Abstract The Indo-Pacific swimming crab Charybdis hellerii (A. Milne-Edwards, 1867) is native to the Indo-Pacific Ocean, and occurs as an invasive species along the Eastern American coast, where it was probably introduced in ballast water. The present study evaluated the osmoregulation and salinity tolerance of C. hellerii in Paranaguá Bay, Paraná, Brazil, an estuary under the constant threat of introduction of non-native species. The crabs were abruptly submitted to salinities of 10, 20, 30 (control), and 40 PSU for 24h. Hemolymph osmolality showed either an increase or a decrease following the changes in water salinity, presenting a pattern of hyper-osmoconformation. Muscle water content was stable at salinities from 20 to 40 PSU, and increased after exposure to 10 PSU indicating cellular swelling. At 10 PSU a physiological limit was reached, indicated by the failure of the capacity to regulate tissue water content. Although adult C. hellerii tolerate a wide range of salinities for 24 h, its osmoregulatory ability indicates a possible advantage for its establishment in estuarine waters with salinities <20 PSU. The salinity tolerance and osmoregulatory data are important basic information to be used in models to predict and prevent the invasion of the species into new areas of the Neotropics.

Os baiacus marinhos Sphoeroides testudineus e S. greeleyi são freqüentadores de estuários e eficientes osmorreguladores. No entanto, eles diferem quanto à sua capacidade de tolerar a redução de salinidade. Durante a descida de maré S. testudineus permanece no estuário com salinidade reduzida e S. greeleyi retorna para a água do mar. A hipótese testada neste estudo foi a de que a mobilização em curto prazo dos substatos metabólicos armazenados nos órgãos osmorregulatórios poderia explicar esta tolerância diferencial a redução de salinidade. Peixes expostos a 5‰ (por 6 h) foram comparados com os mantidos em 35‰ . O conteúdo branquial e renal de triacilglicerol, proteína e glicogênio foram avaliados, e a atividade ATPásica total foi quantificada para representar o metabolismo do tecido. Osmolalidade, cloreto e glicose plasmáticos, o hematócrito e o conteúdo de água no músculo também foram mensurados. O conteúdo total de triacilglicerol foi maior em S. greeleyi do que em S. testudineus em ambas as salinidades e em ambos os órgãos. O conteúdo renal de glicogênio foi maior em S. greeleyi quando comparado a S. testudineus na duas salinidades. A atividade ATPásica total foi reduzida em 5‰ quando comparada a 35‰ no rim de S. greeleyi, e foi maior nas brânquias de S. greeleyi quando comparada aos valores para S. testudineus em ambas as salinidades. Após a exposição a água do mar diluída, ambas as espécies apresentaram padrão osmorregulatório similar: osmolalidade e cloreto plasmáticos foram reduzidos. Em ambas as espécies a estabilidade do conteúdo de água no músculo indicou capacidade de regular o conteúdo de água intracelular. Os substratos metabólicos estocados nos órgãos osmorregulatórios de ambas as espécies não foram mobilizados durante a exposição de curto-prazo a diluição da água do mar, porém algumas diferenças foram reveladas entre as duas espécies. S. greeleyi apresentou maior reserva metabólica (essencialmente triacilglicerol) em seus órgãos, e suas brânquias apresentaram maior atividade ATPásica total quando comparado a S. testudineus.

The marine-estuarine species of pufferfishes Sphoeroides testudineus and S. greeleyi are very efficient osmoregulators. However, they differ with respect to their tolerance of salinity reduction. During low tide S. testudineus remains in diluted estuarine waters, whereas S. greeleyi returns to seawater (SW). The hypothesis tested here was that the short-term mobilization of metabolic substrates stored in their main osmoregulatory organs would correlate with this differential tolerance. Fishes exposed to 5‰ (for 6 h) were compared to those kept in 35‰. Branchial and renal contents of triglycerides, protein and glycogen were evaluated, and total ATPase activity accounted for the tissues' metabolism. Plasma osmolality, chloride and glucose, hematocrit, and muscle water content were also measured. Total triacylglycerol content was higher in S. greeleyi than in S. testudineus in both salinities and in both organs. Kidney glycogen contents were higher in S. greeleyi than in S. testudineus in 5 and 35‰. Total ATPase activity was reduced in 5‰ when compared to 35‰ in the kidney of S. greeleyi, and was higher in the gills of S. greeleyi than in those of S. testudineus, in both salinities. Upon exposure to dilute SW, both species displayed a similar osmoregulatory pattern: plasma osmolality and chloride were reduced. Again in both species, stability in muscle water content indicated cellular water content control. Although the metabolic substrates stored in the osmoregulatory organs of both species were not mobilized during these short-term sea water dilution events, some differences could be revealed between the two species. S. greeleyi showed more metabolic reserves (essentially triacylglycerols) in these organs, and its gills showed higher total ATPase activity than those S. testudineus.

Temperature as an environmental factor has been a frequent subject of study, since it affects either directly or indirectly all living organisms. The determination of thermal limits (critical thermal minima - CTmin and maxima - CTmax) for the tropical ornamental freshwater teleost Xiphophorus maculatus Günther, 1866 (platyfish) was performed after their acclimation to the following temperatures: 15, 20, 25, and 30ºC, for seven days. After this period, the water temperature was elevated or reduced at a rate of 0.125ºC/min until CTmax and CTmin could be determined as the temperature at which 50% of the animals had lost equilibrium. Mean values for CTmax and CTmin for the acclimation temperatures of 15, 20, 25, and 30ºC were respectively: 39.8,39.8,40.4,41.5ºC (CTmax), and 9.6, 12.8, 13.1, 16.0ºC (CTmin). CTmax and CTmin for X. maculatus were thus affected by acclimation temperature. This tropical species is more heat- than cold- tolerant and would not resist the typical low winter temperatures of southern Brazil. Platyfish can adapt to natural environments in regions of mean annual temperatures around 20-25ºC or be kept in aquaria with other ornamental species that accordingly prefer this temperature range.