Coral-reef ecosystems are experiencing frequent and severe disturbance events that are reducing global coral abundance and potentially overwhelming the natural capacity for reefs to recover. While mitigation strategies for climate warming and other anthropogenic disturbances are implemented, coral restoration programmes are being established worldwide as an additional conservation measure to minimise coral loss and enhance coral recovery. Current restoration efforts predominantly rely on asexually produced coral fragments—a process with inherent practical constraints on the genetic diversity conserved and the spatial scale achieved. Because the resilience of coral communities has hitherto relied on regular renewal with natural recruits, the scaling-up of restoration programmes would benefit from greater use of sexually produced corals, which is an approach that is gaining momentum. Here we review the present state of knowledge of scleractinian coral sexual reproduction in the context of reef restoration, with a focus on broadcast-spawning corals. We identify key knowledge gaps and bottlenecks that currently constrain the sexual production of corals and consider the feasibility of using sexually produced corals for scaling-up restoration to the reef- and reef-system scales.

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Abstract. Quantifying the saturation state of aragonite ( Ar ) within the calcifying fluid of corals is critical for understanding their biomineralization process and sensitivity to environmental changes including ocean acidification. Recent advances in microscopy, microprobes, and isotope geochemistry enable the determination of calcifying fluid pH and [CO ]/K sp ) has proved elusive. Here we test a new technique for deriving Ar based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of Ar from 10 to 34, and we found a strong dependence of Raman peak width on Ar with no significant effects of other factors including pH, Mg/Ca partitioning, and temperature. Validation of our Raman technique for corals is difficult because there are presently no direct measurements of calcifying fluid Ar available for comparison. However, Raman analysis of the international coral standard JCp-1 produced Ar of 12.3 ± 0.3, which we demonstrate is consistent with published skeletal Mg/Ca, Sr/Ca, B/Ca, δ 11 B, and δ 44 Ca data. Raman measurements are rapid (≤ 1 s), high-resolution (≤ 1 µm), precise (derived Ar ± 1 to 2 per spectrum depending on instrument configuration), accurate ( ±2 if Ar < 20), and require minimal sample preparation, making the technique well suited for testing the sensitivity of coral calcifying fluid Ar to ocean acidification and warming using samples from natural and laboratory settings. To demonstrate this, we also show a high-resolution time series of Ar over multiple years of growth in a Porites skeleton from the Great Barrier Reef, and we evaluate the response of Ar in juvenile Acropora cultured under elevated CO 2 and temperature.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Quantifying the saturation state of aragonite (&amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt;) within the calcifying fluid of corals is critical for understanding their biomineralisation process and sensitivity to environmental changes including ocean acidification. Recent advances in microscopy, microprobes, and isotope geochemistry allow determination of calcifying fluid pH and [CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2&amp;#8722;&lt;/sup&gt;], but direct quantification of &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; (where &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; =[CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2&amp;#8722;&lt;/sup&gt;][Ca&lt;sup&gt;2+&lt;/sup&gt;]/&lt;i&gt;K&lt;sub&gt;sp&lt;/sub&gt;&lt;/i&gt;) has proved elusive. Here we test a new technique for deriving &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; from 10 to 34, and found a strong dependence of Raman peak width on &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; that was independent of other factors including pH, Mg/Ca partitioning, and temperature. Validation of our Raman technique for corals is difficult because there are presently no direct measurements of calcifying fluid &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; available for comparison. However, Raman analysis of the international coral standard JCp-1 produced &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; of 12.3&amp;#8201;&amp;#177;&amp;#8201;0.3, which we demonstrate is consistent with published skeletal Sr/Ca, Mg/Ca, B/Ca, &lt;i&gt;&amp;#948;&lt;/i&gt;&lt;sup&gt;44&lt;/sup&gt;Ca, and &lt;i&gt;&amp;#948;&lt;/i&gt;&lt;sup&gt;11&lt;/sup&gt;B data. Raman measurements are rapid (&amp;#8804;&amp;#8201;1&amp;#8201;s), high-resolution (&lt;&amp;#8201;1&amp;#8201;&amp;#956;m), precise (derived &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; &amp;#177;1 to 2), and require minimal sample preparation; making the technique well suited for testing the sensitivity of coral calcifying fluid &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; to ocean acidification and warming using samples from natural and laboratory settings. To demonstrate this, we also show a high-resolution time series of &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; over multiple years of growth in a &lt;i&gt;Porites&lt;/i&gt; skeleton from the Great Barrier Reef, and we evaluate the response of &amp;#937;&lt;sub&gt;&lt;i&gt;Ar&lt;/i&gt;&lt;/sub&gt; in juvenile Acropora cultured under elevated CO&lt;sub&gt;2&lt;/sub&gt; and temperature.&lt;/p&gt;