Kristen L. Marhaver

Reversing coral reef decline requires reducing environmental threats while actively restoring reef ecological structure and func-tion. A promising restoration approach uses coral breeding to boost natural recruitment and repopulate reefs with geneticallydiverse coral communities. Recent advances in predicting spawning, capturing spawn, culturing larvae, and rearing settlers haveenabled the successful propagation, settlement, and outplanting of coral offspring in all of the world’s major reef regions. Never-theless, breeding efforts frequently yield low survival, reflecting the type III survivorship curve of corals and poor condition ofmost reefs targeted for restoration. Furthermore, coral breeding programs are still limited in spatial scale and species diversity.Here, we highlight four priority areas for research and cooperative innovation to increase the effectiveness and scale of coralbreeding in restoration: (1) expanding the number of restoration sites and species, (2) improving broodstock selection to maximizethe genetic diversity and adaptive capacity of restored populations, (3) enhancing culture conditions to improve offspring healthbefore and after outplanting, and (4) scaling up infrastructure and technologies for large-scale coral breeding and restoration. Pri-oritizing efforts in these four areas will enable practitioners to address reef decline at relevant ecological scales, re-establish self-sustaining coral populations, and ensure the long-term success of restoration interventions. Overall, we aim to guide the coral res-toration community toward actions and opportunities that can yield rapid technical advances in larval rearing and coral breeding,foster interdisciplinary collaborations, and ultimately achieve the ecological restoration of coral reefs.

Larval settlement in wave-dominated, nearshore environments is the most critical life stage for a vast array of marine invertebrates, yet it is poorly understood and virtually impossible to observe in situ. Using a custom-built flume tank that mimics the oscillatory fluid flow over a shallow coral reef, we show that millimeter-scale benthic topography increases the settlement of slow-swimming coral larvae by an order of magnitude relative to flat substrates. Particle tracking velocimetry of flow fields revealed that millimeter-scale ridges introduced regions of flow recirculation that redirected larvae toward the substrate surface and decreased the local fluid speed, effectively increasing the window of time for larvae to settle. In agreement with experiments, computational fluid dynamics modeling and agent-based larval simulations also showed significantly higher settlement on ridged substrates. These findings highlight how physics-based substrate design can create new opportunities to increase larval recruitment for ecosystem restoration.

ABSTRACT: The widespread loss of stony reef-building coral populations has been compounded by the low settlement and survival of coral juveniles. To rebuild coral communities, restoration practitioners have developed workflows to settle vulnerable coral larvae in the laboratory and outplant settled juveniles back to natural and artificial reefs. These workflows often make use of natural biochemical settlement cues, which are presented to swimming larvae to induce settlement. This paper establishes the potential for inorganic cues to complement these known biochemical effects. Settlement substrates were fabricated from calcium carbonate, a material present naturally on reefs, and modified with additives including sands, glasses, and alkaline earth carbonates. Experiments with larvae of two Caribbean coral species revealed additive-specific settlement preferences that were independent of bulk surface properties such as mean roughness and wettability. Instead, analyses of the substrates suggest that settling coral larvae can detect localized topographical features more than an order of magnitude smaller than their body width and can sense and positively respond to soluble inorganic minerals such as silica (SiO2) and strontianite (SrCO3). These findings open a new area of research in coral reef restoration, in which composite substrates can be designed with a combination of natural organic and inorganic additives to increase larval settlement and perhaps also improve post-settlement growth, mineralization, and defense.

Assisted gene flow (AGF) is a conservation intervention to accelerate
species adaptation to climate change by importing genetic
diversity into at-risk populations. Corals exemplify both the need for
AGF and its technical challenges; corals have declined in abundance,
suffered pervasive reproductive failures, and struggled to adapt to
climate change, yet mature corals cannot be easily moved for breeding,
and coral gametes lose viability within hours. Here, we report
the successful demonstration of AGF in corals using cryopreserved
sperm that was frozen for 2 to 10 y. We fertilized Acropora palmata
eggs from the western Caribbean (Curaçao) with cryopreserved
sperm from genetically distinct populations in the eastern and central
Caribbean (Florida and Puerto Rico, respectively). We then confirmed
interpopulation parentage in the Curaçao–Florida offspring
using 19,696 single-nucleotide polymorphism markers. Thus, we
provide evidence of reproductive compatibility of a Caribbean coral
across a recognized barrier to gene flow. The 6-mo survival of AGF
offspring was 42%, the highest ever achieved in this species, yielding
the largest wildlife population ever raised from cryopreserved
material. By breeding a critically endangered coral across its range
withoutmoving adults, we show that AGF using cryopreservation is
a viable conservation tool to increase genetic diversity in threatened
marine populations.