Whale fall

A whale fall occurs when the carcass of a whale has fallen onto the ocean floor, typically at a depth greater than 1,000 m (3,300 ft), putting them in the bathyal or abyssal zones.^[1] On the sea floor, these carcasses can create complex localized ecosystems that supply sustenance to deep-sea organisms for decades.^[1] In some circumstances, particularly in cases with lower water temperatures, they can be found at much shallower depths, with at least one natural instance recorded at 150 m (500 ft) and multiple experimental instances in the range of 30-382 m (100-1300 ft).^[1] Whale falls were first observed in the late 1970s with the development of deep-sea robotic exploration.^[2] Since then, several natural and experimental whale falls have been monitored^[1][3] 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.^[4]

Deep sea whale falls are thought to be hotspots of adaptive radiation for specialized fauna.^[1] Organisms that have been observed at deep-sea whale fall sites include chordates, arthropods, cnidarians, echinoderms, mollusks, nematodes, and annelids.^[1][5] New species have been discovered, including some potentially specializing in whale falls.^[1] 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.^[1] 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.^[6] This estimate implies an average spacing of 12 km (7.5 mi) and as little as 5 km (3.1 mi) along migration routes. They hypothesize that this distance is short enough to allow larvae to disperse/migrate from one to another.^[6]

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.^[1] Carcasses may also float due to decompositional gases, keeping the carcass at the surface.^[7] The bodies of most great whales (which includes sperm whales and many species of baleen whale^[8]) are slightly denser than the surrounding seawater, and only become positively buoyant when the lungs are filled with air.^[9] 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.^[1] Once in the deep-sea, cold temperatures slow decomposition rates, and high hydrostatic pressures increase gas solubility, allowing whale falls to remain intact and sink to even greater depths.^[7]