The fertilisation and recruitment dynamics of scleractinian corals on South Africa's high-latitude reefs
Hart, Justin R.
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2 The production of coral offspring and their survival through early ontogeny to sexual maturity 3 are both vitally important for the persistence of coral-dominated reefs. Understanding factors 4 which affect these processes is important where limited connectivity occurs. This is the case 5 on the high-latitude Two-mile Reef (TMR) at Sodwana Bay in South Africa. A combination of 6 in situ and ex situ experimental work was conducted, investigating factors which affect the pre7 and post-settlement stages of corals. In vitro experiments conducted on two representative 8 scleractinian broadcast spawning corals, Acropora austera and Hydnophora exesa, revealed 9 that fertilisation success in both species diminishes significantly with a reduction in sperm 10 concentration and water salinity. Fertilisation success was highest for A. austera at 106 sperm 11 ml-1 (56.46% ± 0.83, mean ± SE), and at 105 sperm ml-1 (38.76% ± 1.29) for H. exesa. At 104 12 sperm ml-1 there was a significant reduction in fertilisation of 80% and 58% for the respective 13 species. Additionally, fertilisation success of A. austera and H. exesa decreased significantly 14 by 56% and 79% respectively, when salinity was reduced by 7.06 psu. Ex situ settlement 15 experiments were then conducted to assess the settlement of their larvae in response to the 16 presence of two crustose coralline algae (CCA) a Hydrolithon sp. and Mesophyllum sp. and 17 filtered sea water (FSW) control. Settlement in the presence of Mesophyllum was not 18 significantly different from FSW, but a significant trend was observed in the presence of 19 Hydrolithon, where settlement of both corals peaked. While H. exesa post-settlement success 20 was also greatest in the Hydrolithon treatment (55.00 ± 10.47%), this was not the case for A. 21 austera, which obtained highest post-settlement success in the Mesophyllum treatment (21.67 22 ± 7.23%). However, these trends were not significant. Acropora austera and H. exesa were 23 still capable of settlement 69 and 75 days after fertilisation respectively in FSW. In general, 24 more settlement occurred on container surfaces than on CCA fragments. Results from this 25 study suggest that the inducing effect of CCA is coral and CCA taxon-specific, and that A. 26 austera planulae are more stringent in their settlement requirements. 27 28 In situ experiments were conducted by attaching settlement tiles to concrete Y-frames on TMR 29 for six months to assess how coral recruitment differs according to method of settlement tile 30 attachment onto concrete y-frames and how the exclusion of herbivores and predators affects 31 coral recruitment onto tiles. A new, grooved settlement tile was designed to provide refuge microhabitats on the top surface of the tiles. In total, 32 579 recruits were detected on the 33 settlement tiles, with pocilloporids dominating the recruit composition (64%). 34 35 Grooved tiles were also used to assess whether coral recruit density varied between different 36 microhabitats adjacent to the tile edge (a narrow, 5 mm gap; a wide, 15 mm gap; and tiles raised 37 above the gap). Most recruitment occurred on the vertical edges and towards the edge perimeter 38 of grooved tiles regardless of treatment. The majority of recruitment on the top surface of tiles 39 occurred in the grooves (74.17%). Coral recruit densities differed significantly between the 40 three edge microhabitats, with recruit density significantly less on tiles adjacent to narrow gaps. 41 Raised tiles and tiles with a wide gap had two- and three-fold more recruits (644.33 ± 149.43 42 and 979.29 ± 170.88 recruits m-2) than tiles with a narrow gap (311.05 ± 80.82 recruits m-2). 43 This suggests that the microhabitat associated with the method of tile attachment can have a 44 significant effect on recruitment. Finally, the effect of large herbivores and predators on coral 45 recruitment and the benthic communities was assessed by placing exclusion cages over tiles. 46 Recruit densities had a two-fold reduction when herbivores and predators were excluded. 47 Additionally, CCA cover was also significantly reduced on caged tiles, and the percentage of 48 erect foliose algae, encrusting macroalgae, and turf algae was significantly greater compared 49 to uncaged tiles. This indicates that grazers may assist coral recruitment on TMR. 50 51 This study provided the first assessment of fertilisation success in corals at high-latitude in 52 South Africa and the results are related to information on gene flow and reef resilience. The 53 importance of suitable settlement microhabitats and grazers are also highlighted and stress the 54 need for a multi-faceted management approach to coral conservation. Furthermore, from an 55 experimental point of view, the methodological techniques used to quantify in situ recruitment, 56 such as settlement surface design and attachment technique, may have important implications 57 in quantifying recruit densities and settlement preferences. Such differences must be 58 considered when comparing the results of recruit densities in studies using dissimilar 59 techniques. 60