Soft coral reproductive phenology along a depth gradient: Can “going deeper” provide a viable refuge?

Abstract

Many species across a wide range of taxa and habitats display phenological shifts and differences in response to both environmental gradients and climate change. Moreover, the wide-scale decline of numerous ecosystems is leading to increasing efforts to identify zones that might serve as natural refuges from various disturbances, including ocean warming. One such refuge was suggested to be that of the deep coral reefs, but whether depth can provide coral populations with a viable and reproductive refuge remains unclear.

Speciation across depth gradients in reef corals

“…5) along steep environmental gradients can arise from immigrant inviability 44,115 (i.e., heterozygous disadvantage) or strong habitat selection 116 promoting adaptive divergence between locally adapted phenotypes 117 . Similarly, differential temperature regimes across depths may influence the onset of the gametogenesis causing desynchronization of reproductive cycles leading to temporal reproductive isolation across depths 12,118 .…”

“…Segregation across depth between closely related lineages 8,9 is a particularly promising area of study for understanding the possible role of ecological speciation in light-dependent corals (i.e., with photosynthetic symbionts). Corals living at different depths are exposed to distinct light levels, which affects not only photosynthetic physiology but also many aspects of gamete release and larval settlement cued by light, such as small differences in the timing of spawning, namely a narrow and discrete temporal window (i.e., minutes or hours apart) between species 10 , both of which are well documented for some groups [11][12][13] . However, key remaining questions are: 1) whether genes displaying divergence between nascent depth-segregated species are tied to adaptation to different depths, 2) whether these same genes are linked to reproduction contributing to reproductive isolation, 3) whether natural selection has shaped the evolution of these genes, and 4) whether processes operating within species fuel divergence between species.…”

“…/www.ncbi.nlm.nih.gov/gene/?term=110042296 Fig.S5| Example of a cline fitting shape typical of a neutral random locus at intermediate allele frequency across linear distances and among sampling depths. Vertical dashed lines mark collection depth in meters(6,9,12,18,19).…”

A comparison of mesophotic and shallow sponge holobionts resilience to predicted future temperature elevation

“…Our results suggest that a temperature increase of up to 6°C for 35 days might not affect the survival of D. erythraeanus, which survived throughout the experiment under the different thermal treatments. However, the experiment was conducted during the autumn, when the ambient temperature was generally lower (mean 5 m: 23.4°C, 45 m: 23.2°C) than the maximum temperature that these sponges experience seasonally during the summer (max temp 5 m: 28.9°C, 45 m: 27.1°C, Shlesinger et al, 2018;Liberman et al, 2022). Therefore, although the moderate temperature treatment (~26°C) exceeded the typical autumn temperatures that these sponges encounter, it was not beyond the temperature range they are exposed to throughout the year.…”

Octocorals in the Gulf of Aqaba exhibit high photosymbiont fidelity

“…MCE community structure is affected by the exponential decrease of light across depth (Kahng et al, 2019) and by the corals' reliance on photoautotrophic carbon production via their endosymbionts (Rädecker et al, 2021). Most coral species at these depths are adapted to certain environmental conditions and can only thrive within a narrow depth range (i.e., "depth-specialists"; Liberman et al, 2022). A few corals, however, are able to thrive across a wider depth range (i.e., "depth-generalists").…”