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Ex situ co culturing of the sea urchin, Mespilia globulus and the coral Acropora millepora enhances early post-settlement survivorship.
Craggs J
,
Guest J
,
Bulling M
,
Sweet M
.
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Reef restoration efforts, utilising sexual coral propagation need up-scaling to have ecologically meaningful impact. Post-settlement survival bottlenecks, in part due to competitive benthic algae interactions should be addressed, to improve productivity for these initiatives. Sea urchins are keystone grazers in reef ecosystems, yet feeding behaviour of adults causes physical damage and mortality to developing coral spat. To investigate if microherbivory can be utilised for co-culture, we quantitatively assessed how varying densities of juvenile sea urchins Mespilia globulus (Linnaeus, 1758), reared alongside the coral Acropora millepora (Ehrenberg, 1834) effected survival and growth of coral recruits. Spawning of both species were induced ex situ. A comparison of A. millepora spat reared in three M. globulus densities (low 16.67 m-2, medium 37.50 m-2, high 75.00 m-2) and a non-grazed control indicated coral survival is significantly influenced by grazing activity (p < 0.001) and was highest in the highest density treatment (39.65 ± 10.88%, mean ± s.d). Urchin grazing also significantly (p < 0.001) influenced coral size (compared to non-grazing control), with colonies in the medium and high-densities growing the largest (21.13 ± 1.02 mm & 20.80 ± 0.82, mean ± s.e.m). Increased urchin density did however have a negative influence on urchin growth, a result of limited food availability.
Figure 1. Competitive benthic interactions causing juvenile coral mortality. (A) Peyssonnelia squamaria rapidly over grows juvenile coral; (B) filamentous algae encroaching on Acropora millepora; (C) cyanobacteria and diatom growth causing onset of tissue loss (<) in Acropora millepora; (D) sediment accumulation around the peripheral edge of a juvenile Acropora millepora; (E) Unidentified crustose coralline algae overgrowing Acropora hyacthinus primary polyps on 19/04/16; (F) 25/04/16; (G) 4/05/16; (H) 9/05/16. Scale 1 mm.
Figure 3. (A) Effects of Mespilia globulus grazing density on Acropora millepora spat survivorship at 180 days from linear regression models (LM); (B) A. millepora colony diameter from linear regression with GLS extension; (C) M. globulus basal diameter from LM at 180 days based. [Non-grazing control, low grazing density (four urchins = 16.67 m−2), medium grazing density (nine urchins = 37.50 m−2) and high grazing density (18 urchin = 75.00 m−2)].
Figure 4. Kaplan-Meier survival curves of newly settled Acropora millepora recruits exposed to differing levels of grazing pressures [non-grazing control (black solid line), low grazing density four juvenile Mespilia globulus (16.67 m−2) (green dotted line), medium grazing density nine juvenile M. globulus (37.50 m−2) (red dashed line), high grazing density 18 juvenile M. globulus (75.00 m−2) (blue dash dot line)] and grown over 180 days (mean ± s.e.m).
Figure 5. Kaplan-Meier survival showing effects of Mespilia globulus grazing density on Acropora millepora spat survivorship over 180 days. (A) Pairwise log-rank tests differences between treatment survival curves; (B) Result of the cox proportional hazard models illustrating exponent coefficient and significance of proportional risks of mortality between individuals across treatments. [Non-grazing control, low grazing density (four urchins = 16.67 m−2), medium grazing density (nine urchins = 37.50 m−2) and high grazing density (18 urchin = 75.00 m−2)].
Figure 6. Acropora millepora colonies at 180 days post settlement showing coral size comparison between (A) non-grazing control and (B) high urchin density (75.00 m−2). Scale = 5 mm.
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