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First published online June 29, 2007
Journal of Experimental Biology 210, 2518-2525 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.005637

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Seasonal control of particle clearance by isolated gills from the clam Mercenaria mercenaria

Louis F. Gainey, Jr

Department of Biological Sciences, The University of Southern Maine, Portland, ME 04104, USA

View larger version (47K): [in this window] [in a new window]   Fig. 1. Diagrammatic anatomy of Mercenaria mercenaria [adapted from Gainey and Greenberg (Gainey and Greenberg, 2003) and Gainey et al. (Gainey et al., 2003)]. (A) Clam on the half shell; anterior is towards the left. (B) Cross section of a clam. (C) Details of a water tube: with the musculature relaxed (left) and contracted (right); the water tubes run dorso-ventrally within the gill. The water tube muscles are within the walls of the horizontal blood vessels (the vessels are not shown here). (D) Details of a relaxed demibranch (as in C, left). This cross section is slightly out of the horizontal plane; thus, the filaments to the left of the dashed line are at the level of the interfilamentar tissue junctions that contain the longitudinal muscles whereas the filaments to the right of the line are at the level of the ostia and horizontal blood vessels. The walls of the horizontal blood vessels contain both the water tube muscles and a dense network of serotonergic nerves; neither of these is shown. Abbreviations: bv, blood vessel; fc, frontal cilia; gf, gill filament; hbv, horizontal blood vessel; lc, lateral cilia; lfc, latero-frontal cirri; lm, longitudinal muscle; o, ostium; s, septum; wt, water tube; wtm, water tube muscle; 5-HTn, serotonergic neuron.

View larger version (110K): [in this window] [in a new window]   Fig. 2. Clearance of graphite by an isolated piece of gill. (A) Start; (B) after 10 min; (C) after 20 min. In C, note the graphite/mucus threads on the gill filaments and food groove (arrow). Scale bar, 2 cm.

View larger version (5K): [in this window] [in a new window]   Fig. 3. The seasonal effect of 5-hydroxytryptamine (5-HT) on clearance rates. Data are the ratios of the treatment rate to the basal rate. The treatment rate is the clearance rate after a 5 min exposure to 5-HT, and the basal rate is the initial, unstimulated clearance rate (see text). Data are means ± s.e.m. Closed triangle (C) is the mean ratio for untreated controls: i.e. gills that were not exposed to 5-HT during the treatment run. Sample sizes ranged from 4 to 9 at each concentration of 5-HT. At 10–5 mol l–1 5-HT the means are significantly different (P<0.05).

View larger version (4K): [in this window] [in a new window]   Fig. 4. The change in the mean (± s.e.m.) interfilament space when isolated gills were exposed to 5-hydroxytryptamine (5-HT). Treatments: 0, initial measurements on untreated, quiescent gills; 10–5 mol l–1 and 5x10–5 mol l–1, the same pieces of gill after exposure to increasing concentrations of 5-HT. Sample size: N=8 measurements made on each of two pieces of gill. All of the means are significantly different from each other (P<0.05).

View larger version (4K): [in this window] [in a new window]   Fig. 5. The mean percentage of the original cross-sectional area of water tubes in isolated gills exposed to 5-hydroxytryptamine (5-HT). Sample size: N=2 measurements made on each of six pieces of gill. Data are means ± s.e.m. The mean at 10–5 mol l–1 5-HT is significantly larger than that at 5x10–5 mol l–1 (P<0.05).

View larger version (5K): [in this window] [in a new window]   Fig. 6. The effect of the nitric oxide generator DEANO (10–6 mol l–1) on clearance. Data are ratios of the treatment rate (rate of clearance after a 15 min exposure to DEANO) to the unstimulated basal rate. Data are means ± s.e.m. Treatments: none, untreated controls (N=39); DEANO (w), gills exposed to DEANO during the winter (N=17); DEANO (s), gills exposed to DEANO during the summer (N=18). The mean during the winter is significantly greater than the untreated controls (P<0.05).

View larger version (5K): [in this window] [in a new window]   Fig. 7. The effect of the nitric oxide synthase inhibitor L-NAME (10–5 mol l–1) on 5-hydroxytryptamine (5-HT)-induced clearance rates during the winter. Treatments are either gills that were exposed to 5-HT or gills exposed to both 10–5 mol l–1 L-NAME and 5-HT; the ratios are as described in the text. Data are means ± s.e.m.; sample size ranged from 6 to 14. At 10–5 mol l–1 5-HT, the means are significantly different (P<0.05).

View larger version (6K): [in this window] [in a new window]   Fig. 8. The seasonal effect of dopamine (DA) on 5-hydroxytryptamine (5-HT)-induced clearance rates. To induce maximal clearance during the initial run, gill tissues were first exposed to 10–5 mol l–1 5-HT for 5 min. Then, between the initial and treatment runs, the gills were exposed to DA for 5 min. Data are the means (± s.e.m.) of the ratios of the treatment rate to the maximal rate. Sample sizes ranged from 4 to 7 at each concentration of DA. The regression lines are significantly different (P<0.05).

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