Mesophotic coral reef

Lore & Background

Mesophotic coral ecosystems occur in tropical and subtropical waters, normally between 30 and 40 meters up to 150 meters in depth. They serve as a transition zone between shallow and deep-sea environments, creating specialized and unique biodiversity. Community zonation is driven primarily by light and temperature, broadly corresponding to the upper mesophotic (30 m to 60 m) and lower mesophotic (60 m to 150 m). In some areas, such as American Samoa, mesophotic reefs contain 80% of corals while the rest inhabit shallow reefs. These ecosystems are distributed globally but remain mostly understudied, with limited basic information on taxonomic composition, depth range, habitat preferences, and abundance.

The oldest known mesophotic coral ecosystems have been described from the Silurian of Sweden, and such ecosystems are also known from the Devonian. The oldest scleractinian-dominated mesophotic ecosystems are known from the Triassic. Historically, research on mesophotic coral reefs was limited until the 20th century due to the difficulty of observing them. Advances in technologies such as remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs) have enabled further research and monitoring.

Reader's Guide

Mesophotic coral reefs are significant as a crucial part of coral reef systems, serving as a potential refuge area for shallow coral reef taxa such as coral and sponges. Many species of fish and corals are endemic to MCEs, making these ecosystems a crucial component in maintaining global diversity. The mesophotic zone also serves as a transition zone between shallow and deep-sea environments, creating specialized and unique biodiversity. Due to recent anthropogenic impacts, it was thought that MCEs are less affected by human development and climate change and could be used as a source for reseeding shallow water coral species. However, recent analyses show that mesophotic ecosystems face significant impacts from climate change. Research on these reefs remains highly biased by location and region, with the best studied areas being the northern coast of Jamaica, the Bahamas, the northern Gulf of Mexico, and Puerto Rico. The Pacific mesophotic coral reefs remain understudied, and little to no research has been conducted in the regions of South East Asia and India.

Did You Know?

• The term 'mesophotic' comes from Latin 'meso' (middle) and 'photic' (light).
• In the Red Sea, two species of Alveopora are exclusively found in the mesophotic region.
• Sponges become the dominant species at depths over 50 meters in mesophotic reefs.
• Internal waves can cause temperature fluctuations in mesophotic reefs from 10 to 20 °C.

Naming, Depth, and Global Reach

The term mesophotic draws from Latin roots meaning middle and light, capturing the essence of ecosystems that exist in waters where light penetration is reduced yet still present. The preferred modern designation, mesophotic coral ecosystem, was adopted to replace ambiguous labels such as deep coral reef communities or twilight zone, which had become muddled through interchangeable use. These ecosystems typically span from roughly 30 to 40 meters down to about 150 meters in tropical and subtropical waters, encompassing a gradient from brightly lit upper zones to areas where sunlight barely penetrates. While distributed globally, they remain largely understudied, with basic information on taxonomic composition, depth ranges, habitat preferences, and species abundance still limited. Community zonation within these reefs is primarily governed by light availability and temperature, broadly dividing the upper mesophotic layer from the lower mesophotic layer. In some locations, such as American Samoa, mesophotic reefs harbor up to eighty percent of all corals, while the remaining twenty percent occupy the shallow zone above thirty meters.

Endemic Life and the Transition Zone

Mesophotic coral ecosystems function as a biological bridge between shallow reef habitats and the deep sea, fostering a specialized biodiversity that sets them apart from both. Many coral, sponge, and fish species are endemic to these mid-depth waters, having been outcompeted in shallower zones by faster-photosynthesizing taxa. In the Red Sea, two Alveopora species exist exclusively in the mesophotic range, while other unique inhabitants include Symbiodinium, Brachyuran crabs, Porites astreoides, and Acropora tenella. Predatory fish such as sharks, groupers, and snappers call the mesophotic zone home but migrate daily into shallower waters to hunt. These corals also carry zooxanthellae and require light, distinguishing them from deep-water coral species despite overlapping depth ranges. The oldest known mesophotic ecosystems date to the Silurian period in Sweden, with scleractinian-dominated examples appearing in the Triassic, underscoring their ancient evolutionary significance.

Reef Builders: Sponges, Algae, and Corals

Beyond corals, which form the structural backbone of mesophotic reefs, two other groups play pivotal ecological roles. Sponges rank as the second most dominant taxon in many coral reefs and are largely dependent on planktonic food, both dissolved and particulate, drawn in as water passes through their bodies. They are primarily mixotrophic or heterotrophic, prefer colder water, and thus become crucial inhabitants of the lower mesophotic zone. Their filter-feeding activity generates benthic diversity and creates essential habitat for invertebrates and fish. Many sponge species in mesophotic ecosystems differ from their shallow-water relatives and remain undescribed. Macroalgae, encompassing red, brown, and green forms, contribute by capturing carbon and producing oxygen. Their distribution in mesophotic reefs is shaped by temperature gradients, with a preference for warmer conditions, and by the availability of carbon, nitrogen, and photosynthetic potential. Competition between macroalgae and coral communities can contribute to coral bleaching, highlighting the delicate balance within these mid-depth habitats.

Research Frontiers and Conservation Concerns

For decades, mesophotic coral ecosystems were considered relatively insulated from human development and climate change, leading researchers to view them as potential refuges where shallow-reef corals and sponges could find shelter. They were even proposed as sources for reseeding degraded shallow-water reefs. However, recent analyses have revealed that these mid-depth habitats face significant climate-change impacts, complicating their role as sanctuaries. Advances in remotely operated underwater vehicles and autonomous underwater vehicles have opened new windows into these hard-to-reach environments, enabling more detailed research and ongoing monitoring. Despite this technological progress, fundamental knowledge gaps persist: taxonomic composition, precise depth ranges, habitat preferences, and the abundance and distribution of species remain poorly documented across most of the globe. The fact that these ecosystems have existed since at least the Silurian period, yet remain mostly understudied, underscores both their scientific importance and the urgency of filling these knowledge gaps before conditions shift further.