Interactions between the deposit feeding gastropod Amphibola crenata and the microbial community in intertidal mudflats were studied in laboratory and field experiments in two New Zealand estuaries. The study was mainly designed to reveal the effect of deposit feeding on bacterial and microalgal production and assess the importance of these microorganisms to the nutrition of the snail. The secondary aim was to compare the influence of Amphibola to external factors which regulate microbial production on the mudflat.

Short-term effects of deposit feeding on bacterial production were examined by monitoring the recolonisation of Amphibola faeces by bacteria. Long-term effects on bacterial production were studied in artificial enclosures in the field where the effect of snail density on bacterial numbers and activity was monitored. These same enclosures were also used to study the effect of grazing by the snail on standing crop and productivity of the epibenthic algae. Assimilation of bacterial carborn by Amphibola was experimentally measured, and the contribution of bacterial and microalgal carbon to the snail’s carbon budget was estimated. The effect of microbial biomass on the feeding behaviour of Amphibola was also examined.

It was found that a pulse in bacterial production occured during the recolonisation of Amphibola faeces by bacteria. This appeared to be similar in magnitude to the amount of bacterial biomass consumed by the snail - approximately 4. 5 mg C/m²/day. Amphibola also had a minor long-term influence on bacterial numbers and activity, but no clear effect on productivity was apparent. Grazing by Amphibola caused a substantial reduction in microalgal standing crop and productivity and affected the species composition of the microalgal community. Both the bacteria and microalgae serve as significant sources of carbon for the snail, but a large additional input of carbon is required to meet its nutritional needs. Other possible sources of carbon for the snail include meiofauna and non-living organic material. Amphibola was also found to alter its feeding rate in response to changes in microbial biomass, in a manner which may improve the return for feeding effort.

In overview, it appears that Amphibola and the sediment bacteria similarly influence the other 's productivity while Amphibola has a greater effect on the microalgae than the microalgae have on Amphibola. Ultimate control of microbial productivity was concluded to be external to these relationships, with the snail acting only to modify seasonally determined levels of productivity.