Genes encoding for claudin-8 and -27 tight junction proteins in the euryhaline puffer fish (Tetraodon nigroviridis) were identified using its recently sequenced genome. Phylogenetic analysis indicated that multiple genes encoding for claudin-8 proteins (designated Tncldn8a, Tncldn8b, Tncldn8c and Tncldn8d) arose by tandem gene duplication. In contrast, both tandem and whole genome duplication events appear to have generated genes encoding for claudin-27 proteins (designated Tncldn27a, Tncldn27b, Tncldn27c and Tncldn27d). Tncldn8 and Tncldn27 mRNA were widely distributed in Tetraodon, suggesting involvement in various physiological processes. All Tncldn8 and Tncldn27 genes were expressed in gill and skin tissue (i.e., epithelia exposed directly to the external environment). A potential role for claudin-8 and -27 proteins in the regulation of hydromineral balance in Tetraodon was investigated by examining alterations in mRNA abundance in select ionoregulatory tissue of fish acclimated to freshwater (FW) and seawater (SW). In FW or SW, Tetraodon exhibited alterations in Na(+)-K(+)-ATPase activity (a correlate of transcellular transport) typical of a euryhaline teleost fish. Simultaneously, tissue and gene specific alterations in Tncldn8 and Tncldn27 transcript abundance occurred. These data provide some insight into the duplication history of cldn8 and cldn27 genes in fishes and suggest a possible role for claudin-8 and -27 proteins in the osmoregulatory strategies of euryhaline teleosts.