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The impact of acute low salinity stress on Antarctic echinoderms.
Barrett NJ
,
Harper EM
,
Peck LS
.
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Climate change is causing increased coastal freshening in Antarctica, leading to reduced salinity. For Antarctica's endemic echinoderms, adapted to the stable polar environment, the impact of rapid reductions in coastal salinity on physiology and behaviour is currently unknown. Six common Antarctic echinoderms (the sea urchin Sterechinus neumayeri; the sea star Odontaster validus; the brittle star Ophionotus victoriae; and three sea cucumbers Cucumaria georgiana, Echinopsolus charcoti and Heterocucumis steineni), were directly transferred from ambient salinity (34.5‰) to a range of salinity dilutions (29-9‰) for 24 h. All species showed reduced activity and the establishment of a temporary osmotic gradient between coelomic fluid and external seawater. Most species exhibited a depression in oxygen consumption across tolerated salinities; however, at very low salinities that later resulted in mortality, oxygen consumption increased to levels comparable to those at ambient. Low salinity tolerance varied substantially between species, with O. victoriae being the least tolerant (24 h LC50 (lethal for 50% of animals) = 19.9‰) while E. charcoti and C. georgiana demonstrated the greatest tolerance (24 h LC50 = 11.5‰). These findings demonstrate the species-specific response of Antarctica's endemic echinoderms to short-term hypoosmotic salinity events, providing valuable insight into this phylum's ability to respond to an underreported impact of climate change.
Figure 1.
. Oxygen consumption measured over the first 5–7 h after transfer from ambient salinity (34.5‰) in six Antarctic echinoderms: (a) Sterechinus neumayeri, (b) Odontaster validus, (c) Ophionotus victoriae, (d) Heterocucumis steineni, (e) Cucumaria georgiana, (f) Echinopsolus charcoti. Results are shown for one-way ANOVA and post hoc Tukey test, with different letters indicating significant differences (p < 0.05; n = 7 or 8 (a), n = 5 (b, c), n = 8 (d, f), n = 5 or 6 (e) biological replicates for each treatment) between treatments. Bars in (c) show results between 24 and 34.5‰ only (ns, not significant; **p < 0.01). Box plots show medians (black horizontal line), upper and lower quartiles, and maximum and minimum (whiskers). Biological replicates are represented by grey dots, means by blue dots and outliers by black dots. Grey dashed line represents the calculated LC50 value.
Figure 2.
. Activity rates 6 h after transfer from ambient salinity (34.5‰) into low salinity treatments in six Antarctic echinoderms; (a) Sterechinus neumayeri, (b) Odontaster validus, (c) Ophionotus victoriae, (d) Heterocucumis steineni, (e) Cucumaria georgiana, (f) Echinopsolus charcoti. (a–c) Righting ability was converted to an activity coefficient, calculated as AC = 1000/righting time in s; a smaller AC value indicates a longer righting time. (d–f) Number of peristaltic locomotory waves recorded over 1 h. Results are shown for Kruskal–Wallis and post hoc Dunn’s test (a–c), and two-sample Poisson test (d–f) with different letters indicating significant differences (p < 0.05; n = 5 (a–c), n = 6 (e), n = 8 (d, f) biological replicates for each treatment). Box plots show medians (black horizontal line), upper and lower quartiles, and maximum and minimum (whiskers). Biological replicates are represented by grey dots, means by blue dots and outliers by black dots. Grey dashed line represents the calculated LC50 value.
Figure 3.
. Coelomic fluid osmolality of Antarctic echinoderms at 6 and 24 h after transfer from ambient salinity (34.5‰) versus that of the corresponding tank water; (a) Sterechinus neumayeri, (b) Odontaster validus, (c) Ophionotus victoriae, (d) Heterocucumis steineni, (e) Cucumaria georgiana, (f) Echinopsolus charcoti. Number of animal replicates at each experimental salinity treatment were n = 3 at 6 h and n = 7 or 8 at 24 h for S. neumayeri, n = 3−5 at 6 h and n = 5 or 6 at 24 h for O. validus, n = 3 at 6 and 24 h for H. steineni, E. charcoti and C. georgiana (except 9‰ for C. georgiana at 6 h, n = 2). Mean (± s.e.m.) tank water osmolality (mOsm kg−1) in each salinity experiment was: 34.5‰, 1051 ± 6; 29‰, 876 ± 3; 24‰, 721 ± 2; 19‰, 576 ± 2; 14‰, 414 ± 2; and 9‰, 269 ± 3. A second-order polynomial regression line (blue line) and the s.e.m. (shaded grey area) were fitted to the data. The iso-osmotic line is represented by a grey line. Regression coefficients and r
2 values are included to the right of the isosmotic line.
