Fig. 10. Seasonal variation in the distribution of nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC) in the gills of Mercenaria mercenaria. (A) Thick (100 µm) section; muscle fibers visualized with green-fluorescing phalloidin. Muscle is concentrated in the longitudinal muscle (lm), which also sends branches into each gill filament (gf); muscle also occurs in the major blood vessel (bv). (B) The same section as A but visualized with a filter for red fluorescence showing the distribution of immunoreactive (ir) NOS. This section was made in November (early in the season of potentiation) in gills from a Massachusetts clam. (C) Thick section showing the distribution of NOS in a gill taken from a Massachusetts clam in August (no potentiation) 2002. (D) Thin section (12 µm) showing the distribution of NOS in a gill taken from a Florida clam in July (no potentiation) 2001. Note the intense ir-NOS at the base of the gill filaments and in varicose fibers adjacent to the longitudinal muscle. (E) Thick section of a gill in April (potentiation) showing the distribution of ir-sGC, which is concentrated in the longitudinal and septal muscles and the gill filaments. (F) Thick section of a gill in August showing the distribution of ir-sGC, which is concentrated in the gill filaments and, to a lesser extent, in the projections of the longitudinal muscle in the gill filaments. Note the lack of fluorescence in the main body of the longitudinal muscles. All thick sections are 100 µm. hbv, horizontal blood vessel; s, septum; sbv, septal blood vessel; wt, water tube. Scale bars, 50 µm (except for D, 25 µm).

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