Effects of opportunistic feeding by the intertidal gastropods Batillaria zonalis and B. flectosiphonata on material flux on a tidal flat

Abstract

The role of the opportunistically feeding gastropods Batillaria zonalis and B. flecto- siphonata on the material flux within tidal flat ecosystems was studied using a laboratory tide simu- lated system. Gastropods were cultured in the system for 2 wk using natural seawater; in one treat- ment the gastropods were restricted to suspension feeding only, while in the other they were free to engage in suspension and deposit feeding (dual feeding or opportunistic feeding). Clearance rates in both species were higher in the suspension feeding treatment than in the dual feeding treatment, and were maintained on a diet of high concentration particulate nitrogen (PN) in the water column. In the suspension feeding treatment, B. zonalis and B. flectosiphonata exhibited remarkable biodeposition on the surface sediment, as indicated by chlorophyll a (chl a), pheopigments (pheo), sediment PN and total organic carbon. In contrast, deposition was less evident in the dual feeding and control (no gas- tropods) treatments. 'Freshness', as indicated by the chl a:pheo ratio in the surface sediment, was higher in the control than in the other treatments. The removal rate of suspended PN by B. zonalis and B. flectosiphonata was variable, although uptake from the suspended matter was 0.7 to 2.0 times greater than that of deposited matter. We conclude that these opportunistically feeding gastropods play a significant and unique role in biodeposition by accumulating organic materials on the surface sediment during suspension feeding, and in decreasing the biodeposition by ingesting the bio- deposited materials during deposit feeding.