787 The giant mudskipper Periophthalmodon schlosseri is an amphibious gobioid teleost, living in the mangrove swamps of Singapore and peninsular Malaysia. It builds burrows on high grounds in the estuarine mud, which are used as a refuge upon disturbance, or to lay eggs and protect the hatching larvae during the breeding season. P. schlosseri has a very high tolerance of environmental ammonia. It can survive for more than 1 week in 100·mmol·l –1 NH 4 Cl, an ammonia concentration that most other fish species would not survive for a few hours (Peng et al., 1998). It does not produce urea, or store ammonia in its body when exposed to high concentrations of environmental ammonia. Under such The Journal of Experimental Biology 207, 787-801 Published by The Company of Biologists 2004 doi:10.1242/jeb.00788 Periophthalmodon schlosseri is an amphibious and obligatory air-breathing teleost, which is extremely tolerant to environmental ammonia. It actively excretes NH 4 + in ammonia loading conditions. For such a mechanism to operate efficaciously the fish must be able to prevent back flux of NH 3 . P. schlosseri could lower the pH of 50 volumes (w/v) of 50% seawater in an artificial burrow from pH·8.2 to pH·7.4 in 1 day, and established an ambient ammonia concentration of 10·mmol·l –1 in 8 days. It could alter the rate of titratable acid efflux in response to ambient pH. The rate of net acid efflux (H + excretion) in P. schlosseri was pH-dependent, increasing in the order pH·6.0<7.0<8.0<8.5. Net acid flux in neutral or alkaline pH conditions was partially inhibited by bafilomycin, indicating the possible involvement of a V-type H + - ATPase. P. schlosseri could also increase the rate of H + excretion in response to the presence of ammonia in a neutral (pH·7.0) external medium. Increased H + excretion in P. schlosseri occurred in the head region where active excretion of NH 4 + took place. This would result in high concentrations of H + in the boundary water layer and prevent the dissociation of NH 4 + , thus preventing a back flux of NH 3 through the branchial epithelia. P. schlosseri probably developed such an ‘environmental ammonia detoxification’ capability because of its unique behavior of burrow building in the mudflats and living therein in a limited volume of water. In addition, the skin of P. schlosseri had low permeability to NH 3 . Using an Ussing- type apparatus with 10·mmol·l –1 NH 4 Cl and a 1 unit pH gradient (pH·8.0 to 7.0), the skin supported only a very small flux of NH 3 (0.0095·μmol·cm –2 ·min –1 ). Cholesterol content (4.5·μmol·g –1 ) in the skin was high, which suggests low membrane fluidity. Phosphatidylcholine, which has a stabilizing effect on membranes, constituted almost 50% of the skin phospholipids, with phosphatidyleserine and phsophatidylethanolamine contributing only 13% and 15%, respectively. More importantly, P. schlosseri increased the cholesterol level (to 5.5·μmol·g –1 ) and altered the fatty acid composition (increased total saturated fatty acid content) in its skin lipid after exposure to ammonia (30·mmol·l –1 at pH·7.0) for 6 days. These changes might lead to an even lower permeability to NH 3 in the skin, and reduced back diffusion of the actively excreted NH 4 + as NH 3 or the net influx of exogenous NH 3 , under such conditions. Key words: ammonia, excretion, permeability, cholesterol, fatty acid, gill, H + -ATPase, lipid, membrane, membrane fluidity, mudskipper, Periophthalmodon schlosseri, phospholipid, proton pump, skin. Summary Introduction The giant mudskipper Periophthalmodon schlosseri facilitates active NH 4 + excretion by increasing acid excretion and decreasing NH 3 permeability in the skin Yuen K. Ip 1, *, David J. Randall 2 , Timothy K. T. Kok 1 , Cristiana Barzaghi 1 , Patricia A. Wright 3 , James S. Ballantyne 3 , Jonathan M. Wilson 4 and Shit F. Chew 5 1 Department of Biological Sciences, National University of Singapore, 10 Kent Ridge Road, Singapore 117543, Republic of Singapore, 2 Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong, China, 3 Department of Zoology, University of Guelph, Guelph, Ontario, Canada NIG 2W1, 4 CentroInterdisciplinar de Investigação Marinha e Ambiental-CIIMAR, Rua do Campo Alegre 823, 4150-180 Porto, Portugal and 5 Natural Sciences, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore *Author for correspondence (e-mail: dbsipyk@nus.edu.sg) Accepted 7 November 2003