Structure and respiratory function in the gills of the blue crab, Callinectes sapidus (Rathbun)

Aldridge, James Barrett
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  1. The paired Milne-Edwards openings represent the only significant entry points for ventilatory flow. Setae associated with the opening may serve to filter the inhalent water. 2. The ventral surface of the gill sieve is cupped in the vicinity of the Milne-Edwards openings presumably to allow for the smooth flow of water into the hypobranchial chambers, and thus up through the various gills. 3. The average number of lamellae/mm of gill length is 8. Epithelium 10 μm thick, presumably ion regulatory in nature, underlies areas on gills 5 through 8 and accounts for 45% of the total gill area of 710 mm²/g in freshwater adapted crabs and about 10% in salt water adapted animals. The remaining areas on gills 5 through 8 and all of the area on gills 1 through 4 are underlain by thin, presumably respiratory epithelium. 4. Lamellae are about 60 μm thick separated by 70 μm of water space. About 20 μm of the platelet thickness consists of epithelium in regions of thick epithelium and only about 1 μm in regions of thin epithelium. This leaves a width for the hemolymph lumen of 40 μm and 59 μm respectively. 5. Various papillae associated with gill lamellae and contours of the lamellar surface, particularly at the edges, serve to ensure proper lamellar spacing, a minimum of anatomical dead space, and unidirectional flow over the respiratory surface. 6. Presumptive secretory tissue lining pre- and post-branchial vessels is described on the light and electron microscope levels. 7. Structures which histologically resemble neuropiles are found in regions of thick epithelium in the gills 8. A computerized simulation for carbon dioxide exchange predicts that up to 75% of observed V̇ [subscript CO₂] may occur by simple diffusion across the gill. 9. Injection of the carbonic anhydrase inhibitor sodium acetazolamide has no measurably effect on levels of total hemolymph CO₂. 10. Carbonic anhydrase activity is present in gill homogenates from fresh and salt water adapted crabs but not in hemolymph. The greatest activity per unit tissue weight was found in gill homogenates prepared from freshwater adapted gills containing thick epithelium patches. 11. In a 'standard' 250 g crab, a gradient for oxygen across the hemolymph-water barrier of 37 torr is predicted to be required to account for V̇ [subscript CO₂] in the resting animal. 12. The driving pressure across the gill sieve was calculated to be approximately 2 mm of water (from published ventilatory volumes and measured gill sieve dimensions). This value is comfortably within the range of values reported for pressure gradients in the whole ventilatory system