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A whale fall occurs when the Carrion of a whale has fallen onto the ocean floor, typically at a depth greater than , putting them in the Bathyal zone or . On the sea floor, these carcasses can create complex localized that supply sustenance to deep-sea organisms for decades. In some circumstances, particularly in cases with lower water temperatures, they can be found at much Mesopelagic zone, with at least one natural instance recorded at 150 m (500 ft) and multiple experimental instances in the range of . Whale falls were first observed in the late 1970s with the development of deep-sea robotic exploration. Since then, several natural and experimental whale falls have been monitored through the use of observations from submersibles and remotely operated underwater vehicles (ROVs) in order to understand patterns of ecological succession on the deep seafloor.
Deep sea whale falls are thought to be hotspots of adaptive radiation for specialized fauna. Organisms that have been observed at deep-sea whale fall sites include Chordate, Arthropod, Cnidaria, Echinoderm, Mollusca, Nematode, and Annelid. New species have been discovered, including some potentially specializing in whale falls. It has been postulated that whale falls generate biodiversity by providing evolutionary stepping stones for multiple lineages to move and adapt to new environmentally-challenging habitats. Researchers estimate that 690,000 carcasses/skeletons of the nine largest whale species are in one of the four stages of succession at any one time. This estimate implies an average spacing of and as little as along migration routes. They hypothesize that this distance is short enough to allow larvae to disperse/migrate from one to another.
Whale falls are able to occur in the deep open ocean due to cold temperatures and high hydrostatic pressures. In the coastal ocean, a higher incidence of predators as well as warmer waters hasten the decomposition of whale carcasses. Carcasses may also float due to decompositional gases, keeping the carcass at the surface. The bodies of most great whales (which includes and many species of baleen whale) are slightly denser than the surrounding seawater, and only become positively buoyant when the lungs are filled with air. When the lungs deflate, the whale carcasses can reach the seafloor quickly and relatively intact due to a lack of significant whale fall scavengers in the water column. Once in the deep-sea, cold temperatures slow decomposition rates, and high Hydrostatics increase gas solubility, allowing whale falls to remain intact and sink to even greater depths.
Whales and some other large marine animals feed on and follow large aggregations of zooplankton for sustenance. Based on simple trophic structure, this would mean whales and other large zooplankton feeders can be found at higher abundance around areas of high primary production, potentially making them important exporters of carbon to depth through food falls. Biological pump models indicate that a large amount of carbon uptake by the deep sea is not supplied by particulate organic carbon (POC) alone, and must come from another source. Lateral advection of carbon, especially in coastal areas contributes to this deficit in the model, but food falls are also another source of organic carbon for the deep ocean. Various percentages of the food fall contribution to the total carbon flux to the deep ocean have been hypothesized, ranging from 0.3% to 4%.
There is growing evidence that the contribution of food falls to the deep ocean carbon flux is larger than originally proposed, especially on the local scale in areas of high primary productivity. Unfortunately, contributions of food falls to the biological pump are hard to measure and rely on a few serendipitous studies on discovered falls as well as planted carcasses with much of the deep sea carbon flux studies relying on sediment traps.
The first recorded abyssal whale fall was discovered by US Navy bathyscaphe pilots LT Ken Hanson, Master Chief George Ellis and LT Tom Vetter diving in bathyscaphe Trieste II (DSV-1) on 19 February 1977. The skeleton of the carcass, which was completely devoid of organic tissue, remained intact and collapsed flat on the seafloor. The submersible recovered a jawbone and phalanges. The whale was considered to be a gray whale based on the size of the bones and the skeleton, the lack of teeth and its location west of Santa Catalina.
The first whale fall ecosystem, which included a chemoautotrophic assemblage living on the anaerobic breakdown of organic material in whale bones, was discovered by a team of scientists led by University of Hawaiʻi Oceanography Craig Smith in 1987. The DSV Alvin observed the remains using scanning sonar at in the Catalina Basin and collected the first photographic images and samples of animals and microbes from this remarkable community.
Many other whale falls have since been found by more researchers and deep-sea explorers, as well as by naval . The increase in detection is largely due to the use of cutting-edge side-scan sonar which can minutely examine the ocean floor for large aggregations of matter. A 2022 study identified 45 known natural whale falls, 38 implanted ones, and 78 fossil ones, mostly in the Pacific, but a significant number, particularly of fossil ones, in the Atlantic.
A 2023 Scripps survey found at least 7 whale falls in an area of 135 sq mi surveyed off the California coast, with sonar evidence that may indicate up to 60 total in that area.
At whale fall sites it is common to see between three and five trophic levels present, with two main nutritional sources constituting the base of the food web. Adult whale carcasses can house up to five trophic levels, whereas juveniles more typically have three.
Recent studies also show a possible trend of "dual niche partitioning", in which scavengers tend to reach peak densities on the carcass during the day and predators are more present during the night, reducing competition between the two trophic groups. There is also a possible trend in tidal patterns and species occurrence, indicating that tides play a role in niche partitioning as well.
Similar ecosystems exist when other large volumes of nutrient-rich material fall to the sea floor. Sunken beds of kelp create kelp falls, and large trees can sink to create wood falls. In more recent years, shipwrecks have also provided bases for deepwater communities. In ecosystems formed following a whale fall event, there are four stages of ecological succession.
Chordate scavengers are also early inhabitants of whale falls. Some of these relatively large scavengers that have been recorded include hagfish, sleeper sharks, and various Osteichthyes species such as blob sculpin, Dover sole, and snubnose eelpout. Many crustacean species can also be found on whale falls, including Tanner crab and Galatheoidea. Another common crustacean inhabitant of whale falls is Amphipoda, which often show up in relatively high concentrations.
Whale falls also house cnidarians, echinoderms, and mollusks. Sea anemone, Brittle star, and Sea urchin in particular have been recorded at whale fall sites. Additionally, there are many species of Bivalvia, including members of Mytilidae and Vesicomyidae, and of marine Gastropoda, including members of the bone-eating genus Rubyspira. Marine nematodes in the genera Halomonyhystera, Anticoma, and Theristus have also been recorded, though research on them is less extensive than other whale fall taxa.
Of all taxa observed at whale falls, annelids have received the most research focus. Though marine leeches have been observed at whale falls, polychaetas tend to be the focus of much of the annelid research on whale falls. This is in part due to the number of new polychaeta species discovered in these ecosystems. Two common genera are Ophryotrocha, which displays adaptive radiation on whale falls, and the genus Osedax, which are specialists that burrow into bones. Members of Osedax can be found on whale falls across the globe, though different species have been discovered on Atlantic whale falls than on Pacific whale falls.
The discovery of the limpet Osteopelta in an Eocene New Zealand turtle bone indicates that these animals evolved before whales, including possibly inhabiting Mesozoic (251–66 MYA) reptiles. They may have survived in seeps, wood-falls and vents while waiting out the 20 million year gap between the reptiles' extinction and whales' emergence. Another possibility is that these fossils represent a prior, dead-end evolutionary path, and that today's whale fall animals evolved independently.
Overall, the four carcasses observed showed no evidence of progression past the scavenger stage. The size limitations, as well as physiological differences between large elasmobranchs and whales more than likely causes the changes observed in the communities surrounding their respective carcasses. Osedax worms have the ability to extract collagen from bones as well as lipids, enabling them to sustain themselves on bones other than the lipid-rich remains of whales. Although no Osedax were found on the non-mammalian remains in this study, their absence may have been due to the timing of observation, and the Osedax had not yet colonized the carcasses. Various studies on smaller cetaceans and other marine vertebrate food falls come to similar conclusions that these falls bring a large amount of new organic material to depth, but support mostly a scavenger community, as opposed to the diverse assemblage seen at whale falls. This conclusion can be drawn based on the knowledge that large whales have much higher lipid content in their bulk composition and bone marrow, which supports the diverse communities present in succession at whale falls.
Researchers have compared Sauropoda carcasses to modern whale fall events. The largest carcasses would have been energy rich reservoirs, and it has been argued that they may have been the primary resources of many terrestrial carnivorous dinosaurs, which were argued to have been obligate scavengers. A single dead sauropod would have had enough calories to sustain multiple big theropods for weeks or months, and since they were terrestrial, sauropod carcasses didn't float over long distances or sink into the depths so they were more available to local carnivores than modern whale falls.
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