Where ocean titans rest
Text by Selina Tan
Interview by Lulu M
Main Research by Lulu M and Selina Tan
Additional Research by Selina Tan
Every once in a while, in the pursuit of underwater discoveries, you stumble across something unexpected – but deeply fascinating. Such were the sentiments of Dr Nicholas Higgs, a postdoctoral research fellow at the University of Plymouth’s Marine Institute, upon viewing a video recorded by remotely operated vehicles surveying the seafloor around the Southern African nation of Angola for industrial exploration.
Amazingly, the footage shows the remains of a whale shark (Rhincodon typus) and three devil rays (Mobula sp.) sitting on the seabed, at an astonishing depth of 1,200 metres – well beyond the limit of tek divers. Fed on voraciously by a range of fish, each carcass represents an island of food down below. The scavengers did not even have to travel very far for their next meal as all four carcasses were found within approximately one square kilometre of one another, over a period of two years. The video reveals a remarkable concentration of bodies, as previous estimates of nearest-neighbour distances for whale-falls (the incredible habitats created when a massive whale carcass sinks to the deep bottom) were about 16 kilometres in the ocean’s most whale-rich areas.
The discovery of so many ocean giants at this deep-sea graveyard is miraculous in itself. Finding a single large animal at such depths is a significantly rare event; only a handful has ever been discovered in the 50-odd years of deep-sea photography. While scientists have long suspected that the flesh of dead marine creatures might provide a food bonanza for specialist scavengers of the deep, corroborative evidence of such phenomena has been limited. We now know that whale-falls support specialist animals like Osedax worms, which consume the whale skeleton itself. However, despite intensive research on whale-falls, this is the first time ever that the carcasses of other large animals have been observed, let alone discovered together on one isolated portion of the ocean floor.
Researchers are confounded by the mystery of why so many dead sharks and rays were found in one place. Is there something about this site that draws large cetaceans to spend their final moments, thus forming a mass burial ground? Dr Higgs admits that there is no sure way to tell, yet says that accidental strikes from vessels and entanglement in fishing nets are possible causes of death.
“Also, it seems that the waters above this part of the seafloor are a hotspot of ocean productivity that attracts high numbers of large plankton-feeding animals like whale sharks, manta rays and whales,” says Dr Higgs, “so it is natural to get a higher concentration of their carcasses [when they die off] in this area.”
Whale sharks have only recently been documented in oceanic waters off Angola and appear to be more common at depths over 1,000 metres in this region (Weir CR, 2010). However, only the remnants of this one particular whale shark have been found so far, at a depth of exactly 1,210 metres. As well, the occurrence of Mobulid rays (genera: Mobula and Manta) off Angola was poorly investigated until recent hydrocarbon exploration facilitated dedicated surveys for marine megafauna. Additional observations at exploration drilling locations revealed occasional sightings in this region, but there have been none of the “death windfalls” of the kind shown in the recent video footage. After this rare find, scientists are intensively researching the unforeseen benefits of large food-falls – and venturing ever deeper into the realms where ocean titans rest.
[A] The observed whale shark carcass is estimated to measure approximately 7.3 metres, equating to about 3,600 kilogrammes in body mass
[B] The remains of an individual Mobula were found at a depth of 1,233 metres
[C] A second mobulid carcass was found roughly 1.5 kilometres west of B, at a depth of 1,237 metres
[D] A third mobulid carcass was found 180 metres to the east of B, however, its head was pointing towards the north-northwest direction, indicating that it was not simply a re-sighting of B
A Gigantic Ecological Role
The carcasses of large pelagic vertebrates that sink to the seafloor represent a bounty of food to the deep-sea benthos. The natural mortality of the animals that exist in surface waters brings about many unexpected but positive dividends to the environment.
Researchers estimate that the carcasses of the massive whale shark individual and three devil rays found
off Angola could provide about four percent of the total food (synonymous with carbon) that arrives at the region’s deep-sea floor, a significant contribution as most benthic animals (living below 200 metres) are reliant on detritus – mainly dead plankton and faecal pellets produced by zooplankton – from surface waters as their primary food source. While most detritus reaches depths as millimetre-sized particles of marine snow, the remains of large plants, algae and animals arrive as bulk parcels that create areas of organic enrichment.
In productive stretches where a high number of large animals consume more plankton, the efficiency of the ocean ecosystem in removing carbon from the upper ocean is effectively increased. When these huge beasts die, they continue to benefit the environment greatly. Larger creatures have lower rates of predation than smaller ones, so a higher proportion of their biomass is exported to the deep rather than recycled in the pelagic food chain.
After sinking, the carbon locked up in their bodies is effectively removed from the surface-atmosphere system permanently via the biological pump, the ocean’s biologically driven sequestration of carbon to the deep sea. The particles transferred to the sediment remain there for thousands of years, and it is this export of carbon that is responsible for ultimately lowering atmospheric carbon dioxide. One study showed that by rebuilding depleted whale populations, their sinking carcasses would remove 160,000 tonnes of carbon per year, equivalent to planting some 110,000 hectares of forest.
Thus, the surface oceans and the deep are closely linked. Ocean giants can and do isolate a significant amount of carbon to the ocean’s bottom, helping scientists to better understand how our marine ecosystems can help to maintain conditions on Earth.
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