Mesophotic Coral Ecosystems: The Twilight Reefs Below Scuba Depth

There are coral reefs in the twilight. Mesophotic coral ecosystems — from meso, "middle," and photic, "light" — grow in low light in tropical and subtropical waters, at depths from almost 100 feet to over 490 feet. For a long time little was known about them, for an awkward reason: they sit at exactly the wrong depth for the tools scientists had.

Too deep for divers, too shallow for machines

The upper limit of the mesophotic zone matches the depth limit of conventional scuba diving, 130 feet. Yet the zone is too shallow, and too costly, for most deep-diving technology such as remotely operated vehicles and submersibles. Only recent advances in undersea technology have made it possible to investigate these ecosystems properly.

What lives there

Corals, sponges and algae are the dominant communities, and they form the structure of the habitat. The ecosystems are essential fish habitat for some economically and ecologically important species, which use them for spawning, breeding, feeding and growing to maturity.

Why they matter now

Shallow coral reefs are going through significant changes, so it matters to understand these deeper reefs. Mesophotic ecosystems are considered extensions of shallow reefs, and they share species with them. That is why scientists hypothesize that mesophotic corals could serve as sources to reseed or replenish degraded shallow reefs.

They may also matter for medicine. Like shallower reefs, mesophotic ecosystems hold organisms with specialized defenses against predators and microbial infections, and those defenses often yield compounds that can be developed into natural products that benefit human health.

NOAA's Ocean Today has a short video on the mesophotic zone.

Sources

Based on "What are Mesophotic Coral Ecosystems?" from NOAA's National Ocean Service, a work of the United States government in the public domain.