MULTIPURPOSE GILLS - EFFECT OF LARVAL BROODING ON THE FEEDING PHYSIOLOGY OF FRESH-WATER UNIONID MUSSELS

During reproduction, the lateral (outer) demibranchs of the unionid mu ssel Pyganodon cataracta function in brooding females as marsupia in a ddition to serving in gas exchange, feeding, and ion transport. Recent studies indicate that glochidial brooding reduces clearance rates and particle retention efficiencies, but the opaque shell prevents direct observations of suspension feeding structures and makes it difficult to identify the underlying causes of the changes in feeding dynamics. In this study, video endoscopic techniques were used to describe and c ompare, in vivo, the feeding structures and dynamics of brooding and n on-brooding females. Although circulation within the mantle cavity was slightly altered by the enlarged lateral (gravid) gills of brooding f emales, both medial and lateral gills continued to retain and process particles. During brooding, circulation through medial gills was maint ained by the construction of secondary water tubes near the medial and lateral ends of the brood chambers. In vivo monitoring of particles r etained by the frontal surface of the gill indicated that transport ra tes for particles processed by gravid gills of brooders were significa ntly slower than on lateral gills of non-brooders or on medial gills. Similarly, gravid gills were less efficient at retaining small particl es (<6 mu m) than medial or non-gravid lateral gills. These findings a re consistent with the hypothesis that observed reductions in particle clearance rates and retention efficiencies in brooding female mussels are the result of functional changes in the ciliature and flow dynami cs of the marsupial gills. Moreover, similar mechanisms mediating part icle capture and processing on medial demibranchs appear to be unaffec ted by the presence of developing glochidia in the water tubes of the lateral gills.