Two kilometres below the surface of the Indian Ocean, where the seafloor splits apart along ancient volcanic ridges and superheated water blasts upward through cracks in the earth's crust, life has found ways to flourish that would have seemed impossible to a biologist a century ago. Hydrothermal vent communities are among the most extraordinary ecosystems on the planet: dense, thriving, organised entirely around chemical energy rather than sunlight, sustained by bacteria that extract life from poisonous gases instead of air and water.

Scientists have been studying these communities for decades. Yet a research team surveying the Indian Ocean ridge system over a series of expeditions spanning ten years has now found something that nobody had noticed before, or at least nobody had looked for carefully. Buried within the accumulated moulted shells of vent shrimps, in dense colonies packed into deposits on the seafloor beside active volcanic chimneys, they found giant worms up to thirty centimetres long, living in conditions that most animals could not survive for a minute.

The discovery, published in Deep-Sea Research Part I, raises fundamental questions about how energy flows through vent ecosystems and suggests that a significant food source at these extraordinary habitats has been hiding, quite literally, beneath our feet.

Life at the Edge of the Possible

To understand why this discovery matters, it helps to understand what hydrothermal vent communities are and why they are so unusual. When tectonic plates pull apart along mid-ocean ridges, seawater seeps down through cracks in the oceanic crust and is heated to extreme temperatures by the magma below. As it rises back toward the seafloor, it dissolves metals and sulfur compounds from the rock, emerging through chimneys as superheated fluid that can reach temperatures of several hundred degrees Celsius.

This fluid is toxic to most life. But where it meets the cold surrounding seawater and cools, a narrow zone of chemistry exists in which specialised bacteria can oxidise hydrogen sulfide to extract energy. These bacteria are the foundation of the entire vent food web. They are consumed by animals that either eat them directly or carry them as internal symbionts, and those animals in turn are eaten by predators and scavengers. The whole system runs without a single photon of sunlight.

The shrimp species at the centre of this story, known scientifically as Rimicaris kairei, is one of the most characteristic inhabitants of Indian Ocean hydrothermal vents. It lives in dense swarms around active chimneys, hosting its bacterial partners on specialised structures in its gill chamber rather than in its gut, and moulting its exoskeleton regularly as it grows. These moulted shells, called exuvia, do not simply drift away. They accumulate in deposits below the chimney complexes where the shrimps live, forming thick mats of organic material on the seafloor.

It was inside those mats that the worms were found.

Ten Years of Looking

The research team spent a decade visiting hydrothermal vent fields along the southern Central Indian Ridge, the Rodriguez Triple Junction and the northern South East Indian Ridge, conducting 85 dives across eleven active vent sites using remotely operated vehicles equipped with cameras and manipulator arms. The surveys were primarily focused on mineral exploration but routinely recorded the biology of the vent fields they visited.

During these surveys, the researchers identified 17 distinct organic deposits at nine of the eleven vent fields. Nine of these deposits consisted of shells from a large mussel species called Bathymodiolus septemdierum, often mixed with the shells of another mollusc. Seven consisted of accumulated Rimicaris kairei exuvia. One consisted of recently deposited material from the scaly foot gastropod, an extraordinary animal that builds its shell from iron sulfide minerals.

When the team examined the exuvia deposits more closely, they found something unexpected. At two of the seven exuvia deposits, separated from each other by 241 kilometres, they recovered worms belonging to the family Capitellidae. These animals were up to 30 centimetres in length and occurred in dense aggregations within the material, living and feeding in the interior of the deposit rather than on its surface.

The discovery was confirmed most dramatically during the 2023 expedition to a newly discovered vent field. There, the team used footage from their remotely operated vehicle to build a detailed three dimensional reconstruction of one side of an active chimney complex, achieving a resolution of 3.5 millimetres per pixel. The reconstructed exuvia deposit covered 4.8 square metres of seafloor below the chimney, and within it, capitellid worms were observed and sampled directly.

"The capitellids occurred in dense aggregations and may contribute significantly to the food web of active hydrothermal vent fields, representing a possible overlooked food source for benthic and demersal predators."

KEY FACTS

What is a capitellid worm? A member of the family Capitellidae, a globally distributed group of polychaete worms known as deposit feeders. They live within and consume organic rich sediments, playing important roles in breaking down organic matter, stimulating bacterial growth and recycling nutrients. They are often found in organically enriched environments including polluted estuaries, sediments beneath fish farms and, as this study now confirms, hydrothermal vent deposits.

What is an exuvia deposit? An accumulation of the moulted exoskeletons of crustaceans such as shrimps. When vent shrimps like Rimicaris kairei shed their shells, these lightweight structures drift downward and collect in sheltered areas below the chimney complexes where the shrimps congregate. Over time, these accumulations can form substantial organic deposits rich in carbon and nitrogen.

What makes vent shrimp exuvia a good food source? Rimicaris kairei carries episymbiotic bacteria on its body surfaces, which are shed along with the exoskeleton during moulting. These bacteria represent a concentrated source of chemosynthetically produced organic carbon. The exuvia also contain chitin, a structural carbohydrate, and other organic compounds that provide nutrition for deposit feeding organisms able to break them down.

What does it mean for an animal to be an ecosystem engineer? An ecosystem engineer is a species that physically modifies its habitat in ways that create conditions enabling other species to live there. The capitellid worms appear to compact and stabilise the exuvia deposits through their feeding activity, potentially transforming loose, unstable material into a firmer substrate that other animals can colonise. This function, analogous to the way tubeworms at other vent sites create physical structures that shelter diverse communities, may make the capitellids far more ecologically important than their role as consumers alone would suggest.

What the Worms Are Doing There

Capitellid worms are well known to science in other contexts, but those contexts are very different from deep sea hydrothermal vents. On Earth, they are typically found in organically enriched, oxygen depleted sediments: beneath fish farms where waste accumulates on the seafloor, around sewage outfalls, in polluted estuaries. They are opportunists, capable of colonising disturbed and degraded habitats that most animals cannot tolerate, thriving precisely because they can function in conditions of low oxygen and high sulfide that would kill less specialised creatures.

In those familiar settings, capitellids serve a specific and important ecological function. By feeding on and moving through organic rich sediment, they stimulate the growth of the bacteria that decompose organic matter, accelerating nutrient recycling and making resources available to other animals. They are, in the language of ecology, deposit feeders, and their feeding activity changes the physical and chemical properties of the material they inhabit.

The same processes appear to be operating inside the exuvia deposits at the Indian Ocean vents, but in an environment that nobody had previously considered as capitellid habitat. The worms are consuming the accumulated moulted shells of the shrimps, processing the organic material within them and stimulating bacterial decomposition. In doing so, they appear to be altering the physical structure of the deposits themselves.

The researchers noticed that the exuvia deposits where capitellids were present appeared markedly more compact and compressed than deposits at other sites where no capitellids were found. This difference in physical structure suggests that the feeding and burrowing activity of the worms is binding the loose exuvia together, in a process the authors compare to the way mangrove root and fibre mats stabilise coastal sediments. The result is a firmer, more structurally coherent deposit that other animals can colonise more readily.

This potential engineering role is reflected in the diversity of species found at the deposits. In exuvia deposits where capitellids were present, the team counted up to ten different animal species and representatives of all the major feeding strategies known from hydrothermal vent food webs, including detritivores, grazers, predators, scavengers and the symbiont hosting shrimps themselves. In deposits without capitellids, the number of associated species was lower and the range of feeding strategies narrower.

An Overlooked Source of Carbon

The broader significance of these deposits extends beyond the worms themselves. Hydrothermal vent food web studies have long recognised that particulate organic matter, the scattered fragments of dead organisms and biological waste that drift through vent ecosystems, plays an important role in sustaining non-symbiotic fauna. But those studies have treated this material as dispersed and scattered, spread thinly across the vent field.

What the new observations reveal is that this organic material can also concentrate into substantial local deposits, accumulating in specific locations around active chimneys and remaining stable for years. The same exuvia deposit at one vent site was observed and photographed in 2013 and again in 2016, still intact and still inhabited, suggesting a continuous supply of new material from the shrimps moulting above. Another deposit was revisited in 2018 and 2023, and showed evidence of increasing accumulation over that five year period.

This temporal stability has implications for how energy flows through the broader vent ecosystem. A stable, concentrated deposit of organic material represents a reliable food source not just for the animals living within it but for the mobile predators and scavengers that visit from surrounding areas. The fish, crabs, polychaetes and other species observed on the surfaces of these deposits are drawing energy from material that has already been processed by the worms living beneath, in a cascade that the researchers suggest may contribute meaningfully to carbon flow through the entire vent field.

The capitellids themselves may also become a food source. Dense aggregations of large, protein rich worms living in predictable locations would represent an attractive target for predators, potentially routing chemosynthetically produced carbon upward through the food web in ways that existing models of vent energy flow have not accounted for.

A Species That May Be New to Science

One further dimension of the discovery deserves attention. The capitellid worms found in the Indian Ocean exuvia deposits are likely a species new to science. The animals reach lengths of up to 30 centimetres, making them unusually large for their family, and their occurrence specifically within exuvia deposits at deep sea hydrothermal vents represents a habitat that no previously described capitellid species has been recorded from. Formal taxonomic description awaits detailed morphological and genetic analysis of the collected specimens, but the researchers are confident that what they have found is not simply a known species far from home.

The patchy distribution of the worms across the Indian Ocean vent system is itself informative. The two sites where capitellids were confirmed, separated by 241 kilometres, demonstrate that the worms are not a purely local phenomenon. Yet other exuvia deposits that appeared structurally similar were uninhabited. The factors controlling where the worms establish themselves and where they do not remain unclear: local differences in temperature, oxygen concentration, the intensity of vent fluid emissions, or the quality and quantity of available organic matter may all play roles. The dispersal biology of capitellid larvae, which may be able to travel considerable distances in the water column before settling, also shapes which deposits get colonised and which do not.

What is clear is that these worms, wherever they do establish themselves, are changing the habitat around them in ways that support a richer and more complex community than would otherwise exist. In an ecosystem already remarkable for its improbable vitality, they represent an additional layer of ecological organisation that was hiding, until now, in plain sight.

Publication Details: Year of publication: 2025 Journal: Deep-Sea Research Part I Publisher: Elsevier Volume / Article: Volume 220, Article 104489 DOI: https://doi.org/10.1016/j.dsr.2025.104489

Credit & Disclaimer: This article is based on the peer reviewed research paper. All scientific facts, findings, and conclusions presented here are drawn directly from the original study and remain unchanged. This popular science article is intended purely for general educational purposes. Readers are strongly encouraged to consult the full research article for complete survey data, species tables, and detailed ecological analysis.