A whale fall happens when a whale’s body sinks to the ocean floor, usually in areas deeper than 1,000 meters (3,300 feet), which are called the bathyal or abyssal zones. These whale remains can create unique ecosystems on the seafloor that provide food for deep-sea life for many years. In some cases, especially in colder water, whale falls can occur at shallower depths. One example was found at 150 meters (500 feet), and others were studied in depths between 30 and 382 meters (100–1,300 feet). Whale falls were first discovered in the late 1970s when scientists used deep-sea robots to explore the ocean. Since then, researchers have used submersibles and remotely operated underwater vehicles (ROVs) to study whale falls and learn how life changes over time on the seafloor.
Deep-sea whale falls are believed to be important places where new species develop. Animals found at these sites include chordates, arthropods, cnidarians, echinoderms, mollusks, nematodes, and annelids. Scientists have found new species that may live only at whale falls. It is thought that whale falls help increase the variety of life by offering new environments for species to grow and change. Researchers estimate that about 690,000 carcasses or skeletons from the nine largest whale species are in one of four stages of change at any time. This suggests these whale falls are about 12 kilometers (7.5 miles) apart, and sometimes as close as 5 kilometers (3.1 miles) along whale migration paths. Scientists believe this distance is short enough for baby animals to move between whale falls.
Whale falls can happen in the deep open ocean because of cold temperatures and high water pressure. In shallower coastal areas, more predators and warmer water cause whale bodies to break down faster. Whale remains may also float because gases from decomposition can make them buoyant. Most large whales, such as sperm whales and baleen whales, are slightly heavier than the water around them. They only become buoyant enough to float when air fills their lungs. When their lungs empty, the whales sink quickly to the seafloor, often staying whole because there are few scavengers in the water above. In the deep sea, cold temperatures slow decomposition, and high water pressure increases gas solubility, allowing whale falls to remain intact and sink to even greater depths.
Contribution to the biological pump
A typical whale carcass holds about two tonnes of carbon, which is similar to the amount of carbon that reaches one hectare of the deep ocean floor over 100 to 200 years. When this large amount of organic material reaches the seafloor at once, it creates a sudden increase in carbon movement equal to about 2000 years of normal carbon flow in the 50 square meters of sediment directly below the whale carcass. This supports the unique communities that form around whale carcasses but may also affect the biological pump, which moves organic material from the surface ocean to deeper layers.
Whales and other large marine animals eat zooplankton, which gather in large groups. Based on basic food chain relationships, this means whales and similar animals are more likely to be found in areas with high levels of primary production, where they may help move carbon to deeper ocean layers through food falls. Models of the biological pump show that the deep sea takes in a lot of carbon that is not only from tiny organic particles but also from other sources. Movement of carbon along the ocean surface, especially near coasts, helps explain this, but food falls also provide organic carbon to the deep ocean. Scientists have guessed that food falls may contribute between 0.3% and 4% of the total carbon flow to the deep ocean.
New evidence suggests that food falls may contribute more carbon to the deep ocean than previously thought, especially in areas with high levels of primary production. However, measuring the role of food falls in the biological pump is difficult and depends on rare discoveries of whale carcasses and experiments with placed carcasses. Most studies of deep ocean carbon movement rely on tools called sediment traps.
Discovery
The first sign that whale carcasses might support special animal groups happened in 1854 when a new type of mussel was found on a piece of floating whale blubber. By the 1960s, deep-sea fishing boats accidentally found other new types of shellfish, such as limpets called Osteopelta, attached to whale bones.
The first recorded whale carcass on the deep ocean floor was found in 1977 by US Navy pilots LT Ken Hanson, Master Chief George Ellis, and LT Tom Vetter while diving in the bathyscaphe Trieste II. The whale’s skeleton, which had no soft tissue left, was flat on the seafloor. The submersible collected a jawbone and finger bones. Scientists believed the whale was a gray whale because of the bone size, lack of teeth, and its location near Santa Catalina.
In 1987, a team led by oceanographer Craig Smith from the University of Hawaiʻi discovered the first whale fall ecosystem. This community included animals that lived on chemicals from the breakdown of whale bones. The submersible DSV Alvin used sonar to study the remains at 1,240 meters (4,070 feet) in the Catalina Basin and collected the first photos and samples of the animals and microbes living there.
Since then, many more whale falls have been found by scientists, explorers, and submarines. Better technology, like side-scan sonar, has helped locate these sites. A 2022 study found 45 natural whale falls, 38 placed by humans, and 78 ancient ones, mostly in the Pacific, with many ancient ones in the Atlantic.
A 2023 study by Scripps Institution of Oceanography found at least 7 whale falls in a 135 square mile area off California’s coast. Sonar data suggests there may be as many as 60 whale falls in that region.
Ecology
Whale falls are found unevenly across the ocean and over time, often near areas where whales travel. Many types of animals live at these sites, and some species, like mussels and vesicomyid clams, have special bacteria that use chemicals like sulfur for energy. Before whale falls were studied, these same bacteria were only known to live in sunken wood or deep-sea vents. Lucinid clams were also only found in areas with carbon-rich sediments or low-oxygen environments. Osedax, a type of deep-sea worm, helps break down whale bones by releasing acid, which allows other animals to access nutrients inside the bones. This process increases the variety of life in the deep sea by making it easier for rare species to live on whale bones. Osedax has a stronger effect on young whale bones, which are less hard than adult bones.
At whale fall sites, there are usually three to five levels of food chains, with two main sources of food forming the base. Adult whale carcasses can support up to five levels, while younger whales typically support three.
Recent research suggests that scavengers (animals that eat dead tissue) are most active during the day, while predators (animals that hunt others) are more common at night. This pattern may reduce competition between these groups. Tides also seem to influence where certain species live, possibly helping them avoid competition.
Similar ecosystems form when other large, nutrient-rich materials fall to the ocean floor, such as sunken kelp forests (kelp falls), large trees (wood falls), or shipwrecks. After a whale fall, ecosystems go through four stages of change.
Many types of animals live at whale falls, including some species that were discovered recently. Microbes, such as sulfur-oxidizing bacteria, sulfate-reducing bacteria, and methane-producing archaea, are among the first to live on whale falls. Sulfate-reducing bacteria like Desulfobacteraceae and Desulfobulbaceae are common, as are methane-producing archaea like Methanomicrobiales and Methanosarcinales. Heterotrophic microbes, which break down materials like collagen, often arrive first and help prepare the environment for chemosynthetic microbes. These microbes form mats that support larger animals, such as certain types of worms.
Chordate scavengers, like hagfish, sleeper sharks, and some fish species, are among the first animals to arrive at whale falls. Crustaceans, such as tanner and galatheid crabs, and amphipods (small crustaceans) are also common. Cnidarians (like sea anemones), echinoderms (like brittle stars and sea urchins), and mollusks (like clams and snails) live at whale falls. Bivalves, such as mussels and clams, and marine snails, like those in the genus Rubyspira, are also found there. Marine nematodes in certain groups have been recorded, though less is known about them.
Annelids, such as polychaete worms, have been studied the most at whale falls. Some species, like Ophryotrocha and Osedax, are especially common. Osedax worms are specialized for living in whale bones and are found worldwide, though different species live in different ocean regions.
Whale falls go through four stages of decomposition, which depend on the size of the whale, water depth, and other factors. Large whales pass through all four stages, while smaller whales or partial carcasses may skip some stages. Smaller cetaceans, like dolphins and porpoises, do not follow the same process because of their size and lower fat content. Osedax worms may also influence these differences.
The first stage involves mobile scavengers, like hagfish and sharks, eating soft tissues. This stage can last months to 1.5 years.
The second stage begins when animals colonize bones and sediments contaminated by organic material from the carcass. This stage can last months to 4.5 years.
The third stage involves bacteria that break down lipids in bones using sulfur compounds instead of oxygen. These bacteria produce hydrogen sulfide, which is toxic, so only specialized microbes survive. The bacterial mats support animals like mussels, clams, and snails. Whale bones contain 4–6% lipids by weight, so this stage can last 50 to 100 years.
Some scientists suggest a fourth stage, called the "reef stage," where only minerals remain in the bones, providing a hard surface for filter feeders and other animals.
A process called methanogenesis (methane production) can occur near whale falls. Methane-producing archaea live in low-oxygen sediments, but they are not usually found with sulfur-reducing bacteria. Whale falls support both types of microbes, showing that the area has enough resources for both. Methane and sulfur concentrations are highest near the carcass, with levels much higher than in surrounding sediments. Methane production happens in sediments, while sulfur reduction occurs in both sediments and bones. The presence of sulfur-reducing bacteria in both areas helps whale falls support deep-sea life for long periods.
Paleontology
Fossils from whale falls in the late Eocene and Oligocene (34–23 million years ago) in Washington and from the Pliocene in Italy show clams that lived in environments that did not rely on chemical processes for energy. Animals that only use chemical processes for survival first appear in the Miocene (23–5 million years ago) in California and Japan. This may be because early whale bones had too little fat. As ancient whales adapted to live in open ocean areas and dive deeper, their bodies changed. They grew larger, had bones with less density, and developed more fat. This increase in fat content helped create communities in the deep sea that rely on chemical processes for energy.
The discovery of a limpet called Osteopelta in an Eocene New Zealand turtle bone suggests that these animals evolved before whales. They might have lived on reptiles from the Mesozoic era (251–66 million years ago). These limpets may have survived in areas with natural seeps, fallen wood, or vents during the 20 million years between the extinction of reptiles and the rise of whales. Another possibility is that these fossils represent an earlier, unsuccessful evolutionary path, and modern whale fall animals evolved separately.
Anthropogenic effects
Some scientists believe that the whaling industry has affected a process that helps move carbon from the surface to the deep ocean. This process may have been harmed by the removal of many large whales, which reduces the number of whale falls. Whale falls are events where dead whales sink to the ocean floor, providing food and habitat for deep-sea communities. The impact of this change on these communities is not fully understood. However, it is thought that removing large whales may have reduced the total amount of life in the deep sea by more than 30%. Whales carried large amounts of carbon, which was released into the deep ocean during whale fall events. Whaling has therefore reduced the deep sea’s ability to store carbon. Carbon stored in the deep ocean can remain there for hundreds to thousands of years, supporting deep-sea life. Scientists estimate that, in terms of storing carbon, each whale is equal to the carbon storage of thousands of trees.
Contrast with other large food-falls
Studies have also examined the remains of other non-mammalian marine vertebrates that sank to the deep sea. For example, the accidental discovery of a whale shark carcass and three mobulid ray carcasses allowed scientists to compare the communities that form around large elasmobranch falls to those around whale falls. Whale sharks are commonly found in waters about 1,000 meters deep, suggesting that their remains may regularly provide food in areas where they are plentiful. Many eelpouts (Zoarcidae) were found near the whale shark, and some boreholes on the carcass indicated direct feeding by these fish. Another idea is that the eelpouts were waiting for their main prey, such as amphipods and other small bottom-dwelling animals. The three rays were at different stages of decay, leading to different groups of organisms around each. More intact ray carcasses had more scavengers, including species typically found near whale falls, like hagfish. Around the least intact ray, a bacterial mat was seen in the enriched zone, but no clams or mussels, which are common at whale falls, were present.
The four carcasses studied showed no signs of moving past the scavenger stage. The differences in community composition around elasmobranch and whale carcasses are likely due to their size and physiological differences. Osedax worms can extract collagen and lipids from bones, allowing them to survive on bones other than the lipid-rich remains of whales. However, no Osedax worms were found on the non-mammalian remains in this study, which may have been because the observation timing was too early for the worms to colonize the carcasses. Other studies on smaller whales and other marine vertebrate remains also show that these carcasses bring large amounts of organic material to the deep sea but mostly support scavenger communities, unlike the more diverse groups seen at whale falls. This is because large whales have much higher lipid content in their bodies and bones, which supports the varied life stages found at whale falls.
Researchers have compared sauropod carcasses to modern whale falls. The largest sauropod remains would have been rich sources of energy, and some scientists believe they may have been the main food source for many land-dwelling carnivorous dinosaurs, which were thought to only eat dead animals. A single dead sauropod could have provided enough energy to sustain several large theropod dinosaurs for weeks or months. Unlike modern whale falls, sauropod carcasses did not float far or sink deeply, making them more accessible to local land predators.