A team of researchers who journeyed to two low-oxygen ecosystems in the Red Sea have returned from their expedition with some remarkable finds, including the discovery of several surprising deep-sea fish.
Using both remotely operated vehicles (ROVs) and crewed deep-sea vehicles, the scientists observed lightfish (Vinciguerria cf. mabahiss), soldierfish (Ostichthys sp.) and large aggregations of lanternfish (Benthosema sp.) shoaling in the deep coral reef system.
Oxygen-depleted zones in the ocean are relatively well studied in the temperate zone, but little is known about those in the Red Sea and other oxygen-deprived zones in the tropics, say the researchers, who published their findings in the journal PNAS Nexus.
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The team – led by Shannon Klein, a research scientist at the Red Sea Research Center – explored two depressions, both geomorphologically enclosed and characterised by warm temperatures (22ºC) and deep, oxygen-depleted conditions beneath the reef’s surface waters.
This combination of warmth, salinity and the deep, enclosed depressions restricts vertical mixing, which limits oxygen resupply, explain the authors.
In the first depression, the 619m-deep Amq Deep, Klein and colleagues found lace corals and at least three species of fish, including lightfish, soldierfish and lanternfish.
At these depths, lanternfish – a type of fish that produce light to help them navigate in the dark ocean – swim five times slower than lanternfish in oxygenated waters. The researchers say they may migrate upwards at night to re-oxygenate and feed.
In the second depression, the 491m-deep Farasan Deep, the team were again surprised to observe fish swimming along the surface of the sediment. These fish were unable to be identified.
The authors believe that similar zones are likely to occur in abundance in other tropical coastal areas, perhaps with their own adapted fauna.
"Targeted exploration of deep tropical coastal environments is crucial for determining whether similar zones exist beyond the Red Sea and understanding their potential responses to climate change," says the paper, which adds that as seas warm this "could intensify coastal upwelling and water column stratification and further decrease the solubility of oxygen."
Find out more about the study: Deep oxygen-depleted depressions in a Red Sea coral reef sustain resistant ecosystems
Main image credit: National Center for Wildlife (NCW), Kingdom of Saudi Arabia
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