RESUMO
The intertidal sea-urchin Echinometra lucunter (Linnaeus, 1758) has been submitted to diluted sea water (SW) of salinity 25, or concentrated sea water of salinity 45. In addition, ionic challenges have been offered, supplementing 25 SW with Mg2+, Ca2+ or K+, until the concentration of each of these ions would reach the level of full-strength 35 SW (control). Perivisceral coelomic fluid has been sampled after six hours in these treatments for measurements of osmolality and concentrations of Na+, Cl-, Mg2+, Ca2+, and K+. Urchins have been further observed until five days. SW concentration (45) lead urchins to death after two days of exposure, while urchins tolerated five days in 25 SW without any sign of distress. Urchins displayed osmoconformation and ion-conformation for NaCl, and occasional small gradients with respect to the water for Mg2+ (~6% in full-strength SW), and K+ (8.5% in 25 SW), after six hours. These results are consistent with data compiled from the literature, for echinoderms, which frequently show positive gradients (often higher than 25%) for the most relevant ions, between the coelomic fluid and external SW. Supplemented cations have shown mutual interference, mostly affecting their own coelomic fluid concentrations. Under the protocol used here, urchins of the species E. lucunter held gradients for Mg2+, Ca2+, and K+, but not for Na+ or Cl- or osmolality. They were also able to tolerate at least a 30% reduction in sea water salinity for five days, showing reasonable euryhalinity. However, when compared to other echinoderms, E. lucunter is not especially capable of maintaining large ionic gradients with respect to external SW.
RESUMO
The intertidal sea-urchin Echinometra lucunter (Linnaeus, 1758) has been submitted to diluted sea water (SW) of salinity 25, or concentrated sea water of salinity 45. In addition, ionic challenges have been offered, supplementing 25 SW with Mg2+, Ca2+ or K+, until the concentration of each of these ions would reach the level of full-strength 35 SW (control). Perivisceral coelomic fluid has been sampled after six hours in these treatments for measurements of osmolality and concentrations of Na+, Cl-, Mg2+, Ca2+, and K+. Urchins have been further observed until five days. SW concentration (45) lead urchins to death after two days of exposure, while urchins tolerated five days in 25 SW without any sign of distress. Urchins displayed osmoconformation and ion-conformation for NaCl, and occasional small gradients with respect to the water for Mg2+ (~6% in full-strength SW), and K+ (8.5% in 25 SW), after six hours. These results are consistent with data compiled from the literature, for echinoderms, which frequently show positive gradients (often higher than 25%) for the most relevant ions, between the coelomic fluid and external SW. Supplemented cations have shown mutual interference, mostly affecting their own coelomic fluid concentrations. Under the protocol used here, urchins of the species E. lucunter held gradients for Mg2+, Ca2+, and K+, but not for Na+ or Cl- or osmolality. They were also able to tolerate at least a 30% reduction in sea water salinity for five days, showing reasonable euryhalinity. However, when compared to other echinoderms, E. lucunter is not especially capable of maintaining large ionic gradients with respect to external SW.
RESUMO
The intertidal sea-urchin Echinometra lucunter (Linnaeus, 1758) has been submitted to diluted sea water (SW) of salinity 25, or concentrated sea water of salinity 45. In addition, ionic challenges have been offered, supplementing 25 SW with Mg2+, Ca2+ or K+, until the concentration of each of these ions would reach the level of full-strength 35 SW (control). Perivisceral coelomic fluid has been sampled after six hours in these treatments for measurements of osmolality and concentrations of Na+, Cl-, Mg2+, Ca2+, and K+. Urchins have been further observed until five days. SW concentration (45) lead urchins to death after two days of exposure, while urchins tolerated five days in 25 SW without any sign of distress. Urchins displayed osmoconformation and ion-conformation for NaCl, and occasional small gradients with respect to the water for Mg2+ (~6% in full-strength SW), and K+ (8.5% in 25 SW), after six hours. These results are consistent with data compiled from the literature, for echinoderms, which frequently show positive gradients (often higher than 25%) for the most relevant ions, between the coelomic fluid and external SW. Supplemented cations have shown mutual interference, mostly affecting their own coelomic fluid concentrations. Under the protocol used here, urchins of the species E. lucunter held gradients for Mg2+, Ca2+, and K+, but not for Na+ or Cl- or osmolality. They were also able to tolerate at least a 30% reduction in sea water salinity for five days, showing reasonable euryhalinity. However, when compared to other echinoderms, E. lucunter is not especially capable of maintaining large ionic gradients with respect to external SW.
RESUMO
Populations of Asterina stellifera Möbius, 1859 are declining in southern Brazil and today the species is on the Brazilian list of endangered species. Here we experimentally investigate the ecological role played by A. stellifera as a predator on the rocky substrate community by increasing densities of this species. Starfish taken from other, nearby, locations were added to three replicate plots and their density was maintained every 15 days. None of the total of 32 species that were identified in the encrusting community appeared to be influenced by the experiment. Two species, the barnacle Chthamalus bisinuatus Pilsbry, 1916 and the algae Gracilaria verrucosa (Hudson), showed small apparent responses to predation. We found that A. stellifera only exerts weak predation pressure on the community, even at high densities. This result is strikingly different from that observed for similar species in the northern hemisphere and in Australia.
RESUMO
Populations of Asterina stellifera Möbius, 1859 are declining in southern Brazil and today the species is on the Brazilian list of endangered species. Here we experimentally investigate the ecological role played by A. stellifera as a predator on the rocky substrate community by increasing densities of this species. Starfish taken from other, nearby, locations were added to three replicate plots and their density was maintained every 15 days. None of the total of 32 species that were identified in the encrusting community appeared to be influenced by the experiment. Two species, the barnacle Chthamalus bisinuatus Pilsbry, 1916 and the algae Gracilaria verrucosa (Hudson), showed small apparent responses to predation. We found that A. stellifera only exerts weak predation pressure on the community, even at high densities. This result is strikingly different from that observed for similar species in the northern hemisphere and in Australia.
RESUMO
Populations of Asterina stellifera Möbius, 1859 are declining in southern Brazil and today the species is on the Brazilian list of endangered species. Here we experimentally investigate the ecological role played by A. stellifera as a predator on the rocky substrate community by increasing densities of this species. Starfish taken from other, nearby, locations were added to three replicate plots and their density was maintained every 15 days. None of the total of 32 species that were identified in the encrusting community appeared to be influenced by the experiment. Two species, the barnacle Chthamalus bisinuatus Pilsbry, 1916 and the algae Gracilaria verrucosa (Hudson), showed small apparent responses to predation. We found that A. stellifera only exerts weak predation pressure on the community, even at high densities. This result is strikingly different from that observed for similar species in the northern hemisphere and in Australia.