Selection and processing of large suspended algae in the oyster Crassostrea gigas

Abstract

The mechanisms and sites of processing and selection of large natural algal particles were studied in the oyster Crassostrea gigas, which possesses a heterorhabdic pseudolamellibranch gill type. Endoscopic observations of processing 1000 cells ml -1 suspensions of the pennate diatoms Pleurosigma planctonicum and Rhizosolenia setigera were performed, as well as endoscope-directed sampling of 100 cells ml -1 mixed suspensions of intact (25%) and empty (75%) 150 × 200 µm Coscino- discus perforatus. Video endoscopy was used to observe processing on the gill and to allow in vivo sampling of contents of the dorsal and ventral particle tracts, while pseudofaeces was collected from the individual flow-through chambers. Selection indices were calculated for empty C. perforatus at each processing site. Only those P. planctonicum orientated in a dorso-ventral position could enter the principal filaments (PF) for delivery to the dorsal acceptance tract. R. setigera almost never entered the PF, being prevented by its curved and twisted shape. Visual counts of intact versus empty C. perforatus were done on samples from the ambient medium, ventral and dorsal particle tracts, and pseudofaeces. They showed that the percentages of intact and empty cells in both the dorsal and ventral particle tracts were identical to those initially presented (Kruskal-Wallis test; p > 0.05). In con- trast, the pseudofaeces contained over 98% empty cells (Kruskal-Wallis test; p ≤ 0.001, degree of selection ranged from +29 to +34%). These results show that (1) large natural particles within the size range naturally encountered by C. gigas may only be subjected to qualitative selection on the gills if their shape and dimension allow them to enter the PF, and (2) the site of selection for particles unable to enter the PF is not the gill, but rather the labial palps. Selection sites and mechanisms in heterorhabdic bivalves should therefore be studied over the entire range of naturally available particle sizes. Given the periodic dominance of large diatoms in coastal temperate waters, the labial palp processing site could be of considerable importance in the dynamics of seston flow in oyster- dominated ecosystems.