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Cusk Eel
Image courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas


The deep sea, also known as the deep ocean, represents the largest ecosystem on the planet, covering over 60% of the Earth's surface. This mysterious and relatively unexplored region begins approximately 200 meters (660 feet) below the ocean's surface and extends deep to the deepest trenches, such as the Mariana Trench, which plunges to a staggering depth of around 11,000 meters (36,000 feet).

 It is a fascinating and extreme environment characterized by tremendous pressure, absence of sunlight, near-freezing temperatures, low nutrient levels, limited food supply, and unique organisms adapted to survive in these conditions.

Fangtooth fish (Anoplogaster)

This Fangtooth fish (Anoplogaster) was seen at 800 meters (2,625 feet)
Image courtesy of the NOAA Office of Ocean Exploration and Research, Deep-Sea Symphony: Exploring the Musicians Seamounts.

Deep-sea creatures exhibit remarkable evolutionary adaptations, including bioluminescence, gigantism, and flexible bodies or rigid structures that can withstand pressure.

Some of the most iconic species living in the deep sea include anglerfish, gulper eels, vampire squids, giant isopods, and deep-sea octopuses. These organisms have developed intricate skills for hunting, defence, and reproduction in a realm where resources are scarce and encounters with potential allies or prey are infrequent.


Vampire Squid (Vampyroteuthis infernalis)

Image courtesy of Journey into Midnight: Light and Life Below the Twilight Zone/ NOAA


Flapjack Devilfish

Image courtesy of the NOAA Office of Ocean Exploration and Research, 2019 Southeastern U.S. Deep-sea Exploration

Giant isopod

Giant isopod (Bathynomous gigantus) seen during Dive 11 of the 2019 Southeastern U.S. Deep-sea Expl
photo courtesy of NOAA Office of Ocean Exploration and Research



Image courtesy of the NOAA Office of Ocean Exploration and Research, Mountains in the Deep: Exploring the Central Pacific Basin. 

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Photo of Anglerfish (Lophius piscatorius) adapted to camouflage on the ocean floor

Image credit: Valda Butterworth via Shutterstock

The anglerfish, which belongs to the order Lophiiformes, is renowned for its distinctive appearance, featuring a luminous antenna on its head that serves as a lure to attract prey.

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A deep-sea anglerfish (Haplophryne mollis) with two (parasitic) males attached to her body. 

Image credit: Neil Bromhall via Shutterstock

Most species of Lophiformes engage in a mating behaviour known as sexual parasitism, which involves the complete fusion of males and females during the mating process and, in some cases, throughout their lives. This fascinating behaviour has captured the interest of researchers worldwide. There are currently 168 recognized species of Lophiformes, many of which exhibit pronounced sexual dimorphism, where the differences between males and females are readily apparent. In certain Lophiformes species, males can be as small as 1 cm, while females can reach lengths up to 20 cm. During mating, these diminutive males attach themselves to the female's body, initiating a process of skin tissue fusion. Eventually, the male becomes an integral part of the female, relying on her for sustenance.

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Female (left) and male deep-sea anglerfish (Himantolophus sp).

Image credits: Nature Picture Library/Alamy


Bubblegum coral (Paragorgia arborea) at 1257 meters water depth (California)

photo source: NOAA/Monterey Bay Aquarium Research Institute - NOAA Photo Library

Deep sea coral reefs are submerged structures composed of corals that flourish at depths greater than 200 meters (656 feet). Unlike their shallow-water relatives (which depend on sunlight for sustenance through photosynthesis), deep-sea corals have evolved to thrive in complete darkness. They rely on filter feeding to extract nutrients from passing currents, capturing tiny organisms like zooplankton to survive.


Christmas tree coral (Antipathes dendrochristos) grows several hundred meters down in the ocean

photo credit: Mark Amend / NOAA

These submerged oases support a myriad of species, including corals, sponges, crustaceans, molluscs, and various fish species. One of the most striking inhabitants is the black coral (Antipatharia), named for its distinctive dark appearance. These organisms have adapted to harsh conditions, evolving unique traits to withstand the intense water pressure and scarcity of food.
Deep sea coral reefs are some of the oldest living animals on Earth. Some individual coral colonies are thousands of years old, creating underwater ecosystems that have thrived for millennia. Despite their resilience, these reefs are fragile and vulnerable to human activities, such as deep-sea mining and bottom trawling, which can cause irreversible damage.


In the deep-sea environment, where sunlight is scarce or absent, primary producers are often chemosynthetic bacteria and archaea. These microorganisms convert chemicals into organic matter and serve as primary producers.
Vent and seep organisms like tube worms, clams, mussels, and shrimp form symbiotic relationships with the bacteria and archaea. They provide habitat and receive organic compounds from the microorganisms.

Hydrothermal vents and cold seeps are prime examples of environments where chemosynthetic bacteria and archaea flourish. These microbes form the foundation of unique ecosystems, providing organic matter that supports a wide range of organisms.


A biological community of mussels, shrimp, and limpets living at NW Eifuku seamount in the Marianas region

Image courtesy of Submarine Ring of Fire 2014 - Ironman, NOAA/PMEL, NSF

The deep sea food chain also depends on organic matter sinking from surface waters. Detritus, including dead organisms, faecal pellets, and marine snow, consisting of organic particles and debris, serve as important food sources.

Deep-sea filter feeders, such as giant tube worms, clams, and various species of sponges, feed on suspended organic particles and microbes in the water. They are adapted to capture and consume these particles efficiently.

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Acorn worm

Image courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas

Predatory organisms are abundant in the deep-sea food chain. This category includes various species of fish (lanternfish, dragonfish, anglerfish), cephalopods (like squids and octopuses), and crustaceans (such as deep-sea shrimp).

Apex predators, such as deep-sea sharks, whales, and dolphins, consume lower-level predators and scavengers, completing the food chain.


Wreckfish (Polyprionidae)

Image courtesy of the NOAA Office of Ocean Exploration and Research, Windows to the Deep 2019


This synaphobranchid eel was documented preying on a fish during Dive 16 of the Windows to the Deep 2019 expedition

Image courtesy of the NOAA Office of Ocean Exploration and Research, Windows to the Deep 2019


Lanternfish (Myctophidae)

photo credit: Steven Haddock/Monterey Bay Aquarium Research Institute

Lanternfish are small, bioluminescent fish that serve as both predators and prey. They are known for their vertical migrations, moving up to surface waters at night to feed and returning to the depths during the day.


Dragonfish (Idiacanthus atlanticus)

Image courtesy of NOAA Okeanos Explorer Program, Our Deepwater Backyard: Exploring Atlantic Canyons and Seamounts 2014

Dragonfish are small, dark-coloured fish with small eyes and a long barbel, a whisker-like appendage, under their mouths. Their bodies are slender and eel-like, with long, sickle-shaped fins.


Smalltooth sandtiger shark (Odontaspis ferox), believed to be pregnant, seen on Leg 3 of the 2016 Deepwater Exploration of the Marianas expedition.

Image courtesy of the NOAA Office of Ocean Exploration and Research. 

Deep-sea sand tiger sharks have a slow reproductive rate, typically giving birth to only two pups every couple of years. Their unique reproductive strategy, called aplacental viviparity, involves the development of eggs within the mother's uterus, with the pups eventually consuming unfertilized eggs for nourishment. This strategy, along with variations in viviparity and oviparity, reflects the diverse reproductive adaptations seen in sharks for survival in different marine environments.

Chemotrophic communities on the carcass of a 35-tonne grey whale at a depth of 1,674 m

chemoautotrophic whale-fall community in the Santa Cruz basin off southern California at a depth of 1,674 m (5,492 ft), including bacteria mats, vesicomyid clams in the sediments, galatheid crabs, polynoids, and a variety of other invertebrates.
photo credit: Craig Smith NOAA

When a whale dies and sinks, it creates a localized ecosystem. Scavengers like sharks break down the meat, allowing other organisms to enter. As decomposition progresses, additional organisms join the feast, including bone-eating worms (Osedax), various species of crustaceans, and molluscs. These organisms further consume and decompose the remains. Over time, the dead whale bones are gradually cleaned, emptied, and covered with a delicate layer of silvery bacteria, that give the skeleton the appearance of being wrapped in soft layers of sponge. It may take several decades or even a century for the substantial remnants of the whale carcass to disappear completely.


Osedax worms, such as O. antarcticus, accumulate in large numbers on whale bones, spending as long as 10 years feeding from them  
photo credit: Thomas Dahlgren


Polychaete worms (Vigtorniella flokati) 


Deep sea mining is a relatively new and controversial industry that involves the extraction of valuable minerals and resources from the ocean floor, typically at depths ranging from hundreds to thousands of meters. The primary focus of deep sea mining is on polymetallic nodules, polymetallic sulphides, and cobalt-rich ferromanganese crusts, which are rich in valuable metals such as copper, nickel, cobalt, and rare earth elements.


Polymetallic nodules coat fields of the ocean floor and are rich in critical minerals needed to make batteries for electric vehicles

Image courtesy of  NOAA Office of Ocean Exploration and Research

Deep sea mining poses serious environmental and ecological threats, including habitat destruction, species extinction, sediment and chemical pollution, noise pollution, and unknown consequences. It also raises resource depletion, regulatory challenges, climate impacts, and social-cultural issues. Balancing economic benefits with environmental protection remains a complex and ongoing challenge.

The International Seabed Authority (ISA), established under the United Nations Convention on the Law of the Sea (UNCLOS), is responsible for regulating deep-sea mining activities in international waters. However, the regulatory framework is still evolving, and there are ongoing debates about how to balance resource exploitation with environmental protection.

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Location of the Clarion Clipperton Zone

image source: USGS -

The Clarion-Clipperton Fracture Zone is a geological submarine fracture zone in the Pacific Ocean. It has recently gained global attention due to its abundant manganese nodules, essential for various technologies, including electric vehicle batteries. However, this marine area is also a valuable but poorly understood ecosystem.

The central challenge is balancing the increasing demand for mineral resources with preserving this unique marine environment. Limited knowledge about the ecological dynamics of the Clarion-Clipperton Fracture Zone makes it difficult to predict the consequences of mining activities, posing a significant risk of causing irreversible damage to this fragile ecosystem and its inhabitants.

Additionally, there is a concern about the perceived "green" benefits of technologies relying on these minerals. While the transition to sustainable energy sources is crucial for addressing climate change, it's essential to assess the environmental impact of these technologies critically. Sometimes, these "green technologies" may not be as environmentally friendly as they appear, as material extraction and processing can lead to substantial carbon emissions and environmental harm.


A deep-sea chimaera seen during the INDEX 2010: “Indonesia-USA Deep-Sea Exploration of the Sangihe Talaud Region” expedition

Image courtesy of the NOAA Office of Ocean Exploration and Research, INDEX-SATAL 2010

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