The effect of larval brooding on the filtration rate and particle-retention efficiency of Pyganodon cataracta (Bivalvia: Unionidae)

Abstract

Following fertilization, the outer demibranchs of the unionid mussel Pyganodon cataracta undergo extensive morphological changes to accommodate approximately a million shelled glochidia larvae within the ctenidial water tubes which serve as brood chambers. The effect of larval incubation on particle filtration and quantitative selection was determined by comparing clearance rates and particle retention efficiencies of adult mussels during brooding and nonbrooding periods. Particle clearance rates for gravid females were significantly lower than those for nongravid females and were as much as 54% lower than those of males collected simultaneously. Females brooding larvae were less efficient at retaining particles <5 μm than were males or nonbrooding females. Both particle clearance and retention spectra were nearly identical for males and females collected during nonbrooding periods. The effect of larval brooding on the activity of gill cilia on excised gill tissue was assessed by determining the beat frequency of the laterofrontal cirri and the pattern of transport of fluorescently labeled latex microspheres by frontal cilia. The metachronal beat of cirri on marsupial gills was significantly lower than on nonmarsupial gills of both males and females but did not differ between reproductive seasons. Particles were transported faster by frontal cilia on nonmarsupial and nongravid gills vs. gravid marsupial gills, suggesting that the reduction in particle clearance and retention by brooding females resulted from functional changes in the ciliature of the marsupial demibranchs.