Whale fall

Whale fall is the term used for a whale carcass that has fallen to the ocean floor. Whale falls were first observed in the 1980s, with the advent of deep-sea robotic exploration.

When a whale dies in shallow water, its carcass is typically devoured by scavengers over a relatively short period—within several months. However, in deeper water (depths of 2,000 m/6,600 ft or greater), fewer scavenger species exist, and the carcass can provide sustenance for a complex localized ecosystem over periods of decades. Some of the organisms that have been observed at whale falls are giant isopods, squat lobsters, bristleworms, prawns, shrimp, lobsters, hagfish, Osedax (bone-eating worms), crabs, sea cucumbers, octopuses, clams, and even deep-sea sleeper sharks. Whale falls are often inhabited by large colonies of tubeworms. Over 30 previously unknown species have been discovered at whale falls.^{[citation needed]}

Discovery

Skeleton of a 35-ton, 13-m gray whale that has been on the seafloor of the Santa Cruz Basin (around 1700 m deep) for 18 months. Animals visible include swimming, eel-like hagfish, and thousands of amphipods and newly settled, juvenile clams.

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.

Hagfish swimming over ALVIN's instrument basket on the Santa Cruz Basin seafloor. The skeleton of a gray whale is visible in the background. The instruments in the basket are used for collecting samples of seafloor sediments and associated animals.

A whale fall was first observed by marine biologists led by University of Hawaii oceanographer Craig Smith in 1987, discovered accidentally by the submersible Alvin using scanning sonar at 1,240 m (4,070 ft) in the Catalina Basin. Whale falls have since been found by other scientists, and by military submarines. They can be found by using side-scan sonar to examine the ocean floor for large aggregations of matter.

The first sign that whale carcasses could host specialized animal communities came in 1854 when a new mussel species was extracted from a piece of floating whale blubber. Beginning in the 1960s, deep sea trawlers unintentionally recovered other new mollusc species including limpets (named Osteopelta) attached to whale bones.

Discrete ecosystem

A consistent group of organisms inhabits whale falls in all oceans. The mussels and vesicomyid clams from Alvin's discovery belonged to groups that harbor chemosynthetic bacteria which can draw energy from inorganic chemicals. Their only known habitats were sunken wood and hydrothermal vents. The lucinid clams' only other habitats were seeps and anoxic seafloor sediments. Similarly, a snail inhabited only anoxic sediments.

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.

Three stages

The Santa Catalina falls but not others, consistently exhibit three stages of decomposition.

The initial, mobile scavenger stage features hagfish and sleeper sharks that consume the soft tissue at a rate of 40–60 kilograms (88–130 lb) per day, over a period of up to two years.

The second stage, 'enrichment opportunist', also lasts up to two years. A few species of animals colonize the bones and surrounding sediments. They consume the tissue left by the scavengers.

In the final 'sulfophilic' stage, bacteria anaerobically break down the lipids embedded in the bones. Instead of oxygen, they reduce dissolved sulfate SO2−
4 and excrete hydrogen sulfide H₂S, which is toxic to animals other than certain chemosynthetic bacteria. The mussels and clams are nourished by chemosymbiotic bacteria while the limpets and snails graze on bacterial mats. Whale bones are rich in lipids, which represent 4-6% of the whale's body weight. This stage can last 50 to possibly 100 years.

Osedax

The differences regarding the Santa Catalina falls may have to do with insufficient oxygen at the site. Another contributing factor may be the presence of 1-cm (0.4 in) zombie worm (Osedax Latin for bone-devourer) Osedax inhabits whale falls in southern California, Monterey Bay, Sweden, and Japan. Osedax appendages exchange gas with the water column, but retract if disturbed. Adult Osedax have no digestive tract. Instead they tunnel their green, fleshy 'roots' into the bones to feed their symbiotic bacteria.

Stepping stones

Smith and his colleagues suggested in 1989 that such species may use whale falls as stepping-stones to extend their range across multiple chemosynthetic communities. They estimate that 690,000 carcasses/skeletons of the 9 largest whale species are in one of the three stages at any one time. 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.

The fossil record

Whale fall fossils from the late Eocene and Oligocene (34-23 MYA) in Washington and from the Pliocene in Italy include clams that also inhabited non-chemosynthetic environments. Chemosynthetic-only animals do not appear until the Miocene (23 to 5 MYA) in California and Japan. This may be because the lipid content of early whale bones was too low.

The discovery of the limpet Osteopelta in an Eocene New Zealand turtle bone indicates that these animals predate whales, including possibly inhabiting Mesozoic (251-65 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 animals evolved independently.

See also

• Deep sea communities
• Marine snow
• Detritivore

External links

• Craig Smith's paper on whale fall ecology (University of Hawaii)
• 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