Morphology of the respiratory vasculature of the mudskipper Boleophthalmus pectinirostris (Gobiidae: Oxudercinae).

The gross morphology of the circulatory system in the amphibious mudskipper, Boleophthalmus pectinirostris, conforms with the typical teleost configuration, in which gills and systemic vascular beds are connected in series. However, at the microscopic level, the vasculatures of the respiratory organs, the inner epithelium of the bucco-opercular cavity, gills and skin, all show specializations for aerial gas exchange. The epithelium of the bucco-opercular cavity is heavily vascularized by respiratory capillaries that are derived from systemic arteries of the head, mainly branches of the hyomandibular artery and the dorsal opercular artery. The respiratory circuit of the secondary lamellae of the gills consists of 15-17 channels running in parallel, unlike the lacuna-like blood space of aquatic fishes. The most notable specialization is found in the microcirculation of the respiratory papillae in the skin. Each respiratory papilla is supplied by an arteriole that is derived from a systemic artery, mainly the cranial artery in the head and the segmental artery in the trunk. The arteriole divides several times along its course to the apical region of a papilla, where the branches split into approximately 65 capillaries that radiate to the periphery of the papilla. The capillaries twist 5-10 times before they unite to form the venules that encircle maximally half the circumference of a papilla. A variable number of venules merge into a vein, which progressively coalesces with veins from other papillae. There is no morphological specialization that separates oxygen-rich effluent blood of the epithelia of the bucco-opercular cavity and the respiratory papillae of the skin from the oxygen-poor systemic venous blood. The ecophysiological implications of these findings are discussed in relation to the environmental conditions that B. pectinirostris experience during tidal cycles in the warm months and during overwintering.