Abstract
The persistence of natural metapopulations may depend on subpopulations that exist at the edges of species ranges, removed from anthropogenic stress. Mesophotic coral ecosystems (30–150 m) are buffered from disturbance by depth and distance, and are potentially massive reservoirs of coral diversity and fecundity; yet we know little about the reproductive capabilities of their constituent species and the potential for these marginal environments to influence patterns of coral reef persistence. We investigated the reproductive performance of the threatened depth-generalist coral Orbicella faveolata over the extent of its vertical range to assess mesophotic contributions to regional larval pools. Over equal habitat area, mesophotic coral populations were found to produce over an order of magnitude more eggs than nearby shallow populations. Positive changes with depth in both population abundance and polyp fecundity contributed to this discrepancy. Relative larval pool contributions of deeper living corals will likely increase as shallow habitats further degrade due to climate change and local habitat degradation. This is a compelling example of the potential for marginal habitat to be critical to metapopulation persistence as reproductive refugia.
Highlights
Vertical habitat ranges, in the ocean where species distributions may be bounded by the sea surface, and resources such as light are maximum at this boundary
Shallow coral reefs are at escalating risk of habitat degradation[16,17,18], yet mesophotic coral ecosystems (MCEs) between 30 and 150 meters depth are buffered from many stressors, coral thermal bleaching[19,20,21]
The high abundance of depth generalist coral species in MCE reefs combined with the observation that the extent of known MCEs is large[14] and in some areas may surpass the total area of shallow water reefs, could suggest high reproductive output despite potential light limitations
Summary
Vertical habitat ranges, in the ocean where species distributions may be bounded by the sea surface, and resources such as light are maximum at this boundary. Shallow coral reefs are at escalating risk of habitat degradation[16,17,18], yet mesophotic coral ecosystems (MCEs) between 30 and 150 meters depth are buffered from many stressors, coral thermal bleaching[19,20,21] This observation, and the considerable overlap of scleractinian coral species between MCEs (30–50 m) and shallow reefs (< 30 m) in the Caribbean (~70%;14), has given rise to the Deep Reef Refugia Hypothesis (DRRH)[22,23], that posits a depth refugia for shallow water coral species and a stable pool of reproductively capable corals that could assist in shallow water recovery, increasing resilience after disturbance. We reaffirm the potential importance of mesophotic coral reefs as larval resources in regional metapopulations
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