Plumbing the ocean depths


The snailfish is the deepest living fish ever found at a depth of 8,336 meters. – Screenshot via YouTube/The University of Western Australia

AFTER a decade and a half of negotiations, late Saturday evening, March 4, 2023, Member States of the United Nations finally agreed on a text on the first-ever international treaty to protect the high seas, which cover half our planet. Rena Lee, the Singaporean Chair of the conference, held at the UN Headquarters in New York, appropriately stated in her concluding speech: “The ship has reached shore.” This pertinent announcement was greeted with loud and prolonged applause from all delegates .

This landmark agreement aims to cover 30 percent of the world’s oceans by 2030. Our oceans create 50 percent of the oxygen we breathe and absorb 25 percent of the carbon dioxide released by our activities on Earth. The carbon literally “sinks” into the oceans and these ecosystem systems are vital to all of us. Protecting areas in international waters will play a crucial role in building resilience to the impacts of climate change.

How deep are the oceans?

The deep sea refers to the sea depths where sunlight begins to fade at a depth of 200 meters and is therefore characterized by darkness, low temperatures and high pressure. When I first studied geography for high school in the early 1960s, I was taught that the Mariana Trench off the coast of the Mariana Islands in the western Pacific, 10,915 meters below sea level, was the deepest oceanic trench at the time.

With refined sonar techniques, its maximum depth recorded in the Challenger Deep (a section of the same trench) has now been set at 10,920 meters. In other words, if Mount Everest (8,410 meters) were underwater, its summit would still be 2,000 meters below sea level. Today there are five ocean trenches deeper than 10,000 meters, two deeper than 9,000 meters and four deeper than 8,000 meters.

formation of oceanic trenches

These are at the convergence of plate boundaries, where an oceanic plate meets an oceanic plate or an oceanic plate meets another oceanic plate. The denser of the two plates submerges under the lighter one in the so-called subduction zone (from Latin “to lead under”). Usually the denser plate is the oceanic one.

The Mariana Trench is formed by the denser basaltic rocks of the Pacific Ocean Plate dipping beneath the lighter, younger rocks of the Mariana Ocean Plate. Most ocean trenches are found around the Pacific Rim of Fire, but they are also found in the eastern Indian Ocean, the Atlantic, the Mediterranean, and off the west coast of South America.

Life in the ocean depths

It is often claimed that more is known about life on the moon than about the deep sea. These areas are certainly the least explored areas on earth and our knowledge is certainly limited but with modern technology and the use of robotic submersibles we are getting there. This is illustrated no better than in an editorial in the Borneo Post (March 7, 2023) entitled “Drugs from the deep: Scientists explore ocean frontiers,” a fascinating account based on the deep sea by researchers at the Scripps Institution of Oceanography , based in San Diego, California.

The sea depth can be divided into four zones, the depths of which are measured in meters below sea level: the mesopelagic (twilight) zone from 200 to 1,000 meters; the Bathypelagic (midnight) zone from 1,000 to 4,000 meters; the Abyssal Zone from 4,000 to 6,000 meters; and finally the hadal zone, composed of sea trenches from 6,000 to 11,000 meters. Each zone has a different mix of species adapted to its specific luminosity, pressure, and temperature.

The ocean depth can be divided into four main zones. – Diagram from Deepdisco/Wikimedia Commons

Fish, jellyfish, krill and squid can be found in the mesopelagic zone, and the most numerous family of fish is the bristlemouth. The Bathypelagic Zone is an aphotic area with temperatures of 4 degrees Celsius. Living on a minimal amount of food, creatures have slow metabolisms and possess squishy bodies and slimy skin. There live viper fish, monkfish, hagfish, gulper eels, vampire squid and dumbo octopus.

Many of these species are notable for their grotesque forms and very large reflexed teeth and hinged jaws. Their source of energy is “sea snow,” which consists of dead plankton, diatoms, feces, silt, soot, and inorganic matter. The “snowflakes” collide and coagulate as they drift downward, taking several weeks to reach the sea floor.

The Abyssal Zone is characterized by hydrothermal vents caused by volcanic eruptions at the plate boundaries. Very hot water, rich in minerals, emerges from these openings at 400 degrees Celsius. This toxic chemical soup feeds bacteria that get their energy from the water through a process known as chemosynthesis. These bacteria, in turn, convert the chemicals into inorganic molecules that provide food for giant tubeworms found near the hydrothermal vents.

A crustacean that lives around these openings is the yeti, or hairy crab. Its Latin name is Hirsuita Kiwa, named after the Polynesian goddess of shellfish. It is indeed hairy, sporting furry hairs around its pincers. These hairs trap bacteria that feed on the openings, and these bacteria provide the crab with its main food source. At this depth, the pressure is 600 times that at sea level, so creatures such as tripod fish, rattail or grenadier fish, squid and sea cucumbers are highly specialized.

The yeti crab wears furry hairs around its pincers. – Photo by Andrew Thurber – Oregon State University/Wikimedia Commons

The hadal zone recently revealed the deepest living fish ever found at a depth of 8,336 meters – the snailfish – in the Izu Ogasawa Trench off Japan. Scaleless with large teeth, this small pink-skinned fish is mostly gelatinous to avoid expending energy in an environment of starvation or to care for skin and scales. Professor Alan Jamieson of a Western Australian University managed to film this fish using a remote-controlled camera that can withstand pressures 800 times greater than that at the surface of the earth.

Many species of fish have large tubular eyes to cope with scarce light at depth, and others possess something called bioluminescence. This is a chemical reaction that creates light energy in the body of an organ, but first and foremost a fish must contain luciform, a molecule that reacts with oxygen to create light. Lanternfish use bioluminescence to lure their prey toward their mouths and to attract mates. Deep-sea worms that live on the ocean floor produce a flash of this light to deter impending predators.

Threats to life in the deep sea

Global warming combined with increasing acidification of our oceans has led to the death of many organisms and species of fish. Deep-sea trawling has destroyed a variety of habitats through its methodical raking of the seabed and brought alien-looking fish to the fish markets that appear inedible!

Deep-sea marine biology has also been affected by deep-sea mining of ores, oil and gas, which has disrupted fish populations. Hopefully the new United Nations treaty will help preserve our open oceans. 19th-century French novelist Jules Verne certainly awakened the world to the wonders of the deep sea in his famous book 20,000 Leagues Under the Sea and continues to inspire 21st-century explorers.

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