Whale fall - WikiHQ

right|thumb|A [[Chemotroph#Chemoautotroph|chemoautotrophic whale fall community in the Santa Cruz basin off southern California at a depth of , including bacteria mats, vesicomyid clams in the sediments, galatheid crabs, polynoids, and a variety of other invertebrates.]]

A whale fall occurs when the carcass of a whale has fallen onto the ocean floor, typically at a depth greater than , putting them in the bathyal or abyssal zones. On the sea floor, these carcasses can create complex localized ecosystems that supply sustenance to deep-sea organisms for decades. 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. New species have been discovered, including some potentially specializing in whale falls. 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. The bodies of most great whales (which includes sperm whales 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. This amount of organic material reaching the seafloor at one time creates a pulse equivalent to about 2000 years of background carbon flux in the 50 square meters of sediment immediately beneath the whale fall. 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. to 4%. with much of the deep sea carbon flux studies relying on sediment traps.

Discovery

thumb|The skeleton of a gray whale lies on the [[Monterey Bay|Santa Cruz Basin seafloor as a hagfish swims into view of the US Navy's deep-sea submersible Alvin.]]

The earliest indication that whale carcasses could host specialized animal communities occurred in 1854 when a new mussel species was extracted from a piece of floating whale blubber. 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.

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.

Ecology

thumb|A [[whale bone being recovered from the Santa Catalina Basin floor five years after experimental emplacement. The bone surface contains patches of white bacterial mats and a squat lobster. Hydroids have sprouted on the loop of yellow line attached to the bone. Members of Osedax have more dramatic effects in juvenile skeletons, which are not as well-calcified as adult skeletons.

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.

Of all taxa observed at whale falls, annelids have received the most research focus. Though marine leeches have been observed at whale falls, Smaller cetaceans, such as porpoises and dolphins, do not undergo the same ecological succession stages due to their small size and lower lipid content.

Stage 1

The initial period begins with "mobile scavengers" such as hagfish and sleeper sharks actively consuming soft tissue from the carcass. Consumption can be at a rate of per day. Concentration gradients of both sulfide and methane can be found around whale falls, with the highest concentration coming within one meter of the carcass, which is several orders of magnitude higher than the surrounding sediment concentrations. Methanogenesis appears to only occur in sediments as opposed to sulfur reduction, which occurs both in sediments and on the bones of the carcass.

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. However, it is suggested that the removal of large whales might have reduced the total biomass of the deep sea by more than 30%.

Contrast with other large food-falls

There have also been studies based on the carcasses of other, non-mammalian marine vertebrates that have fallen to the deep sea. In particular, the chance discovery of a whale shark carcass and three mobulid ray carcasses led to observations on the communities that form surrounding large elasmobranch falls as opposed to whale falls. Whale sharks inhabit waters of roughly 1,000 meters depth regularly, which suggests it could be a regular form of food fall in areas where it is abundant. Many eelpouts (Zoarcidae) were found surrounding the whale shark with some evidence of direct feeding as boreholes were observed on the carcass. Another theory suggests that the eelpouts were waiting for their main prey, amphipods and other small benthic animals. The three rays found were at different stages of decomposition, leading to varying assemblages found surrounding the individuals. 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.

References

External links

Smith and Baco 2003 paper on whale fall ecology (University of Hawaiʻi)
Article from NOAA's Undersea Research program (NURP)
Robin Meadows, "A Whale of a Tale"
(Science Daily), University of California, Berkeley, "Fossil Whale Puts Limit On Origin of Oily, Buoyant Bones In Whales" 14 September 2007