a consequence of reduced pumping and not the cause as suggested by Garigan et al. (2002). Our observations of the deteriorative changes in the basal bulb reveals an involvement of almost all region of the bulb- the outer surface, muscles and the grinder. The most spectacular changes are in the muscles in all parts of the bulb that may severely impair the pumping mechanism. Further the distortion of the grinder, brought about by muscle degeneration or mechanical damage caused by continuous pumping may also be a cause of bacterial plugging of the basal bulb.