Diva J. Amon, Adrian G. Glover, Helena Wiklund, Leigh Marsh, Katrin Linse, Alex D. Rogers, Jonathan T. Copley, The discovery of a natural whale fall in the Antarctic deep sea, Deep Sea Research Part II: Topical Studies in Oceanography, Volume 92, August 2013, Pages 87-96, ISSN 0967-0645, http://dx.doi.org/10.1016/j.dsr2.2013.01.028.
The ocean depths can be harsh for the decomposers (microorganisms) we find in our kitchens and backyards, which need warmth, moisture, and oxygen. The temperature in the deep-sea is low, less than 3o C. The concentration of dissolved oxygen can be high or low. The biota of the deep-sea depend on food that falls from above. Most of this food comes as organic matter the size of particles, continuously falling like snow. Large food falls come as animal carcasses – whales, sharks, and large fish.
A whale fall is an oasis in a vast wasteland. The carcass of a cetacean provides flesh, shelter, and substrate and can produce a habitat that is distinct from its surroundings for decades. As the carcass decays, the species structure of the ecological community also changes, much like grasses that invade burnt land that was once a forest . The changes between ecological communities found in a whale fall reflect the stages of decomposition of the whale carcass.
Only six natural whale falls have been discovered. Most are located in the depths ranging from 150 to over 4,000 meters. Scientists use Remote Operated Vehicles (ROVs) or submersibles to study them. These devices are usually equipped with highly specialized cameras that take pictures and send a live video feed to the ship.
Diva Amon and her fellow scientists were interested the factors that drive the initial colonization and the shift of the ecological community in a whale fall. They hypothesize that what lives in or on the bone is controlled by the bone’s lipid content, known as the ‘oil gradient’ hypothesis. If some bones contain more lipids than others, then based on the ‘oil gradient’ hypothesis we can predict what organism will most likely exist on the anatomical type of bone. The more fat content a bone has the greater the sulfide production, which leads to the bone being colonized by sulfide-loving organisms.
Diva Amon and her fellow scientists used DNA barcoding of the bones to identify the whale, videography and mapping to study the site and its inhabitants, and took samples of the macro-fauna and a few bones. The images and video footage were analyzed by 1) identifying the anatomical bone type (mandible, skull, etc), 2) visually identifying the fauna, and 3) quantifying the abundance of organisms and calculating the bone surface area using images as well as data from the scaling lasers collected from the ROV (think of it as a ruler, but you are using a laser instead).
The whale skeleton was found in the South Sandwich Arc, a tectonically active region located south of the Polar Front in the East Scotia Sea 59°41.671′S, 28°21.089′W at a depth of 1447 meters. By pure luck, it was found during a video survey of the area.
Amon and her team believed that the skeleton was in the third stage, a time when the bones become a sulfide-rich environment due the microbial activity within them.
The whale was an Antartic minke (Balaenoptera bonaerensis). Most of the bones were highly degraded, with spongy bones exposed. Some bones such as the mandibles were fragmented in several places. Small bone fragments were also found within the sediment, however a large portion of it was exposed. Lipids were still present in the bone. Amon and her colleagues attempted to date the bones using the isotopic pair (210Pb/226Ra). They were not successful due to contamination, but they estimated the age of the skeleton to be 4 to 64 years old.
Most of the organisms found could be classified in four groups: 1) bacterial mats, 2) Osedax, 3) peracarides (a large group of crustaceans), and 4) gastropods (includes snails and slugs).
Lepetodrilus is a genus of small, deep-sea snails (gastropods) and hydrothermal vent limpets. Osteopeltidae (Mollusca:Gastropoda) are a new family of limpets associated with whalebone in the deep-sea. Pyropelta are small deep water limpets live near hot hydrothermal vents. Jaera and Lysianassidae are crustaceans. Ophryotrocha are gastropods.
The scientists were particularly interested in the Osedax, a genus of deep-sea siboglinid that are also called ‘boneworms’ or ‘zombie worms’. First found in 2002 in an expedition in Monterey Canyon, they are red and a colony on whale bones looks like a furry red carpet. Boneworms are related to tubeworms, critters that live around deep-sea hydrothermal vents. They grow green appendages much like tree roots that burrow into the bone, except instead of taking up water these ‘roots’ use acid-secreting enzymes digest fat and proteins from the bone. Even more peculiar, the male Osedax acts like a pair of gonads for the female – they live within the female worm and seem to be stuck in the larval stage of development. Only about a millimeter long, they are made of two parts, one that makes semen and another that is just yolk. More weirdness can be found here. The females release their eggs when food runs out. Like a vagabond, the eggs drift until they find a nice whale bones to call home.
The scientists observed that the sediment surrounding the bone was nearly black. This shows that the environment surrounding the bones were anoxic, ideal for the microbes within the bone that degrade the lipids. The microbial mats are also indicative of chemoautotropic production fueled by the sulfide.
The density of bacterial mats, Pyropelta, and Osedax differ significantly between the twelve types of anatomical types of bones. Pyropelta and bacterial mats were generally are found together but Osedax were generally not found on areas of bone with bacterial mats. They only appeared to grow on areas of bones where there were no bacterial mats. It made sense that Pyropelta was found with the microbial mats, since the genus is comprised of bacterial grazers.
The bacterial mats were less prevalent on bones predicted to have lower lipid content (such as the cervical vertebrae, thoracic vertebrae and small unidentified bone fragments) and had the most coverage on bones that had the highest lipid content (humerus and lumbar). Osedax were generally found on bones with less lipid content and were rare on bones such as the humerus. It is thought that because the Osedax burrows into the bone, it exposes the inside to oxygen and hinders microbes that need an anaerobic environment to degrade the lipids. From the scientists’ observations, it appears that the ‘oil gradient’ hypothesis is applicable to this particular skeleton.
A whale fall is a complex, localized ecosystem that changes over time. By understanding the fate of a carcass, scientists can develop a clearer picture of how life is sustained in the most remote reaches of our planet.