The distribution and behaviour of Patiriella mortenseni and P. regularis in the extreme hyposaline conditions of the Southern New Zealand Fiords

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 b30‰. 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 lethalexposure 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. © 2008 Elsevier B.V. All rights reserved.