Abstract
Coral reefs worldwide are facing increasing stress due to drastic changes in their environment. Mesophotic coral ecosystems (MCEs) have been considered as a potential refuge from several major stressors, such as warm-water bleaching events. However, their role as a subsequent source of larvae remains unclear for many species, particularly as genetic differentiation of corals over depth has frequently been observed. In 1998 and 2001, two severe bleaching events around Okinawa Island in Japan resulted in major changes to the shallow reefs, including the local “extinction” of species such as Seriatopora hystrix at Sesoko Island. However, recently this species was found to be present in abundance at mesophotic depths in the area, despite no clear signs of recovery being observed in the adjacent shallow waters. Here, we assessed the genetic diversity of Seriatopora from this deep population and provide a comparison with populations from shallow to mesophotic depths in other parts of the Ryukyu archipelago, to understand their depth specificity and their importance in genetic diversity conservation of affected shallow populations. High levels of genetic diversity were observed in both shallow and mesophotic Seriatopora populations for both the nuclear (internal transcribed spacer 2, ITS2) and the mitochondrial (hypervariable open reading frame, ORF) marker, with no clear partitioning of haplotypes over depth or across locations in the archipelago. Both ITS2 and ORF suggest the presence of potential cryptic species and the discrepancy between the markers could reflect hybridization or incomplete lineage sorting. Although associated endosymbionts (Symbiodinium spp.) all shared the same mitochondrial haplotype (cytochrome oxidase subunit 1, COI), the nuclear ribosomal ITS2 revealed slight potential habitat partitioning between the genotypes, with a small decrease of C59 Symbiodinium types below 10 m depth and a mirrored increase in C1/C78a-related types. The relative absence of depth-specific host lineages and the substantial overlap between shallow and deep Symbiodinium types indicate that Okinawan MCEs may act as a refuge preserving genotypic diversity of bleaching-sensitive coral such as Seriatopora, and on the long-term may have the potential to contribute to shallow-water recolonization of this species.
Highlights
Over the last decades, major concerns have arisen from the increasing amount of stress sustained by coral reefs because of climate change and other human impacts
The latter has gained much attention as it could provide a “local refuge” adjacent to threatened shallow communities, given that many coral reef systems extend across a wide depth range. These deeper sections of coral reefs have remained poorly studied, recent advancements in underwater technologies are facilitating access to these otherwise logistically challenging environments (Kahng et al, 2010, 2014). This has sparked a recent interest and led to a definition of Mesophotic Coral Ecosystems (MCEs) as coral reef habitat occurring at depths greater than 30 m (Hinderstein et al, 2010)
Based on the original hypothesis that deeper reefs could act as a thermal refuge (Glynn, 1996; Riegl and Piller, 2003), the Deep Reef Refuge Hypothesis (DRRH) was extended to include other disturbances and the subsequent potential of deep reef habitat to act as source of larvae for shallow reef areas post-disturbance (Bongaerts et al, 2010a)
Summary
Major concerns have arisen from the increasing amount of stress sustained by coral reefs because of climate change and other human impacts (reviewed by HoeghGuldberg et al, 2007; Fabricius, 2011). The occurrence of mass-bleaching events driven by elevated temperatures and high solar irradiance has led to a dramatic decrease of coral coverage worldwide (Hoegh-Guldberg, 1999) In this context, certain reef environments have been identified that have the potential to act as a thermal refuge, such as areas of upwelling, offshore banks and areas at moderate depth (Glynn, 1996; Riegl and Piller, 2003). Certain reef environments have been identified that have the potential to act as a thermal refuge, such as areas of upwelling, offshore banks and areas at moderate depth (Glynn, 1996; Riegl and Piller, 2003) The latter has gained much attention as it could provide a “local refuge” adjacent to threatened shallow communities, given that many coral reef systems (e.g., fringing, barrier and atolls) extend across a wide depth range. A recent study demonstrated that the potential for vertical connectivity can differ greatly between species within a single reef location indicating that reseeding potential may be relevant for individual species rather than representing a broader ecosystem-wide phenomenon (Bongaerts et al, 2017)
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