Multipurpose Gills: Effect of Larval Brooding on the Feeding Physiology of Freshwater Unionid Mussels

During reproduction, the lateral (outer) demibranchs of the unionid mussel Pyganodon cataracta function in brooding females as marsupia in addition 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 compare, in vivo, the feeding structures and dynamics of brooding and non-brooding females. Although circulation within the mantle cavity was slightly altered by the enlarged lateral (gravid) gills of brooding females, both medial and lateral gills continued to retain and process particles. During brooding, circulation through medial gills was maintained by the construction of secondary water tubes near the medial and lateral ends of the brood chambers. In vivo monitoring of particles retained by the frontal surface of the gill indicated that transport rates for particles processed by gravid gills of brooders were significantly slower than on lateral gills of non-brooders or on medial gills. Similarly, gravid gills were less efficient at retaining small particles (<6 ptm) than medial or non-gravid lateral gills. These findings are 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 dynamics of the marsupial gills. Moreover, similar mechanisms mediating particle capture and processing on medial demibranchs appear to be unaffected by the presence of developing glochidia in the water tubes of the lateral gills. Additional key words: glochidia, endoscopy, suspension feeding, filtration Though the gills of suspension feeding bivalves are typically identified as structures for feeding, gas exchange, and ion transport, the ctenidia of many marine and freshwater species also incubate larvae during reproductive periods. Brooding mechanisms vary considerably among species. Most freshwater bivalves, including members of the Corbiculidae and Unionidae, lack planktonic larval stages and incubate developing embryos within internal spaces between gill lamellae (Ortmann 1911; Britton & Morton 1982; Kat 1984; Mackie 1984). In other freshwater and marine bivalve molluscs, developing young are attached to pallial structures or restricted to specialized brood masses, papillae, or sacs (Heard 1977; Bartlett 1979; Richardson 1979; Mackie 1984; Kabat 1985). Conversely, some species, including oysters of the genus Ostrea, brood within the infrabranchial cavity but the young are not confined or physically attached to the mother and move about freely in the inter-demibranchial spaces (Chaparro et al. 1993). Although numerous studies have examined the mechanics of the bivalve pump and filter (for review see J0rgensen 1990), the effect of larval incubation on the gills' customary roles as feeding and respiratory structures has been ignored, except for the observation of Walne (1972) that clearance rates of the oyster Ostrea edulis are lower during brooding periods. In the Unionidae, modifications of ctenidia for brooding, including the location and arrangement of larvae (glochidia) within brood chambers, the degree of swelling of the lamellae, and the duration of larval incubation, vary among species and often serve as important taxonomic characters (Ortmann 1911; Heard & Guckert 1971). In most species, the water tubes of either the lateral (outer) demibranchs or all four demibranchs serve as ovisacs. In others, the marsupium is restricted to a portion of the lateral gills, often forming bulges or sulci along the ventral aspect of the demiThis content downloaded from 207.46. 13.21 on Tue, 27 Sep 2016 04:23:58 UTC All use subject to http://about.jstor. org/terms