THE EFFECT OF LARVAL BROODING ON THE FILTRATION-RATE AND PARTICLE-RETENTION EFFICIENCY OF PYGANODON-CATARACTA (BIVALVIA, UNIONIDAE)

Following fertilization, the outer demibranchs of the unionid mussel P yganodon cataracta undergo extensive morphological changes to accommod ate approximately a million shelled glochidia larvae within the ctenid ial water tubes which serve as brood chambers. The effect of larval in cubation on particle filtration and quantitative selection was determi ned by comparing clearance rates and particle retention efficiencies o f adult mussels during brooding and nonbrooding periods. Particle clea rance rates for gravid females were significantly lower than those for nongravid females and were as much as 54% lower than those of males c ollected simultaneously. Females brooding larvae were less efficient a t retaining particles < 5 mum than were males or nonbrooding females. Both particle clearance and retention spectra were nearly identical fo r males and females collected during nonbrooding periods. The effect o f larval brooding on the activity of gill cilia on excised gill tissue was assessed by determining the beat frequency of the laterofrontal c irri and the pattern of transport of fluorescently labeled latex micro spheres by frontal cilia. The metachronal beat of cirri on marsupial g ills 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 nongravi d gills vs. gravid marsupial gills, suggesting that the reduction in p article clearance and retention by brooding females resulted from func tional changes in the ciliature of the marsupial demibranchs.