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

Jia-Ho Shiu,Yoko Nozawa,Viet Do Hung Dang,Chia-Ling Fong

doi:10.1038/s41598-020-77494-0

Abstract

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.

Highlights

As the global decline in coral reefs becomes more evident[1,2,3,4], the significance of our knowledge concerning factors contributing to coral resilience increases[5]
We found that Diadema sea urchins were the dominant herbivore in southern Taiwan and that Diadema density positively correlated with juvenile coral density and coral r ecovery[26]
The present study confirms the findings of previous studies and elucidates both the positive and negative influence of Diadema grazing on the coral recruitment process

Summary

Introduction

As the global decline in coral reefs becomes more evident[1,2,3,4], the significance of our knowledge concerning factors contributing to coral resilience increases[5]. The sudden decline of D. antillarum was the result of an outbreak of disease in 1983–1984, leading to algal blooms due to low functional redundancy of grazers; a consequence of overfishing herbivorous fishes This hampered coral recovery and a phase shift from a coral-dominant to algal-dominant state occurred along many Caribbean reefs[21,22]. We found that Diadema sea urchins were the dominant herbivore in southern Taiwan and that Diadema density positively correlated with juvenile coral density and coral r ecovery[26] These findings motivated us to examine the effect of Diadema grazing on algal control and the coral recruitment process, as a primary driver of the observed correlations. Coral recruit density and growth and the survival of small coral fragments (proxy of coral juveniles) were examined under three density conditions of D. savignyi (0, 8, 16 indiv. m−2)

Methods

Results

Discussion

Conclusion