Depth related parameters, specifically light, affect different aspects of corals physiology, including fluorescence. Green fluorescence protein (GFP)-like pigments found in many coral species have been suggested to serve a variety of functions, including photo-protection and photo-enhancement. Using fluorescence imaging and molecular analysis, we further investigated the role of these proteins on the physiology of the coral Stylophora pistillata and its algal partners. Fluorescence was found to differ significantly between depths for larvae and adult colonies. Larvae from the shallow reef presented a higher GFP expression and a greater fluorescence intensity compared to the larvae from the mesophotic reef, reflecting the elevated need for photo-protection against high light levels characteristic of the shallow reef, thus supporting the "sunscreen" hypothesis. Additionally, given the lower but still occurring protein expression under non-damaging low light conditions, our results suggest that GFP-like proteins might act to regulate the amount of photosynthetically usable light for the benefit of the symbiotic algae. Moreover, we propose that the differences in GFP expression and green fluorescence between shallow and deep larvae indicate that the GFPs within coral larvae might serve to attract and retain different symbiont clades, increasing the chances of survival when encountering new environments.
As the devastating impacts of global climate change and local anthropogenic stressors on shallow-water coral reefs are expected to rise, mesophotic coral ecosystems have increasingly been regarded as potential lifeboats for coral survival, providing a source of propagules to replenish shallower reefs. Yet, there is still limited knowledge of the capacity for coral larvae to adjust to light intensities that change with depth. This study elucidates the mechanisms underlying plasticity during early life stages of the coral Porites astreoides that enable survival across broad depth gradients. We examined physiological and morphological variations in larvae from shallow (8–10 m) and mesophotic (45 m) reefs in Bermuda, and evaluated differences in survival, settlement patterns and size among recruits depending on light conditions using a reciprocal ex situ transplantation experiment. Larvae released from mesophotic adults were found to have significantly lower respiration rates and were significantly larger than those derived from shallow adults, indicating higher content of energetic resources and suggesting a greater dispersal potential for mesophotic larvae compared to their shallow counterparts. Additionally, larvae released from mesophotic adults experienced higher settlement success and larger initial spat size compared to larvae from shallow adults, demonstrating a potential connection between parental origin, offspring quality, and recruitment success. Although both shallow and mesophotic larvae exhibited the capacity to survive and settle under reciprocal light conditions, all larvae had higher survival under mesophotic light conditions regardless of parental origin, suggesting that conditions experienced under low light may enable longer larval life, further extending the dispersal period. These results indicate that larvae from mesophotic Porites astreoides colonies are likely capable of reseeding shallow reefs in Bermuda, thereby supporting the Deep Reef Refugia Hypothesis.
Mesophotic reefs are found in water depths greater than 30m and as such are, for the most part, removed from most direct wave activity, notably in the Mediterranean Sea. There, they are protected from anthropogenic impact and serve as a possible refuge for marine biodiversity in a warming ocean. Yet, like any reef, their growth is modulated by the distribution of the sediment they produce and supply of nutrients. Here we explore the mechanisms governing the sediment distribution around a coralligenous mesophotic reef system as a case study of the long-term governing mechanisms in this environment. A detailed survey of a mesophotic reef offshore Israel was carried out using acoustic imaging from an autonomous underwater vehicle. This survey generated a highly detailed (cm-scale) resolution dataset with wide area coverage of the sediment distribution patterns around the reef. The mesophotic reefs are built as a series of asymmetric ridges with biogenic sediment filling the depressions between them. Two types of sediment bedforms were identified in the sediment fill - elongated linear bedforms aligned north-south; and intersecting smaller bedforms aligned NW-SE and NE-SW, forming chevron-like shapes. Both types of bedforms are composed of coarse-grained material (sand to gravel) and their distribution is partially related to their location relative to the hard substrate. The bedform present and the ridge asymmetry points to a prominent role of a south-to-north bottom current. These current supplies nutrients to the reefs, but also loads sediment on their southern side, leading to asymmetry. Additionally, the chevron bedforms are attributed to interactions with the rocky substrate and internal waves. These internal waves likely also play a role in modulating the reefs’ growth. The geometries and growth of mesophotic reefs, and the distribution of the sediment around them, are controlled by the combination of both internal waves and bottom currents.
Bioinvasions have the potential to provoke cascade effects that can disrupt natural ecosystems, and cause ecological regime shifts. The Mediterranean Sea is particularly prone to bioinvasions as the changing water conditions, evoked by climate change, are creating advantageous conditions for Lessepsian migrants from the Red Sea. Recently, in May 2023, a new alien species was documented in the Mediterranean Sea - a soft coral of the genus Dendronephthya. This discovery was made by divers conducting Long Term Ecological Research surveys, along the coast of Israel, at a depth of 42 m. Genetic and morphological testing were utilized to confirm the species identity as Dendronepthya hemprichi, an Indo-Pacific coral, common in the Red Sea. According to life history traits of this species such as accelerated attachment to available surfaces and fast growth, we expect it to rapidly expand its distribution and abundance across the Mediterranean.
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