. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 82 2a 0) S, MEDLER AND H. SILVERMAN. relaxed contracted Figure 2. Intertilament distance, (a) Relaxed gills from each of the species (Corbicitln fluminea. Dreissena polymorpha. and Mercenaria mercenaria) had an intertilament distance of about 20 /xm, but this distance was significantly reduced to less than 10 upon muscle contraction (mean ± SE:;; = 5 ). (b, c) Frontal face (SEMI of a C. fluminea gill: (b) relaxed gill; (c) gill from the same animal following muscle contraction. Discussion Two of the most important facto
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 82 2a 0) S, MEDLER AND H. SILVERMAN. relaxed contracted Figure 2. Intertilament distance, (a) Relaxed gills from each of the species (Corbicitln fluminea. Dreissena polymorpha. and Mercenaria mercenaria) had an intertilament distance of about 20 /xm, but this distance was significantly reduced to less than 10 upon muscle contraction (mean ± SE:;; = 5 ). (b, c) Frontal face (SEMI of a C. fluminea gill: (b) relaxed gill; (c) gill from the same animal following muscle contraction. Discussion Two of the most important factors affecting bivalve pumping potential are the dimensions of the intertilament passages and the exit loss (derived from the kinetic energy carried by the water jet leaving the excurrent siphon) (Fos- ter-Smith. 1976: J0rgensen et 1986, 1988; J0rgensen and Riisgard. 1988; J0rgensen, 1989; Riisgard and Larsen, 1995). Both of these factors are ultimately controlled by muscle tone, since integral gill muscles control gill dimen- sions and exit loss is controlled by the muscles affecting siphon dimensions. It is reasonable to expect higher pump- ing rates when the gill muscles are fully relaxed, because in this condition the passageways for water flow are signifi- cantly more open than when the muscles contract. J0rgensen and colleagues have long held that pumping activities are correlated with the degree of valve gape and the associated changes of the gill (J0rgensen et 1986, 1988: J0rgensen and Riisgard. 1988; J0rgensen, 1989. 1990). Their interpre- tation is that gill dimensions are controlled secondarily to the contraction of muscles within the gill axis and that the muscles of the gill axis contract when the adductor muscles reduce valve gape (Jorgensen et ai. 1988; J0rgensen, 1989, 1990). We agree with this general description, but would refine it by stipulating that intertilament distance and other gill dimensions are controlled directly by integral smooth muscles
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