Deep Heat: A Comparison of Water Temperature, Anemone Bleaching, Anemonefish Density and Reproduction between Shallow and Mesophotic Reefs

French Polynesia is experiencing increasing coral bleaching events in shallow waters triggered by thermal anomalies and marine heatwaves linked to climate change, a trend that is replicated worldwide. As sea surface thermal anomalies are assumed to lessen with depth, mesophotic deep reefs have been hypothesized to act as refuges from anthropogenic and natural disturbances, the ‘deep reef refugia hypothesis’ (DRRH). However, evidence supporting the DRRH is either inconclusive or conflicting. We address this by investigating four assumptions of the DRRH focusing on the symbiotic association between anemones and anemonefish. First, we compare long-term temperature conditions between shallow (8 m) and mesophotic sites (50 m) on the island of Moorea from 2011–2020. Second, we compare the densities of the orange-fin anemonefish, Amphiprion chrysopterus between shallow and mesophotic (down to 60 m) reefs across three archipelagos in French Polynesia. Finally, we compare the percentage of anemone bleaching, as well as anemonefish reproduction, between shallow and mesophotic reefs. We found that the water column was well mixed in the cooler austral winter months with only a 0.19 °C difference in temperature between depths, but in the warmer summer months mixing was reduced resulting in a 0.71–1.03 °C temperature difference. However, during thermal anomalies, despite a time lag in warm surface waters reaching mesophotic reefs, there was ultimately a 1.0 °C increase in water temperature at both 8 and 50 m, pushing temperatures over bleaching thresholds at both depths. As such, anemone bleaching was observed in mesophotic reefs during these thermal anomalies, but was buffered compared to the percentage of bleaching in shallower waters, which was nearly five times greater. Our large-scale sampling across French Polynesia found orange-fin anemonefish, A. chrysopterus, in mesophotic zones in two high islands and one atoll across two archipelagos, extending its bathymetric limit to 60 m; however, orange-fin anemonefish densities were either similar to, or 25–92 times lower than in shallower zones. Three spawning events were observed at 50 m, which occurred at a similar frequency to spawning on shallower reefs at the same date. Our findings of thermal anomalies and bleaching in mesophotic reefs, coupled with mainly lower densities of anemonefish in mesophotic populations, suggest that mesophotic reefs show only a limited ability to provide refugia from anthropogenic and natural disturbances.

Depth moderates loss of marine foundation species after an extreme marine heatwave: could deep temperate reefs act as a refuge?

Marine heatwaves (MHWs) have been documented around the world, causing widespread mortality of numerous benthic species on shallow reefs (less than 15 m depth). Deeper habitats are hypothesized to be a potential refuge from environmental extremes, though we have little understanding of the response of deeper benthic communities to MHWs. Here, we show how increasing depth moderates the response of seaweed- and coral-dominated benthic communities to an extreme MHW across a subtropical–temperate biogeographical transition zone. Benthic community composition and key habitat-building species were characterized across three depths (15, 25 and 40 m) before and several times after the 2011 Western Australian MHW to assess resistance during and recovery after the heatwave. We found high natural variability in benthic community composition along the biogeographic transition zone and across depths with a clear shift in the composition after the MHW in shallow (15 m) sites but a lot less in deeper communities (40 m). Most importantly, key habitat-building seaweeds such as Ecklonia radiata and Syctothalia dorycarpa which had catastrophic losses on shallow reefs, remained and were less affected in deeper communities. Evidently, deep reefs have the potential to act as a refuge during MHWs for the foundation species of shallow reefs in this region.

Octopods of deep reefs off Curaçao, southern Caribbean including description of one newly discovered species

Recent exploratory research on poorly studied deep reefs in the Caribbean Sea has yielded substantial new information about the diversity of deep-reef fishes and some invertebrates, but the diversity of deep-reef cephalopods has not been assessed. Using images, videos, and specimens collected with the aid of a manned submersible, as well as DNA sequences derived from the specimens, we surveyed the cephalopod diversity of the mesophotic and rariphotic reef communities in Curaçao, southern Caribbean. Among the 50 records comprising 15 specimens, 39 photos, and 15 videos (of which six specimens matched video records), we found Octopus hummelincki Adam, 1937, Pteroctopus cf. tetracirrhus Delle Chiaje, 1830, Scaeurgus unicirrhus Delle Chiaje, 1841, Paroctopus mercatoris Adam, 1937, and Lepidoctopus joaquini Haimovici and Sales, 2019. In addition, we found one group of octopods, with two specimens that did not correspond to any known species. We describe this as a new species. One specimen of Lepidoctopus joaquini was hectocotylized on two arms.

A new species of Lipogramma from deep reefs of Roatan, Honduras (Teleostei, Grammatidae)

A new species of Lipogramma is described from submersible collections at 122–165 m depth off the coast of Roatan, Honduras, in the western Caribbean. The new species is distinguished from all other species in the genus by its bright blue coloration on the head, nape, and dorsal portion of the trunk beneath the spinous dorsal fin, a prominent round black blotch below the origin of the spinous dorsal fin, and a high number of gill rakers. A molecular phylogeny based on mitochondrial and nuclear genes shows that the new species belongs to a clade containing L.levinsoni, L.regia, and L.anabantoides. At Roatan, submersible observations of this and other Lipogramma species indicate clear, interspecific habitat partitioning by depth and substrate.

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.