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Abstract: Echinoderms are a stenohaline phylum, however some species can tolerate varying degrees of exposure to hyposaline conditions. These exposures typically are limited in duration to a few hours to no more than a day or two, and hyposalinity levels range from ~8‰ to <30‰. The unique climatological and hydrographic conditions in Fiordland of southwest New Zealand produce a near-freshwater layer (Low Salinity Layer = LSL) on top of marine (34‰) water. The relative depth of the LSL varies both tidally and with rainfall. This halo-boundary presents a particularly stressful osmotic challenge to organisms found in shallow water. A series of transects in Doubtful Sound and the more northern Milford Sound quantified differences in density and bathymetric distribution of two species of seastars. The surveys revealed that Patiriella mortenseni occurs primarily below the LSL whereas Patiriella regularis is found well within the range of the LSL. Video observations of P. regularis over a rising tide were correlated with salinity measurements and showed a strong correlation between increased salinity and general vertical displacement upwards of the seastars observed moving. Activity coefficients (AC) were used to estimate the degree of neuro-muscular coordination and stress levels of these seastars in two hyposalinity experiments. Reciprocal transplants showed that P. regularis performed equally well in deeper, marine-salinity waters, as in shallow hyposalinity conditions, whereas P. mortenseni showed significantly impaired performance in the hyposaline conditions. These performance estimates were refined in a second experiment by quantifying AC of both species in a seawater dilution series. To rule out the possibility of heretofore unknown osmoregulatory capacity, we exposed both species to 0‰, 5‰, 15‰, 25‰, and 34‰ and recorded the osmolarity of the body fluid after 24 h. P. mortenseni died at salinities below 25‰. In contrast, P. regularis survived and the body fluid was nearly isotonic with the surrounding water. Two final experiments established the hyposalinity-tolerance limits of P. regularis. In a righting-time (RT) experiment we exposed P. regularis to hyposaline water of 5‰ for: 12, 24, 36, 48, 60, 84, and 133 h and recorded righting times immediately after re-immersing them in sea water. All individuals survived and the mean RT ranged from 18 min to 27 h (control group=1.6 min). Finally, a lethal-exposure experiment to 0‰ (deionized water) confirmed that P. regularis can withstand far greater levels of extreme hyposalinity than has been reported for any other echinoderm. [Copyright &y& Elsevier]

Sea surface temperature (SST) influences the physiology of marine invertebrates, and, in turn, their geographical distribution. The Diadematidae sea urchin Centrostephanus rodgersii has expanded its range in Eastern Australia as a result of increased winter sea temperatures, and there is interest in the potential for the species to increase its range and abundance in New Zealand. We investigated the extent to which embryonic and larval thermal tolerance of fertilization, cleavage, hatching, gastrulation and appearance of plutei may control the distribution of the adult populations in New Zealand. We also compared these individuals with an Australian population in northern New South Wales at Coffs Harbour from a region just north of where the Eastern Australian Current gives rise to the Tasman Front current to New Zealand. The aim of the present study was to find evidence for adaptation in fertilization and the developmental stages to New Zealand winter sea temperatures. Fertilization was not affected by temperature over a broad thermal range (12.0°C–26.4°C and 12.8°C–29.5°C for the New Zealand and the Coffs Harbour population respectively). The first cleavage, hatching, gastrulation and appearance of plutei were more sensitive to temperature and occurred within a well-defined thermal window (17.2°C–23.5°C in New Zealand and 17.3°C–24.5°C in Coffs Harbour). The sites where C. rodgersii adults have been reported in New Zealand lie on the 15°C winter isotherm, consistent with our findings with regard to the lower temperature for pluteal development for the New Zealand population. We found evidence for a shift in thermal tolerance between the two populations of C. rodgersii examined, with fertilization and each developmental stage showing an upper thermal limit consistently shifted by ≈1°C towards lower temperatures in New Zealand individuals. Our findings indicate that the thermal window for larval development of C. rodgersii may contribute to its present distribution in New Zealand. In addition, given that larval development in New Zealand presently occurs at the colder limit of the thermal window of C. rodgersii, there may be the potential for a future expansion of its distributional range under future ocean warming scenarios. [ABSTRACT FROM AUTHOR]