Journal of Experimental Marine Biology and Ecology Ž . 262 2001 243–253 www.elsevier.nlrlocaterjembe Ionic regulation in aglomerular tropical estuarine pufferfishes submitted to sea water dilution Viviane Prodocimo, Carolina Arruda Freire ) Departamento de Fisiologia, Setor de Ciencias Biologicas, UniÕersidade Federal do Parana, ˆ ´ ´ Centro Politecnico, Curitiba, Parana, 81531-990, Brazil ´ ´ Received 6 March 2001; received in revised form 4 April 2001; accepted 15 May 2001 Abstract Two common tropical estuarine pufferfishes were used in this study. The main species was Sphoeroides testudineus Linnaeus, 1758, a very abundant species in the estuaries of Paranagua ´ Ž . Ž . Ž Bay Parana, Brazil , found in waters of salinity between 0‰ tidal creeks and 34‰ tidal ´ . plains . The second species was S. greeleyi Gilbert, 1900, a species limited in distribution to an Ž . area of higher salinity ;30‰ than S. testudineus. The present work thus aimed at evaluating the capacity of ionic regulation of both species of pufferfishes when submitted to salinity decrease, relating the results with both species’ distribution in nature. Ion regulation curves for sodium Ž q . Ž y . Ž 2q . Na , chloride Cl , and magnesium Mg ions after 6 h and 15 days of exposure of the abundant S. testudineus to the salinities of 30‰, 20‰, 10‰, and 5‰ were elaborated, as well as for Cl y and Mg 2q after 6 h and 15 days of exposure of both species to the extreme salinities of y Ž . 2q Ž . 35‰ and 5‰. Both species kept their plasma Cl ;120–160 mM , and Mg ;1.3 mM q Ž . q Ž concentrations stable, as did S. testudineus for Na ;130 mM . Na measured only for S. . y Ž . Ž . testudineus and Cl were either hyper-regulated in 5‰ or kept iso-ionic ;7–10‰ , but more Ž . 2q often hypo-regulated 20–35‰ . In contrast, Mg was strongly hypo-regulated in all salinities. According to their distribution records in nature, S. greeleyi was less able to tolerate strong sea water dilution, showing mortality after 5 days in 5‰ water. These estuarine pufferfishes are thus efficient regulators of plasma ionic concentrations in diluted sea water, as expected from their occupation of estuaries. The experiments have supported the distribution records of both species in the estuarine complex and resident estuarine species were thus characterized with respect to their osmoregulatory capacities. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Estuarine fish; Ionic regulation; Pufferfish; Sphoeroides greeleyi; Sphoeroides testudineus ) Corresponding author. Tel.: q 55-41-361-1712; fax: q 55-41-266-2042. Ž . E-mail address: cafreire@bio.ufpr.br C.A. Freire . 0022-0981r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0022-0981 01 00293-3