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Multiyear trend in reproduction underpins interannual variation in gametogenic development of an Antarctic urchin.
De Leij R
,
Peck LS
,
Grange LJ
.
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Ecosystems and their biota operate on cyclic rhythms, often entrained by predictable, small-scale changes in their natural environment. Recording and understanding these rhythms can detangle the effect of human induced shifts in the climate state from natural fluctuations. In this study, we assess long-term patterns of reproductive investment in the Antarctic sea urchin, Sterechinus neumayeri, in relation to changes in the environment to identify drivers of reproductive processes. Polar marine biota are sensitive to small changes in their environment and so serve as a barometer whose responses likely mirror effects that will be seen on a wider global scale in future climate change scenarios. Our results indicate that seasonal reproductive periodicity in the urchin is underpinned by a multiyear trend in reproductive investment beyond and in addition to, the previously reported 18-24 month gametogenic cycle. Our model provides evidence that annual reproductive investment could be regulated by an endogenous rhythm since environmental factors only accounted for a small proportion of the residual variation in gonad index. This research highlights a need for multiyear datasets and the combination of biological time series data with large-scale climate metrics that encapsulate multi-factorial climate state shifts, rather than using single explanatory variables to inform changes in biological processes.
Figure 1. Location of Hangar Cove study site at Rothera Point, Adelaide Island, Antarctica (67°33′54.2"S 68°07′13.1"W) and insert map showing Rothera Point on Western Antarctic Peninsula. Marine environmental data are collected at the site south west of Rothera Point as part of the British Antarctic Survey Rothera Time Series monitoring programme (RaTS). Large-scale map indicates the position of Rothera Research Station on Adelaide Island, on the Western Antarctic Peninsula. Figure modified from Grange et al. (2011).
Figure 2. Monthly changes for Sterechinus neumayeri in (A) gonad index for males (blue) and females (red); (B) mean equivalent circular diameter (ECD) of oocytes present in female gonads; (C) percentage (%) of gonad tissue in females composed of nutritive phagocytes; (D) percentage frequency (%) of male gonad maturity stages where frequencies are smoothed by the function y ~ x using the local regression smoother (LOESS) method. The smoothing span was chosen to reflect seasonal changes. Data as box plots are displayed with the central line in the boxes representing the median value, the upper and lower hinges representing the 25th and 75th percentiles, and the upper/lower whiskers representing the largest/smallest value, no further than 1.5 times the interquartile range from the hinge. All data outside these ranges are plotted as points.
Figure 3. Monthly female gonad index as proportional area of nutritive phagocytes (NP) and oocytes. Monthly data for proportions are ± the standard error of the NP or oocyte equivalent GI based on replicate months. Chlorophyll data are represented on the secondary y-axis and have been averaged from the time series (2012–2018), ± standard error.
Figure 4. Smoothers of the effect of the three non-parametric terms: time, Southern Oscillation Index (SOI), chlorophyll and Southern Annular Mode (SAM), on the gonad index of Sterechinus neumayeri, from the optimal GAM model. Shaded area represents a 95% confidence interval and data points represent raw gonad index data. The magnitude of change in gonad index as a response to the change in the x-variable is represented on the y-axis as the square-root transformed gonad index. For the axis ‘time’, year intervals are plotted on every 1st January. Green represents males and yellow represents females.
Figure 5. Seasonal cycle of gonad index for males (solid green line) and females (solid yellow line), extracted from decomposition analysis, overlaid on seasonal cycle of chlorophyll (dotted black line) extracted from decomposition analysis.
Figure 6. Long-term changes in the percentage frequency (%), represented as a density plot, of maturity stages in the male sample population from March 2012–March 2017. Frequency densities derived from (LOESS) method. The smoothing span was chosen to reflect long-term changes rather than seasonal variability.
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