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Ultrastructure and morphometry of the gills of Latimeria chalumnae, and a comparison with the gills of associated fishes

The gross morphology of Latimeria gills is characterized by well developed interbranchial septa that extend almost to the tips of the filaments of each hemibranch and among living fish resembles most closely that of the gills of the lungfish, Neoceratodus. Morphometric studies have shown that Latimeria has a very small gill surface area (ca. 18 mm2/g body mass). The total length of the gill filaments is low and comparable with that of other fishes caught at similar depths (200 m) off Grande Comore. These fish also have smaller gill areas than those of shallow water species collected during the British-French-American expedition. The second gill arches of embryonic and very small Latimeria have a similar number of filaments to those of the adults and regression analysis suggests that filament length increases more gradually with body size in Latimeria than in most other fish, except for some Pacific fish collected from depths of 1300 m. Latimeria gills were examined in the electron microscope and compared with those of Neoceratodus. In both species the basic structure is similar to that of other fishes, having a water-blood barrier consisting of two epithelial layers, a basement membrane and pillar cell flange layers. The outer surface of the epithelium is covered with microvilli and microridges beneath which are a series of bodies reminiscent of those found in elasmo-branch fish. In Latimeria the spaces between the two epithelial layers contained lymphocytes of several types that were similar to those present in the blood channels. As in other fish secondary lamellae, the marginal channels are lined by endothelial cells containing typical osmiophilic granules, but, unlike in Latimeria and all other fish examined, such bodies were also present in the pillar cells of Neoceratodus. The distance between water and red blood cells in Latimeria is greater (6-8 μm) than in most fish and this, together with the low gill surface area, shows that this fish is ill-equipped for high oxygen uptake. A very sluggish mode of life is indicated and excessive exercise would result in hypoxic stress. The gills thus combine features related to the phylogenetic relations of Latimeria with others that it shares with unrelated fish living in similar habitats.