An experiment to see how deep-sea creatures might react to the presence of an uncommon food source—alligator carcasses—has resulted in some fascinating new science.
As many of you will recall, Gizmodo covered some of this research in April of last year. The marine biologists responsible for the experiment have finally published their long-awaited results in the open-access science journal PLOS One. The authors, led by Craig McClain and Clifton Nunnally from Louisiana Universities Marine Consortium (LUMCON), have provided some intriguing new details about the project.
Alligators and the deep seafloor may seem an unlikely combination, but these reptiles sometimes wander out far from shore, whether in search of new food sources or because they were flushed out by extreme weather, such as hurricanes. In some instances, however, these wayward alligators die while out at sea, and their bodies drift down to the seafloor.
For the creatures that live on the seafloor, these are precious gifts from above, as locally sourced meals are scarce and insufficient.
“In the deep-sea, there is no light, so there is no photosynthesis,” River Dixon, a PhD Fellow in the McClain Lab at the University of Louisiana-Lafayette and a co-author of the new study, said in an email to Gizmodo. “This means that the typical organisms that form the base of a food web, plants or plant-like organisms called phytoplankton in marine systems, are not present. Instead, food for animals in the abyss arrives via what we call ‘marine snow,’ aggregations of bits of decaying animals, faeces, and other detritus raining down from the overlying waters.”
But as Dixon explained, this ‘marine snow’ can’t possibly be enough to sustain the population below, leading to what Dixon calls a “carbon conundrum.”
“Much effort has been put into investigating whales, large fishes, and even plants like wood and kelp as a potential source for this missing carbon,” said Dixon. “We have seen alligators and crocodiles utilising marine habitats more in recent years... so we decided to do this experiment to investigate the impact of a large reptile carcass on deep-sea food webs and large reptile carcasses as a potential carbon pathway to the deep.”
For the experiment, three alligator carcasses (Alligator mississippiensi) were placed along the slope of the northern Gulf of Mexico at depths reaching 2,000 meters (6,600 feet). Heading into the experiment, the LUMCON team predicted that the tough hide of the alligators would be difficult for scavengers to munch through and that the carcasses would sit idle for a prolonged period of time.
They were wrong. Not only did various animals of the deep make quick use of this unusual food source, the experiment also resulted in the discovery of an entirely new species.
At the site of the first alligator drop, the carcass was inundated with giant isopods, who were able to penetrate the hide in under 24 hours. These football-sized creatures didn’t seem to be bothered by the reptile’s thick skin, with unsettling video showing the giant isopods ripping away at the flesh in an orgy of culinary delight. Incredibly, giant isopods can go years between meals.
The researchers visited the second alligator 51 days after it was dropped, by which time it had been stripped of all its flesh. All that remained was the skeleton, aside from a lone amphipod that was still zipping around in search of one last meal. The orientation of the bones, including the curved spine, was just as the researchers left it, but the gator’s head was upturned, probably the result of scavenger activity.
The amphipod was not the only creature seen at this drop. A brownish fuzz on the bones was later confirmed to be a newly discovered species from the Osedax genus—a group of bone-eating worms sometimes referred to as zombie worms.
“Osedax feed on the lipids within the bones of many types of vertebrates,” said Dixon. “We confirmed it as a new species through comparing the DNA of the animals that we collected to the DNA of known Osedax species. We found that the DNA was different enough to qualify our samples as a new species.”
This discovery is noteworthy because it’s the first time any Osedax have been seen in the Gulf of Mexico, and it’s the first recorded instance of the worms munching on alligator bones.
“With further experimentation, we may be able to figure out if this new Osedax species is a reptile specialist or is more broadly found on vertebrate carcasses in the Gulf,” added Dixon.
The third alligator drop was a bit different. Returning to the site eight days after it was laid down, the researchers were stunned to find... nothing. The carcass was gone. The only things left were a depression on the seafloor showing where the animal once rested and a marking device.
Dixon said she and her colleagues were shocked by the sight, or lack thereof.
“When we are in the process of an ROV dive, we are in a shipping container on the deck of a ship, huddled around a TV screen showing the video feed from the robot on the seafloor,” she said. “As we were nearing the spot where we had placed the alligator carcass just a week prior, we weren’t seeing anything. We were all looking at each other like, are we in the right place?? Surely it can’t be totally gone! But the marking device we had put next to the carcass was still there, and when we got up to it, we saw the depression that the carcass had made. We knew we were in the right spot, but we were definitely surprised, so we started canvassing the area.”
Eventually, the LUMCON researchers discovered drag marks, which led them to the weight and rope that had been tied to the carcass. Dixon said they were left 9 meters (30 feet) from the original spot—and the rope was completely bit through. Absolutely no trace of the alligator could be found.
“Whatever did that had to be huge; the carcass and weight combined to be over 36 kilograms (80 pounds), and its shape and length made it quite unwieldy,” said Dixon. “With some calculations we were able to figure out that the bite strength needed to cut cleanly through our rope was consistent with that of a large shark.”
Specifically, the researchers suspect the deed was done by a Greenland shark or sixgill shark, which are capable of eating an alligator whole.
So, three alligator falls resulted in three different observations, showing the adaptability of bottom-dwelling sea creatures, who clearly made quick work of a creature quite alien to their marine environment.