When they sink to the bottom of the ocean, the carcasses of large marine vertebrates provide a bounty of food for deepsea scavengers -- but these large, natural food-falls have rarely been observed. A few whale carcasses have been found, and now, for the first time, researchers have discovered four large “fish-falls” on the seafloor. There lies the remarkable remains of a whale shark and three eagle rays.
The carcasses were discovered by chance. A UK team led Nicholas Higgs of Plymouth University analyzed observations from remotely operated vehicle video surveys of the seafloor on the Angola continental margin in the Atlantic. The industrial Subsea 7 Hercules ROV collected the footage for BP Angola and partners between 2008 and 2010.
The carcasses supported communities of scavenging fish (mostly eel-looking fish called eel pouts from the family Zoarcidae). Many of these actually appear to live on, or at least near, the remains. Based on a global dataset of scavenging rates, the team estimated that the carcasses provided food for weeks and even months. Each of the carcasses fed between 13 and 54 individuals.
"What really dominated were eel pouts. These normally sit around the carcass and wait for smaller scavengers -- amphipods -- to come along, and they will eat them," Higgs says. Amphipods are little shrimp-like crustaceans, and they’re always hanging out around dead whales -- though none were seen at these sites. Lots of eel pouts were sitting around the carcasses. "They seemed to be guarding it," he adds.
Here are the carcasses. The whale shark (Rhincodon typus) is A, and the rest are eagle rays (genus Mobula). And be sure to check out this video of scavengers feasting on the dead ray below (with eel pouts wiggling about), as well as a video of the whale shark.
The whale shark was found at a depth of 1,210 meters, resting dorsal-side up on the seafloor. The only parts that remained were the fleshy head, pectoral fins and girdle, and part of its spine. They think it was about 7.3 meters long and about 3,600 kilograms. The rays were found around 1,230 meters deep. These are typically between 1 to 3 meters in diameter and can weigh up to 280 kilograms. One of them (C, pictured) was being attended by 54 eel pouts. Several of those fish were actively feeding on the remnants of flesh inside the skeleton, but most of them were inactive -- typical “roosting” behavior, when they wait for small invertebrate scavengers to show up so they can eat them.
The causes of death are unknown, though they think each have been dead for a month or two when they were filmed. Using best estimates of carcass mass, the team calculated that the carcass of large fishes provide about 4 percent of the food that falls down to the seafloor region.
"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,” Higgs says. The ecosystems of fish- and whale-falls seem to be different, the team found. Whale carcasses host complex ecosystems: Sharks are the first to show up, followed by smaller opportunists like crabs and amphipods. They’ve also got bone-eating worms and specialist bacteria who break down fats.
The team didn’t see any evidence of whale-fall type communities with these cartilaginous carcasses. Their flesh are primarily muscular, without the fatty blubber layer of whales; their skeletons don’t contain lipid-rich bone marrow and they're unmineralized, so they degrade more rapidly.
The work was published in PLoS ONE last week.
Image: 2014 Higgs et al.