Unnerving footage from the oil and gas industry revealed the carcasses of four colossal marine creatures resting in a small patch of seafloor near the coast of Angola, according to a study published in PLOS ONE.
Surrounding the dead rays and an expired whale shark were a cloud of scavengers — feasting on the meal of a lifetime.
Disturbing graveyard reveals much about life on Earth
"There's been lots of research on whale-falls, but we've never really found any of these other large marine animals on the sea bed," said lead study author Nick Higgs from the Universit of Plymouth's Marine Institute.
When whales die in the ocean, their carcasses quickly attract scavengers like sharks — who strip much of the whale's meat. Eventually, the whale carcass sinks to the sea bed, where smaller hungry creatures like crabs and shrimp-like critters called amphipods join in the feast.
Not even the bones go to waste, as osedax — also called "zombie worms" — consume the giant bones, as specialized bacteria break down and eat fats.
New footage shows 50 scavenging fish per carcass
However, the new footage enabled scientists to see how the frenzy of consumption goes down when other big animal carcasses strike the sea bed — in a video recorded with remotely operated vehicles (ROVs), which were initially performing a survey of the seafloor around Angola for industrial purposes.
The deceased creatures were discovered between 2008 and 2010 — strewn across a one-square-kilometer area of the sea bed. The creatures had remained dead for roughly one to two months.
Upon discovering the grim scene, researchers saw mainly scavenging fish — up to 50 per carcass.
'Absence of evidence isn't evidence of absence'
"We found three to four different types — but what really dominated were eel pouts," said Higgs. "These normally sit around the carcass and wait for smaller scavengers called amphipods to come along, and they will eat them."
"There were lots of these fish sitting around the carcasses — they seemed to be guarding it," added Higgs. But the researchers didn't find the other animals known to frequent rotting sea-bed carcasses, like bone-eating worms, the rays, and dead whale sharks.
"Absence of evidence isn't evidence of absence," remarked Higgs. "[B]ut the ecosystem does seem different to whale falls."
Decomposing large sea creatures enrich local sediment for years
This is remarkable because when large carcasses like whales fall to the seafloor, they enter a crucial part of a natural ecosystem. Different stages of decomposition of large sea creatures support varying successions of biological communities in the ocean.
Scavenging creatures eat the soft tissue in mere months, and organic fragments — also called detritus — provide enrichment for sediments in the area for more than a year, according to a blog post on the National Oceanic and Atmospheric Administration (NOAA).
Large sea creature carcasses basis for ocean ecosystem
Whale skeletons can feed biological communities for years — or even decades —as both a rigid surface for organic colonization and a source of sulfides from the decaying organic compounds of whale bones.
Microbes then live off of the energy released from subsequent chemical reactions — and become a foundation for ecosystems until the food is fully consumed. In the depths of the ocean, this constitutes a new food ecosystem, chemically powering single- and multi-cell organisms.
Whale fall scavengers also fuel deep-ocean ecosystem
Researchers estimated the carcasses of whale sharks and rays found in the video taken near the coast of Angola could provide roughly 4% of the total food that annually arrives on the local seafloor.
"These large carcass falls can be quite common and support quite a few fish in terms of the amount of food coming down there," said Higgs, in a BBC report. "[T]here may be easily enough to support fish populations."
So while the absence of the full range of sea-bed scavengers was apparent in the researchers' study — it's not only the whales, but the creatures who consume them who also eventually fall backward in the food chain, and become a source for biological communities in the dark abyssal depths of the ocean.