What is the Mesophotic Zone?
Between the warm, bright, and turbulent waters that host shallow-water coral reefs and the pitch black, near freezing, steady waters that are home to deep-sea corals, there is a long-overlooked zone called the mesophotic. The name literally means middle (meso) light (photic), and it is an area of the seafloor between approximately 100 feet (30 meters) to 500 feet (150 meters). These depths are too deep for typical scuba divers to reach and often too shallow to explore with deep-sea submersibles or larger remotely operated vehicles (ROVs). These challenges have made this zone an awkward middle ground that is under-represented in scientific literature and thus poorly understood. Technological advances like the development of smaller, lower cost remotely operated vehicles are making these mesophotic habitats more accessible, and their importance is beginning to be better understood by scientists who are better able to access this “middle zone” of the ocean where light transitions to darkness.
Where is the Mesophotic Zone?
The upper boundary of the mesophotic zone corresponds to the conventional SCUBA diving limit at around 100 feet (30 meters). Below that depth divers must have special training and use specialized equipment to safely visit. Less than 10 percent of the light at the surface makes it down to these depths. With so little light, sometimes these habitats are called twilight reefs. The deeper boundary of the mesophotic zone is determined by the depth at which there is still enough light present for photosynthetic corals to live. Beyond that depth begins the rariphotic zone.
When light hits the water, it immediately begins to be absorbed and reflected, and its energy begins to decrease. On average, at around 500 feet (150 meters) light levels decrease to 1 percent of what they are at the surface, which is the minimum life needs to maintain photosynthesis. However, different ocean conditions can alter how far light can move through the water. For example, light can travel farther where the water is clearer. So, in the clear blue waters off the coast of Hawaii the mesophotic zone may start and end deeper in the water. But in an area like the continental shelf offshore from Texas to Florida, where the flow of the Mississippi River makes the water murkier, light cannot reach as far and the mesophotic zone is typically shallower.
What Lives in the Mesophotic Zone?
In the mesophotic zone, corals (both reef-building stony corals and soft corals), sponges, and seaweed (macroalgae) make up the habitat that all other life in this zone relies upon. In the upper areas of the mesophotic zone, stony corals that have shelf-like shapes dominate. Farther down, soft octocorals, black corals, and other non-reef-building corals become more common. Calcified green algae, pink coralline algae, and sponges also contribute important habitat structure in the mesophotic zone. In mesophotic habitats off the southwest coast of Florida, you can find over 100 different species of sponges. These sponges filter water and host rich communities of symbiotic bacteria, some of which may be helpful to humans.
The corals, sponges, and algae that create the habitat of the mesophotic zone support a diverse array of fish, echinoderms (sea stars, brittle stars, and feather stars), mollusks, crustaceans, and worms. Many animals found in shallow-water reefs can be found in the upper parts of the mesophotic. For example, reef fish species like colorful damselfish and butterflyfish can be found flitting around mesophotic corals. Some animals spend only a part of their lifetime in mesophotic habitats. In Atlantic waters and offshore from Texas to Florida, commercially important fish like grouper and snapper will spend their younger years in shallow waters, but as adults they will move deeper and even gather in large numbers in mesophotic habitats to reproduce. In the Pacific, shelled cephalopods called nautiluses will spend their days in deep waters but at night will come up to mesophotic reefs to hunt for food. Some animals, like the Aphrodite anthias fish, are only found in the mesophotic zone.
How do Animals Adapt to the Middle?
Life forms that are reliant on the sun to survive have developed unique adaptations to live in places where sunlight is scarce. Coral species that live at both shallow and mesophotic depths will even change how they grow depending upon where they live. At mesophotic depths, corals that rely on photosynthesis by symbiotic algae more commonly have shelf-like, encrusting, and plate-like shapes to allow the capture of as much light as possible. In shallow water, great star coral (Montastraea cavernosa) colonies grow in a boulder shape, but at mesophotic depths they grow in a flatter, plate-like shape to help it capture every ray of light.
Corals also change the type and amount of symbiotic algae, zooxanthellae, that they house in their tissues to take advantage of the spectrum of light that is most available at mesophotic depths. Light comes in different wavelengths: red, orange, yellow, green, blue, indigo, and violet. As light travels down through the water column the colors with longer wavelengths like red, orange, and yellow are filtered out. By the time light makes it to the mesophotic zone, only the short blue, indigo, and violet wavelengths of light are left. At mesophotic depths more uncommon species of zooxanthellae are found and some have special pigments that are better at capturing the available spectrum of light.
Threats to the Mesophotic Zone
In some ways mesophotic habitats are more protected from many of the threats that shallow-water reefs face. One of the largest threats to corals that host symbiotic algae is coral bleaching, which occurs when water temperature rises, stressing corals and causing them to expel their symbiotic algae. Mesophotic corals that host symbiotic algae are more protected during heat waves because deeper waters take longer to heat up and remain cooler. When coral die-offs occur due to bleaching, deeper corals of the same species that survive might help areas recover by being a source of new coral larvae. Many direct human threats like fishing and pollution can be less severe because mesophotic habitats are farther from shore and harder to access than shallow-water reefs. In some areas, like Southwest Puerto Rico, commercially important fish like black grouper, goliath grouper, and Caribbean reef sharks are only found in large groups in mesophotic habitats where they face less fishing pressure.
While mesophotic corals are more protected from threats like ocean warming, fishing, and human interaction than shallow-water reefs, they are still vulnerable to many other threats. Oil and gas activities can be harmful to mesophotic corals, especially when spills happen. In 2010, the Deepwater Horizon oil spill off the coast of Louisiana, Mississippi, and Alabama impacted large areas of mesophotic coral habitats. Mesophotic coral species like Swiftia exserta and Muricea pendula were found after the spill coated in a mixture of oil, the oil dispersant used during spill response efforts, and dead plant and animal material. Invasive species are also a threat to some mesophotic reefs. On the southeastern coast of the United States, invasive lionfish can be found on reefs as far down as 1,000 feet.
As the importance of mesophotic habitats and the threats they face are better understood, scientists and conservationists around the world are beginning to prioritize new restoration and conservation efforts. There has been more interest in protecting mesophotic habitats by including them in Marine Protected Areas, notably in the north sea and the Mediterranean. In 2021, the Flower Garden Banks National Marine Sanctuary, which is a marine sanctuary around 80 miles off the coast of Texas and Louisiana and has extensive mesophotic habitats, added 14 new banks to the sanctuary.Currently there are efforts to restore the mesophotic habitats injured by the Deepwater Horizon oil spill through the Mesophotic and Deep Benthic Communities projects. These restoration projects, led by the National Oceanic and Atmospheric Administration and the Department of the Interior, focus on mapping and habitat modeling, coral propagation, habitat assessment, and active management of mesophotic and deep Gulf habitats. Part of this work also involves testing new methods to help mesophotic corals recover by growing them in aquariums, testing materials to encourage coral settlement, and investigating the best ways to propagate mesophotic corals in the wild.
At the Smithsonian
The National Museum of Natural History’s Department of Invertebrate Zoology is partnered with the Mesophotic and Deep Benthic Communities restoration projects to help restore mesophotic habitats injured during the Deepwater Horizon oil spill. A team of scientists at the Smithsonian is helping to identify the corals, sponges, crabs, mollusks, and other invertebrates that live in the areas that were impacted by the spill to better understand what a healthy mesophotic ecosystem looks like. Scientists are also looking at the museum’s historical collections to see what kinds of animals were found in impacted areas before the spill. The Smithsonian also supports the projects through identifying new species, documenting how they are related through taxonomy, maintaining archives and specimens that are shared with scientists worldwide, and reaching audiences through education and outreach.