Do Hung Dang V et al. (2020), Grazing effects of sea urchin Diadema savignyi ...

ECB-ART-49472

Sci Rep 2020 Nov 23;101:20346. doi: 10.1038/s41598-020-77494-0.

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Grazing effects of sea urchin Diadema savignyi on algal abundance and coral recruitment processes.

Do Hung Dang V , Fong CL , Shiu JH , Nozawa Y .

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Herbivores control algae and promote coral dominance along coral reefs. However, the majority of previous studies have focused on herbivorous fish. Here we investigated grazing effects of the sea urchin Diadema savignyi on algal abundance and coral recruitment processes. We conducted an in situ cage experiment with three density conditions of D. savignyi (0, 8, 16 indiv. m-2) for three months during the main coral recruitment season in Taiwan. Results demonstrated a strong algal control by D. savignyi. At the end of the experiment, average algal cover was 95% for 0 indiv. m-2, compared to 47% for 8 indiv. m-2 and 16% for 16 indiv. m-2. Average algal biomass at 8 indiv. m-2 declined by one third compared to 0 indiv. m-2 and almost zero at 16 indiv. m-2. On the other hand, a negative grazing effect of D. savignyi was observed on coral recruitment processes. Notably, at 16 indiv. m-2, the density of coral recruits declined and mortality of small coral fragments (proxy of coral juveniles) increased. Our results confirm findings of previous studies and indicate the need to balance both positive (strong algal control) and negative (physical damage) influences of Diadema grazing to facilitate the coral recruitment process.

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	Figure 1. (a) Study location of the inclusion cage experiment at Nanwan bay, Kenting, Southern Taiwan. The area enclosed by the dashed line indicates Kenting National Park. The map was created using the package, maps ver. 3.3.0 with the software, R ver. 3.6.1 (https://www.rdocumentation.org/packages/maps/versions/3.3.0). (b,c) Image of the inclusion cage experiment: (b) two of the five experimental cage groups, each consisting of three cages for three D. savignyi density conditions on the top of plastic container base; (c) close-up of two cages with 8 indiv. m−2 (right) and 16 indiv. m−2 conditions (left) of D. savignyi. Each cage had 12 experimental terracotta plates (10 cm × 10 cm) fixed on the mesh bottom.
	Figure 2. The effect of three density conditions of Diadema savignyi on algal cover during the 3-month inclusion cage experiment. Average ± SE are shown. Stars indicate statistical significance at p < 0.01 () and p < 0.001 (*); (a) between the three density conditions from the 1st to 3rd month, (b) between 8 and 16 indiv. m−2 at the 1st month and between the three density conditions at the 2nd and 3rd month, and (c) between the three density conditions at the 1st month. Datapoints were slightly shifted horizontally to avoid overlapping.
	Figure 3. The effect of three density conditions of Diadema savignyi on algal biomass at the end of the 3-month experiment. Average + SE are shown. Stars denote statistical significance at p < 0.05 () and p < 0.001 (**).
	Figure 4. The effect of the three density conditions of Diadema savignyi on coral recruitment density (a) on grooved surface plates and (b) plane surface plates at the end of 3-month experiment. Average + SE are shown. Stars denote statistical significance at p < 0.05 () and p < 0.01 (*). No coral recruits occurred on plane plates in the 16 indiv. m−2 condition.

References [+] :

Bellwood, Confronting the coral reef crisis. 2004, Pubmed

	Figure 1. (a) Study location of the inclusion cage experiment at Nanwan bay, Kenting, Southern Taiwan. The area enclosed by the dashed line indicates Kenting National Park. The map was created using the package, maps ver. 3.3.0 with the software, R ver. 3.6.1 (https://www.rdocumentation.org/packages/maps/versions/3.3.0). (b,c) Image of the inclusion cage experiment: (b) two of the five experimental cage groups, each consisting of three cages for three D. savignyi density conditions on the top of plastic container base; (c) close-up of two cages with 8 indiv. m−2 (right) and 16 indiv. m−2 conditions (left) of D. savignyi. Each cage had 12 experimental terracotta plates (10 cm × 10 cm) fixed on the mesh bottom.
	Figure 2. The effect of three density conditions of Diadema savignyi on algal cover during the 3-month inclusion cage experiment. Average ± SE are shown. Stars indicate statistical significance at p < 0.01 () and p < 0.001 (*); (a) between the three density conditions from the 1st to 3rd month, (b) between 8 and 16 indiv. m−2 at the 1st month and between the three density conditions at the 2nd and 3rd month, and (c) between the three density conditions at the 1st month. Datapoints were slightly shifted horizontally to avoid overlapping.
	Figure 3. The effect of three density conditions of Diadema savignyi on algal biomass at the end of the 3-month experiment. Average + SE are shown. Stars denote statistical significance at p < 0.05 () and p < 0.001 (**).
	Figure 4. The effect of the three density conditions of Diadema savignyi on coral recruitment density (a) on grooved surface plates and (b) plane surface plates at the end of 3-month experiment. Average + SE are shown. Stars denote statistical significance at p < 0.05 () and p < 0.01 (*). No coral recruits occurred on plane plates in the 16 indiv. m−2 condition.