The wonders of the deep seas

Print edition : January 14, 2022

The remotely operated vehicle (ROV) Deep Discoverer images a newly discovered hydrothermal vent. Photo: Courtesy of NOAA Ocean Explorer

Ocean layers. Photo: Courtesy of NOAA Ocean Explorer

An octopus at a seep site. Photo: Courtesy of NOAA Ocean Explorer

The autonomous underwater vehicle Sentry. Photo: Courtesy of NOAA Ocean Explorer

Alvin uses one of its manipulator arms to take samples from the sea floor. Alvin is a three-manned American deep-sea submersible fitted with sophisticated equipment. Photo: Courtesy of NOAA Ocean Explorer

A seamount at a depth of 2,465 metres. Photo: Courtesy of NOAA Ocean Explorer

Glass sponges are visible in the foreground of this sponge community found at a depth of about 2,360 m. Photo: Courtesy of NOAA Ocean Explorer

A translucent egg case with a catshark embryo actively swimming inside it. Photo: Courtesy of NOAA Ocean Explorer

A copepod family Aetideidae) laden with eggs. Photo: Courtesy of NOAA Ocean Explorer

The octocoral Iridigorgia with squat lobsters on it, in the north-western Gulf of Mexico. Photo: Courtesy of NOAA Ocean Explorer

The curlicue shape is a characteristic of Iridogorgia. Photo: Courtesy of NOAA Ocean Explorer

A hydrothermal vent chimney. Photo: Courtesy of NOAA Ocean Explorer

A vent emitting droplets of liquid carbon dioxide. Photo: Courtesy of NOAA Ocean Explorer

A seep site with clusters of live Bathymodiolus mussels (left side, foreground, and background). Photo: Courtesy of NOAA Ocean Explorer

An aggregation of Lamellibrachia sp. tubeworms providing a habitat for many smaller animals. Photo: Courtesy of NOAA Ocean Explorer

The giant tubeworm Riftia pachyptila. Photo: Courtesy of NOAA Ocean Explorer

Sea cucumbers (Chiridota heheva) with chemosynthetic Bathymodiolus mussels at a cold seep. Photo: Courtesy of NOAA Ocean Explorer

An aggregation of ice worms inhabiting methane hydrate. These worms eat chemoautotrophic bacteria using chemicals in the hydrate. Photo: Courtesy of NOAA Ocean Explorer

The bone-eating worm Osedax. Photo: Courtesy of Greg Rouse

Methane bubbles flow in small streams out of the sediment on an area of sea floor. Quill worms, anemones and patches of microbial mat can be seen in the periphery. Photo: Courtesy of NOAA Ocean Explorer

Tubeworms associated with seeps. These worms are related to but differ from the giant tubeworms found around hydrothermal vents. Photo: Courtesy of NOAA Ocean Explorer

The site of a whale fall. When a whale dies and sinks to the sea floor, it is feast time for several organisms for several years to come. Photo: Courtesy of Craig Smith, University of Hawaii.

A purple squat lobster with stalked barnacles attached to it. This lobster is a scavenger and is among the first creatures to arrive at the scene of a whale fall. Photo: Courtesy of NOAA Ocean Explorer

A squat lobster perching on an undescribed genus of bamboo coral. This lobster is a scavenger and is among the first creatures to arrive at the scene of a whale fall. Photo: Courtesy of NOAA Ocean Explorer

A coral garden. Like their shallow warm-water cousins, cold-water corals provide a habitat for several deep-sea creatures. Photo: Courtesy of NOAA Ocean Explorer

A deep-sea red crab hangs out on a bubblegum coral. Photo: Courtesy of NOAA Ocean Explorer

A hard rock area with a very high coral diversity on a seamount complex. Photo: Courtesy of NOAA Ocean Explorer

A sea pen (Pennatulacea) on the soft sediment of the sea floor. Photo: Courtesy of NOAA Ocean Explorer

Bright yellow parasitic zoanthids encrusting a glass sponge. Photo: Courtesy of NOAA Ocean Explorer

Sulphide chimneys coated with an iron-based microbial mat at a vent site. Photo: Courtesy of NOAA Ocean Explorer

A yellow bamboo coral. Deep-sea corals obtain their nutrition by trapping tiny organisms that the ocean currents bring to them. Photo: Courtesy of NOAA Ocean Explorer

A sponge covered with hundreds to thousands of tiny anemones also provides a home to several brittlestars (pink), sea lilies (yellow) and a basket star (brown). Photo: Courtesy of NOAA Ocean Explorer

A new species of vent-endemic flatfish. Photo: Courtesy of NOAA Ocean Explorer

Before we think about exploiting the deep seas for their mineral wealth, it is essential that we understand the unique and fragile ecosystems of these dark waters with their wondrous and diverse life forms so that we do not repeat the mistakes that have destroyed terrestrial ecosystems and contributed to climate change.

THE Indian Ocean is a region of high biodiversity, with India, one of the countries in the region, considered a repository of mega biodiversity. India has a coastline of approximately 8,000 kilometres. On January 1, 1966, soon after the conclusion of the International Indian Ocean Expedition in the 1960s, the National Institute of Oceanography, Goa, was established for oceanography-related research. On October 30, 2021, the Ministry of Earth Sciences launched the Samudrayaan project for deep-ocean exploration. A question that comes naturally to the mind is, how much have we advanced our knowledge of the oceans and seas of India from 1966 to 2021 and what do we know of their biodiversity?

The Samudrayaan project will explore the deep sea for minerals. What and how will this exploration impact life in these zones, especially when we have little idea of the organisms that occupy these waters? A 2016 paper published by the United Nations says that “between one-third and two-thirds of marine species may be undescribed”.

Despite the work being carried out by international institutes such as the National Oceanic and Atmospheric Administration (NOAA) of the United States, the Woods Hole Oceanographic Institution (WHOI), Massachusetts, U.S., and others for the last 20 years, our understanding of the geological and biological details of the deep region of the oceans has remained limited for several reasons. Extending from the Arctic Ocean to the Southern Ocean and at depths of 1,000 m (called the bathypelagic zone) to 11,000 m (hadopelagic zone), the deep seas are dark and cold and the pressure there is enormous. The deepest part is the Mariana Trench or Marianas Trench, located in the western Pacific Ocean at a depth of 11,034 m.

The crushing pressure had made it difficult to access this region until the technology making it possible to study the different habitats and the life forms found therein was developed. Historically speaking, the fascination for underwater exploration was driven mostly by the desire to retrieve sunken treasure. But legend has it that Alexander the Great was lowered into the sea in a diving bell so that he could watch the creatures living beneath the water. According to a Persian poem, Alexander, having conquered vast areas of land, wanted next to conquer the oceans. He ordered the building of glass diving bells to explore the oceans. From Aristotle’s writings, we know that diving bells were known from the fourth century B.C.

Undersea vehicles

From diving bells in the fourth century to submarines, followed by bathyspheres and the bathyscaphe Trieste, the journey to study the deep oceans has made rapid strides. There are now remotely operated vehicles (ROVs), unmanned robotic autonomous underwater vehicles (AUVs) and, more recently, Alvin: a three-manned American deep-sea submersible fitted with sophisticated equipment, which has facilitated the discovery of mind-boggling diversity that challenges the very meaning of life in equally unique ecosystems.

Before any mineral exploitation occurs in these dark waters, it is essential that we understand the unique ecosystems so that we do not repeat the mistakes that have destroyed terrestrial ecosystems. India has been working in the field of deep-sea exploration of minerals since 1981 when the research vehicle Gaveshani first found polymetallic nodules in the Indian Ocean. But the biodiversity register of organisms from even the photic zones of the oceans is rather limited. Will the Samudrayaan project inventory the biodiversity? The various missions conducted by countries such as the U.S., Russia and Japan provide us with some idea of the unique biodiversity we are likely to discover in deep-sea benthic regions. The variety of the habitats and ecosystems that influence and drive the diversity of life provides us with some fascinating insights into the extent to which living organisms are capable of adapting themselves to their environment. Here are some examples of organisms surviving in uncommon habitats; if we ignore their story of survival, it will only be at our peril.


Seamounts are underwater mountains, large geologic landforms arising from the ocean floor. They are mostly extinct volcanoes. On the basis of satellite and bathymetric-mapping data, it is estimated that there are more than 100,000 seamounts that are 1,000 m high. It is estimated that the Pacific Ocean alone may have more than 30,000 seamounts. Sadly, only 0.1 per cent of these have been explored. Many are separated from one another by long distances. But all of them are hotspots for biodiversity, much of which may be endemic to the mounts. There is higher species diversity and biomass on a seamount than in the waters around it on the sea floor.

There are many reasons for the high species count. Seamounts provide something hard to come by in the deep ocean: a solid surface that organisms can to cling to, attach themselves to and grow. Many of the deep-sea animals on seamounts are permanently attached to the rocks. As these animals grow and reproduce, they create more three-dimensional structures on which newly arriving drifters can establish their homes. Much like the seeds of a plant, the larval forms of these sessile animals float away in search of a new mount to attach to and continue their life. Sessile organisms cannot move in search of food. They depend on the complex current patterns created by these mounts rising off the water column for their food. The strong ocean currents moving over a seamount provide the animals living on it with a continuous supply of planktonic food. Ocean currents also bring larvae from distant, quite geographically separated, areas. For these drifting larvae, the hard surface of the seamount provides them a place to settle down and grow. Thus, there is a community of unrelated organisms on a seamount, resulting in high biodiversity.

The already famous Galapagos Islands got another feather in their cap in 1977 with the discovery of hydrothermal vents. While exploring an oceanic ridge near these islands at a depth of 2,500 m (8,250 ft), scientists were stunned to see hot mineral-rich fluids emerging from openings on the ocean floor.

Hydrothermal vents

These hydrothermal vents are the geysers and hot springs of the oceans. They occur in volcanically active areas such as the mid-ocean ridges where there is movement of tectonic plates. Here magma rises up close beneath the sea floor. The cracks in the porous rocks allow ocean water to percolate down and get heated by the underlying magma. A series of chemical reactions results as the heated fluids pouring out of the vent meet the cold ocean water. These reactions cause several materials such as sulphur, iron, zinc and copper to precipitate and form metal-rich towers, which is the classic image of a hydrothermal vent.

The explorers who made this discovery were further astonished to see a unique gas-based ecosystem driven by chemosynthetic bacteria thriving around these vents. These are symbiotic bacteria that reside within or on the surfaces of the wide variety of living organisms—clams, mussels, shrimps and giant tubeworms—found around the vents. These bacteria use hydrogen sulphide to produce energy for the metabolic processes of these organisms.

The free-living fauna of hydrothermal vents are mainly composed of invertebrates with only a few fish families, each including a small number of species, found so far. Up to now, 500 species belonging to 12 phyla are the known constituents of the vent fauna, with several more yet to be described.

An interesting study comes from the benthic ecologist Lauren Dykman, a PhD student from the WHOI, who discovered that the hydrothermal vents also support a wide array of parasites. “Diverse, functioning, healthy ecosystems actually have a higher diversity and abundance of parasites,” said Lauren Dykman. “So parasites can be a sign of an ecosystem being healthy and functioning properly.” She notes that these parasites have survived volcanic eruptions, earthquakes and humans mining for ores and minerals. Their hosts are not only fish but crabs, snails and other invertebrates.

New to science

Lauren Dykman has so far come across 29 different kinds of vent parasites, including a leech, a nematode, copepods, acanthocephalans (spiny worms) and digeneans; seven of them were new to science. All of them were uniquely adapted to their environment, with one tiny flatworm called digeneans requiring three different hosts to complete its life cycle.

Hydrothermal vents are important as they help transport minerals and heat from the depths of the earth to oceans, thereby regulating their chemical composition. The significance of the activity of these vents is best exemplified by the copper mines of Cyprus. These were formed because of hydrothermal activities that occurred millions of years ago and were lifted off the sea floor to become dry land. Mineral deposits of commercial value are believed to be present near these vents.

Cold seeps

Cold seeps, like hydrothermal vents, are also places on the ocean floor from where chemically rich fluids and gases emanate. The underlying condition that forms cold seeps differs from the one that forms vents. Cold seeps form at cracks in the earth’s crust. The cracks release buried petroleum-based gases and liquids from deep underground where they were formed over millions of years. These liquids and gases are made up of hydrogen and carbon, mostly methane. Seeps also form at places with geological faults, salt deposits or where canyons cut into fluids or gas-trapped sediments. Sometimes, cold seeps also develop where the warm water of the oceans causes gas hydrates (deposits with methane) to release their gases. They are called cold seeps not because the fluids that emerge from the sea floor are colder than the surrounding water but because these fluids are colder than the scalding hot fluids that come out of vents.

Whereas hydrothermal vents occur at the edges of tectonic plates (whose movements result in processes that can even lead to the formation of new sea floors), cold seeps are found within the plates—leaking fluids and gases that are close to ambient deep seawater temperatures—or above geologic faults. Vents and the ecosystems they support may be created and destroyed as underlying volcanic activity changes over the years. Cold seeps, on the other hand, do not change and may be thousands of years old. This is because they produce a diffuse flow of lower-temperature fluids made up of natural gas and a mixture of hydrocarbons at slower rates for longer periods.

Cold seeps are thus natural-gas-based ecosystems driven mostly by methane. They are an important component of deep-sea ecosystems because they often are the drivers of entire communities that use bacterial chemosynthesis of hydrocarbons (mostly methane) for their nutrition. These chemosynthetic bacteria may be either free-living, forming bacterial mats on the sea floor, or in a symbiotic relationship with tubeworms, mussels, and so on. The tubeworms, clams and mussels that survive here attract predators such as crabs and octopuses to the seep. The food web is thus quite unique.

Significance of life at the vents and seeps

Stephane Hourdez and Chuck Fisher of the NOAA study the tubeworms found in cold seeps. These worms are related to but differ from the giant tubeworms found around hydrothermal vents. Lacking a digestive system, these worms derive their nutrition from methane through special symbiotic bacteria living inside their bodies in an organ called a trophosome. These chemosynthetic bacteria derive their energy from the methane hydrate present in the seeps. The researchers found that the cold seep tubeworms grow very fast in the first 10 years and then slow down for a long time, growing perhaps just a centimetre long in a year. After studying hundreds of such tubeworms, the researchers calculated their ages and found that they could live for more than 200 years, possibly making them the longest-lived animals on the earth (source: NOAA).

The organisms thriving at deep sea vents and seeps are termed extremophiles because their living conditions are dark and freezing cold with high pressures and toxic chemicals. These conditions are comparable with those that are believed to have existed on the earth when early life evolved. Studying the way of life of these organisms, their adaptive abilities, life processes, and so on, can give us more clues to understanding how life evolved and radiated on earth when it was still a hot planet. Rather than searching for the origin of life in hot planets of the universe, it would be better to search the vents, seeps and volcanoes of the deep sea.

There are other benefits too. Molecules, enzymes and metabolic processes unknown to science, which hold great potential for biomedical and commercial applications, have been discovered in these extremophiles. A type of protein found only in deep-sea organisms, called an ice-nucleating protein, is already being used to make ice creams. The enzyme polymerase that drives the polymerase chain reaction and has allowed scientists to generate copies of DNA was first isolated from creatures found in the vents.

The whale fall ecosystem

An opportunistic event called whale fall results in the birth of an ecosystem in the deep seas. Food is scarce for large creatures that live in deep seas. The silt, dead plankton, decomposing animal parts, empty shells, animal poop and other such decaying matter that rain down from the surface (termed marine snow) are their only source of nutrition. So when a whale dies and sinks to the sea floor, it is feast time for several organisms for several years to come. Whales that die on the sea surface may end up on a shore or sink to the bottom. The latter event is called whale fall. The whale carcass provides a habitat and food for diverse communities of marine life, thus creating a unique ecosystem.4,5 Other falls too take place, such as shark fall, wood fall or kelp fall. But none of these are comparable to a whale fall in terms of longevity, extending from several years to decades, and the diverse communities it supports.

A whale fall creates an organic-cum-sulphide-rich island habitat. Feeding happens in series and goes on for years because every part of the whale is eaten and by a variety of animals and microbes. Marine biologists from the Monterey Bay Aquarium Research Institute in California found that one whale-fall community lasted for 50 years.

The first to arrive at the scene are scavengers such as the hagfish, sleeper sharks,6 rattail fish, snow crabs, sea scuds, brittle stars and squat lobsters. They can smell the meat from a distance and arrive to feed on the meaty part of the carcass: muscles and visceral organs. They may feed there for up to two years or more depending on the size of the whale. As they feed, they also enrich the sediment around the whale with nutrients. When they move out, hordes of polychaete worms, molluscs, unusual crustaceans and other invertebrates descend on the remains. They feed on the blubber and the nutrient-rich sediment beneath and around the carcass.

Dr Craig Smith of the University of Hawaii has found that a whale skeleton, depending on its age, holds anywhere between 2 to 24 tonnes of oil in its bones. Approximately, 45,000 worms per square feet have been recorded to blanket the sea floor a year after a whale fall. Marine biologists have found many of these organisms to be unique to the whale fall. Of these, the bone-eating worm Osedax leads the show as other organisms feed on the sediment around the whale skeleton. The worm has no gut, eyes or legs, and the male is a tiny creature residing within the female. She has beautiful red plumes and rootlike appendages with symbiotic bacteria within to digest the lipids. These appendages bore through the bones to reach the marrow for the symbiotic bacteria to breakdown the lipids and release energy for the worms. This can go on for at least two years depending on the size of the whale. The holes drilled into the bone by the Osedax create the openings for microbes to move in, starting the final, or sulphophilic, stage that can last for several years. The first to enter are the anaerobes followed by the chemosynthetic sulphophilic bacteria. The latter appear as a yellow mat covering the bony remains of the whales. The bacterial food web on a whale fall is quite unique, with nearly 200 sulphophilic species having been recorded.

As the bacteria break down the lipids to generate sulphur, many more species of bacteria and species of mussels, snails and worms unique to this habitat are attracted to the fall. Several species seen at this stage are new to science and have yet to be described. The diversity of species seen during this last stage is among the widest known compared with known communities on the deep sea floor.

Deep-sea reefs

Coral reefs bring to mind those shallow water reefs seen through glass-bottomed boats or when snorkelling or scuba diving. But coral reefs not only exist but thrive in the icy cold, dark parts of the ocean at depths of 6,096 m (20,000 ft). Like their shallow warm-water cousins, these cold-water corals provide a habitat for several deep-sea creatures. They exist as individual coral polyps or colonies of polyps of the same individual as well as reefs hosting colonies of one or many species. Deep-sea corals obtain their nutrition by trapping tiny organisms that the ocean currents bring to them.

They are widely distributed despite the unfavourable conditions they inhabit. The ice-cold temperatures and water chemistry make skeleton construction a difficult process. Although they have overcome these difficulties by developing a buffering mechanism to manipulate the water chemistry and build their skeletons, their growth is extremely slow. They are thus highly vulnerable to the slightest changes in their environment. Because of their longevity, some deep-sea corals can serve as archives of past climate conditions, which are important to understanding global climate change. It would be both interesting and instructive to observe how they will adapt to the increasing concentration of CO2 in oceans brought about by climate change. There might just be a lesson on survival for us as we too grapple with climate change.

Community-based ecosystems can also be seen among species of deep-sea sponges that offer a home to several species, both free-living and parasitic. The deep ocean has many more habitats such as brine pools, canyons and abysmal plains, each with their own unique communities that create yet other kinds of ecosystems. The brine pool, for example, is so saline that only microbial communities can survive it. The ecosystems found in the deep ocean are significant in helping us understand life and life processes. As we grapple with the changes in the climate, creatures living in extremely challenging conditions may hold some keys to unlock the processes that humanity can use to adapt to the changing abiotic conditions on land.

Clues to evolution

The existing belief is that life began in water. Evolutionary history tells us about the weird and strange creatures that once populated the earth. Scientists have also begun to hypothesise that life on earth, which began 3.5 million years ago, may have started in some inhospitable environment such as vents. They believe that the clues to the evolution of life on earth may be found there. Molecular and paleoecological studies suggest that whale falls serve as hotspots of adaptive radiation for specialised fauna; falls may well have been the evolutionary stepping stones for vent and seep life and could have facilitated the formation of other new species. The clues we need to search for extraterrestrial life may well lie in species of deep-ocean ecosystems

Samudrayaan project

Worldwide, the deep-sea life of the Indian Ocean is the least researched and studied. Extraction of the mineral deposits from the deep sea is the primary objective of the Samudrayaan project. Specifically mentioned in its resource list are the extraction of gas hydrates and hydrothermal sulphides, among other minerals. There are several yet to be described or studied species for whom these resources are home. The effects of mining on land and the environmental destruction it has led to are starkly visible. The climate change that we are grappling with is directly related to how we have misused land-based resources.

Should not our deep-sea explorations start with studying and documenting biodiversity before we mine and kill these habitats? Oceans are the last frontiers in the fight against climate change. Should we not be taking care of them, nurturing their diversity rather than destroying their fragile habitats in the name of the economy and development? Can there not be a development plan formulated that can take care of the oceans?

In order to join “elite clubs” of the world, we do not have to follow the model the developed, industrialised nations established. The ancient Indian culture that is often spoken about considers all life to be sacred and worthy of protection. Can India not use its Samudrayaan project to show the world that to be developed is to have respect for all life forms? Can we not be pioneers in finding ways to extract and utilise minerals in a sustainable manner that does not destroy but sustains fragile habitats and life, finding ways to coexist with them? Can the first step of the project be about documenting the deep-sea life of the Indian Ocean?

All images unless specified otherwise are courtesy of the National Oceanic and Atmospheric Administration (NOAA), Ocean Explorer, U.S. The author thanks the NOAA as well as Drs Craig Smith and Greg Rouse for permission to use the images.


1. https://www.whoi.edu/know-your-ocean/ocean-topics/seafloor-below/hydrothermal-vents/

2. https://www.whoi.edu/know-your-ocean/ocean-topics/ocean-life/life-at-vents-seeps/

3. https://www.whoi.edu/oceanus/feature/the- evolutionary-puzzle-of-seafloor-life/

4. https://www.youtube.com/watch?v=MPm9WOOY4f4

5. https://marinesanctuary.org/blog/whale-fall-101/- Real time video of whalefall event

6. https://ocean.si.edu/ocean-life/marine-mammals/life-after-whale-whale-falls

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