A COMPARATIVE STUDY ON THE GILLS OF THE MUDSKIPPERS PERIOPHTHALMODON SCHLOSSERI AND BOLEOPHTHALMUS BODDAERTI

Abstract

The gills of Periophthalmodon schlosseri are better adapted to a terrestrial than an aquatic environment. It has fewer and shorter filaments than Boleophthalmus boddaerti. Its branched gill filaments accommodate long filament lengths into shorter branches which can then derive maximal skeletal support from the thick gill rods. The intrafilamentary secondary lamellar fusions reduce coalescence of the respiratory surfaces but renders gaseous exchange via countercurrent mechanism inefficient. In contrast, the gills of B. boddaerti are morphologically better adapted for respiration in water as most of their secondary lamellae are aligned to the respiratory water current. The gill surfaces of both mudskippers are highly convoluted to increase surface area. The gill area : skin area ratio of B. boddaerti is greater than that of P. schlosseri. The gills of P. schlosseri are metabolically more active than those of B. boddaerti. The specific activities of most of the enzymes examined in this study are greater in the gills of the former fish. After exposure to hypoxia or anoxia in vitro, the gills of P. schlosseri produced significantly greater amounts of lactate (13.2 ± 1.79 µmol/g and 17.2 ± 4.7 / µmol/g, respectively) than those from B. boddaerti (3.54 ± 0.91 µmol/g and 10.7 + 2.1 µmol/g respectively). Conversely, the gills of B. boddaerti are able to reduce its metabolic rate to a greater extent than those of P. schlosseri. Indeed, the glycogen content in the gills of B. boddaerti exposed to hypoxia in vitro (4.97 ± 1.34 µmol glycosyl units/g) was not significantly different from those exposed to normoxia (4.73 ± 0.95 µmol glycosyl units/g). Therefore, a reversed Pasteur effect might have occurred in these gills during hypoxic exposure. Regulation of glycolytic flux in the gills of both mudskippers via covalent modification of PFK or PK is unlikely as no change in the kinetic properties of either enzyme extracted from the gills of hypoxia or anoxia exposed fish was observed, The branchial F-2,6-P2 content decreased significantly after exposure to hypoxia and anoxia to restrict channelling of carbohydrates for purposes other than energy metabolism. The differences in glycolytic fluxes in the gills of P. schlosseri and B. boddaerti exposed to hypoxia can be explained by the distribution of glycolytic enzymes between the free and bound fractions. Hypoxia had no effect on the binding of glycolytic enzymes to the particulate fraction in B. boddaerti, but induced an increase in the proportion of bound PFK to accelerate glycolytic flux in the gills of P. schlosseri. Contrary to hypoxia, anoxia increased the percentage of PFK and aldolase bound to the particulate fraction in the gills of B. boddaerti. For P. schlosseri, in addition to those changes induced by hypoxia, anoxia further increased the binding of aldolase and PGK to subcellular particles.