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Temporal variability modulates pH impact on larval sea urchin development: Themed Issue Article: Biomechanics and Climate Change.
Chan KYK
,
Tong CSD
.
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Coastal organisms reside in highly dynamic habitats. Global climate change is expected to alter not only the mean of the physical conditions experienced but also the frequencies and/or the magnitude of fluctuations of environmental factors. Understanding responses in an ecologically relevant context is essential for formulating management strategies. In particular, there are increasing suggestions that exposure to fluctuations could alleviate the impact of climate change-related stressors by selecting for plasticity that may help acclimatization to future conditions. However, it remains unclear whether the presence of fluctuations alone is sufficient to confer such effects or whether the pattern of the fluctuations matters. Therefore, we investigated the role of frequency and initial conditions of the fluctuations on performance by exposing larval sea urchin Heliocidaris crassispina to either constant or fluctuating pH. Reduced pH alone (pH 7.3 vs 8.0) did not affect larval mortality but reduced the growth of larval arms in the static pH treatments. Changes in morphology could affect the swimming mechanics for these small organisms, and geometric morphometric analysis further suggested an overall shape change such that acidified larvae had more U-shaped bodies and shorter arms, which would help maintain stability in moving water. The relative negative impact of lower pH, computed as log response ratio, on larval arm development was smaller when larvae were exposed to pH fluctuations, especially when the change was less frequent (48- vs 24-h cycle). Furthermore, larvae experiencing an initial pH drop, i.e. those where the cycle started at pH 8.0, were more negatively impacted compared with those kept at an initial pH of 7.3 before the cycling started. Our observations suggest that larval responses to climate change stress could not be easily predicted from mean conditions. Instead, to better predict organismal performance in the future ocean, monitoring and investigation of the role of real-time environmental fluctuations along the dispersive pathway is key.
Figure 1. Larval survival decreased under constant pHÂ 8.0, constant pHÂ 7.3 and fluctuating pH between these two levels over 8Â days post fertilization (dpf). Each symbol represents the raw count observed within one replicate jar (aâc). Linear regressions between the observed proportions and time were used to determine the mortality rate (Table S1). Larval growth rate was determined by measuring the total body length (dâf) and postoral arm length (gâi) of 15 haphazardly selected individuals from each treatment daily. Means and standard error of means for each replicate jar were plotted for dâi. Logarithmic regressions between the body/arm length and time were used to determine the growth rate (Table S1). Open symbols represent treatments that started with control (pHÂ 8.0) condition, and solid symbols are for those starting with pHÂ 7.3.
Figure 2. Survivorship (rate of decline in density), total body length and postoral arm length growth rate of larval urchins exposed to constant and fluctuating pH between 8.0 and 7.3. Means and standard errors of means of the three replicate jars per treatment were plotted (Nâ=â18, aâc). Exposure to constant low pH did not significantly increase mortality (pHÂ 7.3 in a) but did compromise the growth rates of arms (pHÂ 7.3 in c). Log response ratio (LnRR) for each treatment relative to the constant pHÂ 8.0 (control) and their corresponding 95% confidence intervals are shown (dâf). The log response ratio relative to constant pHÂ 8.0 (control) affirmed the pattern observed in the direct measurements i.e. increased larval mortality (d) and a significant reduction in arm growth (f) in the acidified treatments. Pairwise comparisons of the LnRR with Z-test showed that the frequency (24- vs 48-h cycle) and the initial condition (clt2low vs low2 clt) can modulate pH impact (dâf). The LnRR and variance of sampling are listed in Table 2.
Figure 3. Landmark analysis shows the overall shape of larval sea urchins changed when exposed to acidification stress. Thirteen landmarks were used (circles in the top right inset, total body length (TBL) and postoral arm length (POL) are also labelled). The body shape (V-shape vs. U-shape, CV1) explained 37.6% of the variance, and relative arm length (CV2) explained 27.4% of the variance. Individuals exposed to fluctuations appeared to have an intermediate form. Each data point represents an individual measured (nâ=â15 for each treatment), and the 90% confidence ellipse of the mean is also plotted. Wrapped wireframe (solid line) illustrates the shape change compared to the mean shape (grey, dotted line).
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