MEPS 178:259-269 (1999)  -  doi:10.3354/meps178259

ABSTRACT: Mechanisms of pseudofeces voidance from the mantle were investigated in 3 of the 4 major bivalve particle processing systems using scanning electron microscopy, to determine what mechanisms permit the isolation of pseudofeces transport in bivalves presenting discrete mantle rejection pathways, and how this contrasts with species which do not possess such pathways. The entire mantle surfaces of Mytilus edulis (homorhabdic filibranch), Mya arenaria and Spisula solidissima (eulamellibranchs), and Placopecten magellanicus (heterorhabdic filibranch) were surveyed and photographed. In both the homorhabdic filibranchs and the eulamellibranchs, the mantle rejection tracts previously located using video endoscopy were characterized by cilia which were extraordinarily long compared to the cilia of the general pallial surface. These long cilia were grouped into closely adhering tufts, herein termed composite cilia. In M. edulis and M. arenaria, the general pallial surface presented shorter simple cilia, whereas in S. solidissima the general pallial surface presented simple cilia dorsally, long composite cilia ventrally, and an intermediate band of short composite cilia. These 3 species all possess a gill ventral particle groove; hence the site of pseudofeces production is the labial palp, and a discrete mantle rejection tract is necessary to transport pseudofeces to the inhalent siphon for expulsion. The long composite cilia within this tract may provide vertical isolation of pseudofeces from the general pallial surface; effective mucociliary transport using such long cilia can only be accomplished if they are grouped, as in all 3 species with mantle rejection tracts. In the heterorhabdic filibranch P. magellanicus, no specialized cilia were observed on the mantle; this corresponds to the absence of a mantle rejection tract in this system, which does not possess a gill ventral particle groove and relies on valve adduction to expel pseudofeces. These results suggest that elevation above the surrounding mantle cilia is the rule in species relying on mantle rejection tracts for the voidance of pseudofeces. These results show the usefullness of cilia mapping in the study of bivalve suspension-feeding mechanisms.

KEY WORDS: Bivalves · Feeding · Pseudofeces · Rejection