An exploratory expedition describes two deep-sea, low oxygen ecosystems in the Red Sea, including some surprising fish.
Persistent oxygen-depleted zones in the ocean are relatively well studied in the temperate zone, but little is known about these unique ecosystems in the tropics. Shannon Klein and colleagues explored two subsurface oxygen-depleted zones in deep reaches of a Red Sea coral reef system with remotely operated vehicles (ROVs) and crewed deep-sea vehicles as part of the Red Sea Decade Expedition of 2022. Both sites are geomorphologically enclosed and characterized by warm temperatures (22ºC) and deep, oxygen-depleted conditions (<2–10.958 μmol O2 kg⁻¹) beneath the reef's surface waters. In the Amq Deep, reaching 619 meters below the surface, the authors found lace corals and at least three species of fish, including lightfish, soldierfish, and large aggregations of lanternfish. Lanternfish in the suboxic deep swim five times slower than lanternfish in oxygenated waters and may migrate upwards at night to reoxygenate and feed. In the 491-meter-deep Farasan Deep, near-anoxic conditions (<2 μmol O2 kg⁻¹) were expected to preclude the presence of aerobically respiring organisms. However, the researchers were surprised to observe fish swimming along the surface of the sediment under these conditions. These fish are unidentified. According to the authors, the warm and saline environment of the Red Sea interacts with deep enclosed depressions to restrict vertical mixing, limiting oxygen resupply at depth—and similar zones are likely to occur in abundance in other tropical coastal areas, perhaps with their own adapted fauna.
