Whale Falls: Oases in The Deep
The worlds deep oceans (below 200 m) were once thought to be devoid of life. In fact, the depths of the oceans below the continental shelf, harbour more life in sediments than do tropical rain forests, or shallow water reefs. Much of the life is limited by the fact that there is limited hard substrate in the deep ocean. However, there may actually be more hard substrate surfaces than previously known. Modern era anthropogenic hard substrates from ship wrecks, cargo containers, all have been deposited along the sea bed below historic and present day shipping routes. Other natural hard substrate is also deposited on the sea bed in the form of rafted gravels and boulders, dropped by icebergs making their voyages from glaciers in to the open oceans. Biological material such as wood is also deposited, after long journeys from the rivers and coastlines past the continental shelfs of the terrestrial world to finally sink, carrying nutrients to the depths. This also includes whale carcasses that form some of the most diverse and transitional species rich point-source organic enrichment; habitat sites within the deep sea.
Chemoautotrophic whale-fall community, including bacteria mats, vesicomyid clams in the sediments, galatheid crabs, polychaete worms, brittle stars, sea anemones and a variety of other invertebrates. The 35 ton Grey Whale was originally implanted on the seafloor at 1674 m depth in the Santa Cruz Basin in 1998. This image was captured 6 years later by Craig Smith from the University of Hawaii.Whale fall communities were first speculated to exist during the so called “golden age of whaling”. Given that an estimated 390,000 carcasses per year fell to the sea floor, prior to the beginning of whaling in the 1800s the 1850s, new whale fall specialist species have been being discovered on whale carcasses starting in 1854, when geologist S. P. Woodward, discovered a mytilid mussel occupying the blubber of a dead whale, floating off the cape of Good Hope. This species later found to be of the genus Adipicola pelagica, was again reported in 1927, on whale remains in the North Atlantic. Then again in 1967 on a whale skull brought up from 439 m off South Africa. Another species of the same genus A. simpsoni was described living in abundance on whale skulls trawled from the sea floor to the west of the UK this species now accepted as idas simpsoni. During the mid 1960s, the increased practice of deep water trawling prompted the discovery of numerous whale carcasses and their associated communities. These discoveries were further advanced by the first explorations of the deep sea by submersibles; during the late 70s, this increase of newly discovered species prompted further experimental studies to ascertain the stages of whale carcass faunal succession and led to the documentation of molluscs such as gastropods, mussels, harpacticoid copepods, sipunculans and polychaetes, all found to have been living on whale bones.
The first of three stages of whale fall succession lasts for months to years, consisting of mobile scavengers drawn to the carcass by their olfactory senses (sense of smell). During this stage, species of deep water sharks of the family Somniosidae, commonly (but not correctly…) all known as sleeper sharks, fish known as rattails, hagfish, and invertebrate scavengers such as Galatheid crabs, and Brittle stars feast on the soft body tissues. Around 40 to 60 Kg average can be consumed each day during this stage; meanwhile, the whale also leaks out fats and fluids that seep in to the surrounding sediments, further enriching the area of the of the fall approximately 50 m2
The second stage begins after all the soft tissue is removed over months and years; and is exemplified by the colonisation of the exposed bones and organically enriched sediments. Vast congregations of crustaceans, polychaete worms and Osedax worms, up to 40,000 m2 gather, while bacteria form dense microbial mats. But during this stage the bones are also changing. As they start to decompose they begin to release sulphide, due to the decomposition of the lipids (fats) within the bone, paving the way for the third stage.
Stage three is the longest stage, and can last for decades. It is also the most species rich of the lot, with an average of 185 macro-fauna: species that can be seen with the naked eye. This stage is known as the sulphophilic, sulphur loving, stage. Categorised by the accumulation of a rich variety of chemoautotropic organisms such as Idas washingtonia (Bivalve Mollusc), Ilyarachna profunda (Isopod), and Gastropods Cocculina craigsmithi Pyropelta corymba and Pyropelta musaica, to name a few. Using the sulphur oxidising bacteria to feed on the sulphide produced by the breaking down of the skeleton, the whale fall communities occupy a tightly defined niche.
These species have rarely been documented in other habitats, reported species diversity and richness outnumbers that of deep sea vents and is comparable to cold water seeps. Furthermore, given that an estimated 390,000 carcasses per year fell to the sea floor, prior to the beginning of whaling in the 1800s, and that within our lifetime the approximate numbers falling to the ocean floor are estimated to be six times less at 65,000 a year. The question should be not are these species whale fall specialists but how many species of whale fall specialist organisms have already been lost due to anthropogenic influence.
(Video credit: Sweet Fern Productions)