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Ecology

Sinking Alligator Carcasses into the Deep Ocean to See What Eats Them

 

The Deep Unknown

The deep ocean is an alien world. Life here must contend with crushing pressures, near-freezing temperatures, and a complete absence of light. At more than a thousand feet deep, it’s also the least explored and least understood ecosystem on Earth. That’s a problem, because the deep makes up about 95% of the global ocean.

HMS Challenger, the host of one of the first deep ocean expeditions (1858), from The Report of the Scientific Results of the Exploring Voyage of HMS Challenger during the years 1873–1876

It was originally thought that life couldn’t survive in the deep ocean without light to create the carbon needed for complex food chains. On land and at the ocean’s surface, light allows photosynthetic plants, algae, and other organisms to build their own sugars, forming the building blocks of an entire ecosystem’s energy cycle. Without light, scientists thought the deepest parts of our ocean wouldn’t have the energy required to sustain even simple living creatures.

That all changed at the turn of the last century when the first deep sea explorations, championed by the HMS Challenger expedition, discovered jellyfish, worms, shrimp, and several other curious species at depths beyond 1,500 feet. These discoveries set the stage for 100 years of exploration and innovation, trips led by Otis Barton, Jacques Cousteau, Don Walsh, and James Cameron, driven by a widespread and popular interest in the strange world at the bottom of the sea.

There’s still a lot we don’t know much about the deep ocean, though. We don’t fully understand how deep-sea currents affect our global climate, how ecosystems are arranged, how might we be able to sustainably harvest from deep-sea populations, or how oil spills, plastic waste, and other kinds of pollution affect deep ocean ecosystems. In the field of community ecology, one outstanding question that carries important ramifications for other fields of science is, “What does everybody eat?”

 

Skeleton of a 35-ton, 13-m Gray Whale on the sea bottom feeding a group of eels, NOAA

Outside of hydrothermal vents, which we now know offer alternative energy sources to the chemosynthetic organisms living around them (some of which can break apart the bonds of hydrogen sulfide, ferrous iron, and ammonia), most of the deep ocean is open dessert. There’s no energy creation here, so most deep ocean lifeforms are scavengers, dependent on the sunken remains of plants, algae, and animals from the surface. Whale carcasses and the deep ocean feeding frenzy the carcasses attract are probably the most well-known example among the public, but scientists have also tracked the fall of much smaller organisms, such as the tiny photosynthetic plankton that make up the base of surface food chains. Knowing how many fascinating and sometimes surprisingly large creatures (such as the Giant Oarfish at the bottom of this article) live at the bottom of the ocean, many scientists have suggested that these sunken food sources must make up much more of the deep ecosystem than we currently know about. A couple of whale carcasses aren’t enough to feed the dazzling variety of lifeforms we now know live at the bottom of our giant ocean.

Tying Weights to Alligators for Science

A deepwater Ophidiid fish off Hawaii, NOAA Office of Ocean Exploration and Research, 2015

Previous work on deep-sea food webs has focused on naturally occurring or experimentally deployed plant remains, such as large tree trunks, or chance encounters with marine animal carcasses. Each of these has revealed a complex deep-sea diversity and illuminated the importance of fallen carbon as an energy source for deep-ocean ecosystems. Whale carcasses in particular, because they are so large and contain so much fat, often host a wider diversity of deep-sea lifeforms (and not just scavengers) and can support them for longer periods. Little work has been done monitoring the results of other animal falls.

Last year, McClain et al. from the University of Louisiana attempted to remedy this gap by sinking three (dead) American alligators, Alligator mississippiensis, into the deep ocean in the Gulf of Mexico. Crocodilians may serve an important ecological role in the deep ocean, similar to sunken cetaceans, because they are globally widespread, exist in some places in high numbers or dense populations, and have large carbon-rich bodies. McClain et al. deployed their alligator carcasses at three different sites, about 2,000 meters deep, and monitored them for 51 days using a marine ROV.

In whale falls, the first few months of activity are dominated by scavenging organisms, like giant isopods and sharks, which devour the whale’s soft tissue very quickly. If the carcass is large enough, other organisms like worms and crustaceans able colonize the bones and enriched sediment around the carcass will move in. From there, chemosynthetic bacteria, other bone-eaters, and predators will move in, forming a diverse ecosystem that can last for decades. In the end, the bones of the whale serve as a substrate for suspension-feeding animals, forming a kind of deep-sea reef.

 

Sunken alligator being eaten by deepwater Giant Isopods (B. giganteus), McClain et. al., 2019

All three of the alligators the researchers deployed developed scavenger populations. The team originally speculated that the tough skin of an alligator compared to a whale might result in longer sitting times before the carcasses would be consumed, but found all three alligators were very quickly eaten. One alligator disappeared entirely, its weight dragged a few feet along the ocean floor; the researchers suspected that a large shark took the entire carcass. One of the other carcasses was completely devoid of soft tissue by the end of the 51-day monitoring period. Upon retrieval, the researchers found evidence of the bone-eating worm Osedax, indicating that the alligator carcasses might support more complex ecosystems in the future. They state their suspicion, though, that the smaller size of the alligators would prevent the formation of the long-lasting and complex ecosystems found on whale falls.

McClain et al.’s work suggests that carcass falls from the surface may be key components of deep-sea food chains and important determinants of ecosystem structure at the bottom of the ocean. Having lost a carcass to a single large scavenger, their work also shows how this surface contribution can enter the deep ocean food web in different ways. Further work could reveal interesting differences in which species congregate at crocodilian vs. cetacean carcasses, and what kinds of carcasses and environments promote more complex or longer-lasting deep-ocean ecosystems. All of this work is really just the tip of the deep ocean iceberg, though, with 95% of the ocean left to survey and so many questions still left to answer. What amazing things will deep-sea marine scientists discover next!?

To stay up to date on deep ocean discoveries, you might want to follow the National Oceanic and Atmospheric Administration Office of Ocean Exploration and Research

A Giant Oarfish in Laos caught during the Vietnam War, then called the “Mekong Dragon”, Leo Smith, US Navy

Source: McClain CR, Nunnally C, Dixon R, Rouse GW, Benfield M (2019) Alligators in the abyss: The first experimental reptilian food fall in the deep ocean. PLoS ONE 14(12): e0225345. https://doi.org/10.1371/journal.pone.0225345

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