Reproduction, abundance and survivorship of two Alveopora spp. in the mesophotic reefs of Eilat, Red Sea

A molecular census of early-life stage scleractinian corals in shallow and mesophotic zones

The decline of coral reefs has fueled interest in determining whether mesophotic reefs can shield against disturbances and help replenish deteriorated shallower reefs. In this study, we characterized spatial (horizontal and vertical) and seasonal patterns of diversity in coral recruits from Dabaisha and Guiwan reefs at Ludao, Taiwan. Concrete blocks supporting terra-cotta tiles were placed at shallow (15m) and mesophotic (40m) depths, during 2016-2018. Half of the tiles were retrieved and replaced biannually over three 6-month surveys (short-term); the remainder retrieved at the end of the 18-month (long-term) survey. 451 recruits were located using fluorescent censusing and identified by DNA barcoding. Barcoding the mitochondrial cytochrome oxidase I (COI) gene resulted in 17 molecular operational taxonomic units (MOTUs). To obtain taxonomic resolution to the generic level, Pocillopora were phylotyped using the mitochondrial open reading frame (ORF), resolving eight MOTUs. Acropora, Isopora, and Montipora recruits were identified by the nuclear PaxC intron, yielding ten MOTUs. Overall, 35 MOTUs were generated and were comprised primarily of Pocillopora and, in fewer numbers, Acropora, Isopora, Pavona, Montipora, Stylophora, among others. 40% of MOTUs recruited solely within mesophotic reefs while 20% were shared by both depth zones. MOTUs recruiting across a broad depth distribution appear consistent with the hypothesis of mesophotic reefs acting as a refuge for shallow-water coral reefs. In contrast, Acropora and Isopora MOTUs were structured across depth zones representing an exception to this hypothesis. This research provides an imperative assessment of coral recruitment in understudied mesophotic reefs and imparts insight into the refuge hypothesis.

The Algal Symbiont Modifies the Transcriptome of the Scleractinian Coral Euphyllia paradivisa During Heat Stress

The profound mutualistic symbiosis between corals and their endosymbiotic counterparts, Symbiodiniaceae algae, has been threatened by the increase in seawater temperatures, leading to breakdown of the symbiotic relationship-coral bleaching. To characterize the heat-stress response of the holobiont, we generated vital apo-symbiotic Euphyllia paradivisa corals that lacked the endosymbiotic algae. Using RNA sequencing, we analyzed the gene expression of these apo-symbionts vs. symbiotic ones, to test the effect of the algal presence on the tolerance of the coral. We utilized literature-derived lists of "symbiosis differentially expressed genes" and "coral heat-stress genes" in order to compare between the treatments. The symbiotic and apo-symbiotic samples were segregated into two separate groups with several different enriched gene ontologies. Our findings suggest that the presence of endosymbionts has a greater negative impact on the host than the environmental temperature conditions experienced by the holobiont. The peak of the stress reaction was identified as 28 °C, with the highest number of differentially expressed genes. We suggest that the algal symbionts increase coral holobiont susceptibility to elevated temperatures. Currently, we can only speculate whether coral species, such as E. paradivisa, with the plasticity to also flourish as apo-symbionts, may have a greater chance to withstand the upcoming global climate change challenge.

Photoacclimation and induction of light-enhanced calcification in the mesophotic coral Euphyllia paradivisa

Corals and their photosymbionts experience inherent changes in light along depth gradients, leading them to have evolved several well-investigated photoacclimation strategies. As coral calcification is influenced by light (a process described as LEC-'light-enhanced calcification'), studies have sought to determine the link between photosynthesis and calcification, but many puzzling aspects still persist. Here, we examine the physiology of Euphyllia paradivisa, a coral species found at a wide range of depths but that is strictly mesophotic in the Red Sea; and also examines the coupling between photosynthesis and LEC by investigating the response of the coral under several controlled light regimes during a long-term experiment. E. paradivisa specimens were collected from 40 to 50 m depth and incubated under three light conditions for a period of 1 year: full-spectrum shallow-water light (approx. 3 m, e.g. shallow-light treatment); blue deep-water light (approx. 40 m, e.g. mesophotic-light treatment) or total darkness (e.g. dark treatment). Net photosynthesis remained similar in the shallow-light-treated corals compared to the mesophotic-light-treated corals, under both low and high light. However, calcification increased dramatically with increasing light intensity in the shallow-light-treated corals, suggesting a decoupling between these processes. Photoacclimation to shallow-water conditions was indicated by enhanced respiration, a higher density of zooxanthellae per polyp and lower chlorophyll a content per cell. The dark-treated corals became completely bleached but did not lower their metabolism below that of the mesophotic-light-treated corals. No Symbiodinium clade shift was found following the year-long light treatments. We conclude that E. paradivisa, and its original symbiont clade, can adapt to various light conditions by controlling its metabolic rate and growth energy investment, and consequently induce LEC.

Biodiversity and Functional Ecology of Mesophotic Coral Reefs

Mesophotic coral reefs, currently defined as deep reefs between 30 and 150 m, are linked physically and biologically to their shallow water counterparts, have the potential to be refuges for shallow coral reef taxa such as coral and sponges, and might be a source of larvae that could contribute to the resiliency of shallow water reefs. Mesophotic coral reefs are found worldwide, but most are undescribed and understudied. Here, we review our current knowledge of mesophotic coral reefs and their functional ecology as it relates to their geomorphology, changes in the abiotic environment along depth gradients, trophic ecology, their reproduction, and their connectivity to shallow depths. Understanding the ecology of mesophotic coral reefs, and the connectivity between them and their shallow water counterparts, is now a primary focus for many reef studies as the worldwide degradation of shallow coral reefs, and the ecosystem services they provide, continues unabated.

Reproduction, energy storage and metabolic requirements in a mesophotic population of the gorgonian Paramuricea macrospina

This study examined the sexual reproductive cycle, energy storage and metabolic requirements of a Mediterranean gorgonian in a mesophotic ecosystem (~70 m depth). Paramuricea macrospina resulted to be a gonochoric internal brooding species with a 1:1 population sex ratio. Oogenesis lasted ~12–14 months, whereas spermatogenesis was significantly shorter, only lasting 6 months. Fertilization occurred during late summer (August) and larval release occurred during autumn (September–October). The organic matter and total lipid content showed a slight seasonal variability. Stable isotopic composition remained constant throughout the year, reflecting a general stability in gorgonian food sources. Conversely, the free fatty acid composition varied seasonally, reflecting changes in P. macrospina energetic demands probably related to gametogenesis and larval brooding. The reproductive ecology and biochemical composition of P. macrospina significantly differ from shallow coastal gorgonian species, reflecting the higher environmental stability of deeper environments.

Can mesophotic reefs replenish shallow reefs? Reduced coral reproductive performance casts a doubt

Mesophotic coral ecosystems (i.e., deep coral reefs at 30-120 m depth) appear to be thriving while many shallow reefs in the world are declining. Amid efforts to understand and manage their decline, it was suggested that mesophotic reefs might serve as natural refuges and a possible source of propagules for the shallow reefs. However, our knowledge of how reproductive performance of corals alters with depth is sparse. Here, we present a comprehensive study of the reproductive phenology, fecundity, and abundance of seven reef-building conspecific corals in shallow and mesophotic habitats. Significant differences were found in the synchrony and timing of gametogenesis and spawning between shallow and mesophotic coral populations. Thus, mesophotic populations exhibited delayed or protracted spawning events, which led to spawning of the mesophotic colonies in large proportions at times where the shallow ones had long been depleted of reproductive material. All species investigated demonstrated a substantial reduction in fecundity and/or oocyte sizes at mesophotic depths (40-60 m). Two species (Seriatopora hystrix and Galaxea fascicularis) displayed a reduction in both fecundity and oocyte size at mesophotic depths. Turbinaria reniformis had only reduced fecundity and Acropora squarrosa and Acropora valida only reduced oocyte size. In Montipora verrucosa, reduced fecundity was found during one annual reproductive season while, in the following year, only reduced oocyte size was found. In contrast, reduced oocyte size in mesophotic populations of Acropora squarrosa was consistent along three studied years. One species, Acropora pharaonis, was found to be infertile at mesophotic depths along two studied years. This indicates that reproductive performance decreases with depth; and that although some species are capable of reproducing at mesophotic depths, their contribution to the replenishment of shallow reefs may be inconsequential. Reduced reproductive performance with depth, combined with the possible narrower tolerance to environmental factors, further suggests that mesophotic corals may in fact be more vulnerable than previously conceived. Furthermore, we posit that the observed temporal segregation in reproduction could lead to assortative mating, and this, in turn, may facilitate adaptive divergence across depth.

Reproductive biology of the deep brooding coral Seriatopora hystrix: Implications for shallow reef recovery

Mesophotic coral ecosystems (MCEs, between 30 and 150 m depth) are hypothesized to contribute to the recovery of degraded shallow reefs through sexually produced larvae (referred to as Deep Reef Refuge Hypothesis). In Okinawa, Japan, the brooder coral Seriatopora hystrix was reported to be locally extinct in a shallow reef while it was found abundant at a MCE nearby. In this context, S. hystrix represents a key model to test the Deep Reef Refuge Hypothesis and to understand the potential contribution of mesophotic corals to shallow coral reef recovery. However, the reproductive biology of mesophotic S. hystrix and its potential to recolonize shallow reefs is currently unknown. This study reports for the first time, different temporal scales of reproductive periodicity and larval settlement of S. hystrix from an upper mesophotic reef (40 m depth) in Okinawa. We examined reproductive seasonality, lunar, and circadian periodicity (based on polyp dissection, histology, and ex situ planula release observations) and larval settlement rates in the laboratory. Mesophotic S. hystrix reproduced mainly in July and early August, with a small number of planulae being released at the end of May, June and August. Compared to shallow colonies in the same region, mesophotic S. hystrix has a 4-month shorter reproductive season, similar circadian periodicity, and smaller planula size. In addition, most of the planulae settled rapidly, limiting larval dispersal potential. The shorter reproductive season and smaller planula size may result from limited energy available for reproduction at deeper depths, while the similar circadian periodicity suggests that this reproductive aspect is not affected by environmental conditions differing with depth. Overall, contribution of mesophotic S. hystrix to shallow reef rapid recovery appears limited, although they may recruit to shallow reefs through a multistep process over a few generations or through random extreme mixing such as typhoons.