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Biology

The backbone of an ecosystem: bone-eating zombie worms control biodiversity at deep-sea whale falls

It’s the circle of life: everything must die. We are familiar with vultures picking on roadkill or mushrooms recycling the remains of organisms in our backyard, but you probably do not think about what happens to the animals that die in the ocean.  You can imagine that smaller animals are picked up by scavengers pretty quickly, but what happens to the animals that are too big to be easily inhaled by sharks or fishes? What happens when a whale dies?

Large animals like whales tend to sink deep down to the bottom of the ocean, which can be miles below the surface. These deepest reaches of our oceans are cold (often reaching near-freezing temperatures) and dark (sunlight cannot penetrate waters deeper than 1000 m – a little over half a mile – below the surface). The majority of life on Earth requires sunlight: primary producers (i.e. phytoplankton and algae) use the sun’s energy to make their own food and these primary producers are in turn eaten by all larger organisms. Without sunlight, deep-sea communities must rely heavily on food that falls down from the surface. This food may be in the form of marine snow (a slurry of mixed live and dead plankton, pieces of other dead animals, poop, and other delicious bits of organic matter) or larger organisms like kelp, tree trunks that have been lost at sea, and, of course, whales.

Whale falls are critical components to deep-sea food webs that give a large surge of food to a relatively barren environment – like a buffet popping up in the middle of the desert – and host a diversity of different organisms. Sharks arrive first to take a bite, opening up the carcass

Figure 1: A white bacterial mat can be seen on this whale skeleton as the bacteria decompose the bones. Image credit: Wikimedia commons

and allowing other, smaller organisms to join the feast. Hagfish, worms, crabs, and all sorts of other animals then congregate around the whale and take advantage of the food. Eventually, after several months or years, the whale’s remains are picked away, leaving only a cleaned skeleton. But the buffet is by no means closed for business yet. Even bones are packed full of nutrients and loaded with fats. The problem, however, is that it is challenging to break open the hard, calcified bones and reach such nutrients. Certain bacteria are able to break down the fats within bones over years to decades (Figure 1). But bacteria are not the only organisms capable or breaking down bones.

Zombie worms, in the genus Osedax, are pretty bizarre worms that have been found decomposing skeletons in the oceans around the world in both shallow and deep waters (Figure 2). These worms burrow into the bone by secreting a type of acid and then digest the fats stored within the bone. Osedax worms do not have a mouth or digestive tract, and appear to rely on a symbiotic relationship with bacteria that help them digest the bone. Not weird enough for you? Male Osedax worms are microscopic and are much smaller than the female worms that are visible to naked eye. In fact, you would not see male worms on their own at all, even if you had a microscope, because in many species the male worms live inside the females!

Figure 2 – A single zombie worm, Osedax, on the right and a numerous Osedax worms colonizing a skeleton on the right. Image credit: Wikimedia commons

While these weird worms have been recognized as critical decomposers of large whale falls, scientists have not been able to characterize just how these zombie worms impact the small ecosystems that crop up around whale falls. In part, our lack of information about Osedax worms and whale fall communities is driven by the fact that it is difficult to explore the deep-sea habitats where whale falls can be witnessed. Furthermore, these whale falls are rare and finding one is like finding a whale in a haystack.

 

How do scientists study something that is so hard to find?

There have been some remarkable studies where scientists sunk cow carcasses or whale carcasses (that they found dead in shallow water) in known areas so that they could watch the succession of organisms that colonized the animal falls over time. For this study, however, the scientists were able to make use of a natural whale fall after opportunistically coming across partial skeletal remains of an Antarctic Minke whale off the coast of Rio de Janeiro. The skeleton consisted of nine vertebrae – five of which had bacteria decomposing the bones and four of which had zombie worms colonizing the bones. The two bone-eating organisms were not found together on any of the nine vertebrae, giving scientists a perfect way to study just how zombie worms impact the community of other organisms colonizing the vertebrae.

To do so, the scientists took videos of the skeleton and the surrounding area and then collected the vertebrae using a manned submersible (the SHINKAI 6500) to study them in the lab. Once collected, the different organisms found on and within the bones were identified and counted. The scientists compared the number of species and individuals of each species they found between the bones that did not have Osedax worms (only had bacteria) and those that were colonized by Osedax worms (but had no bacteria).

The bare-bone results:

When analyzing the species composition in the different vertebrae, the scientists found that the vertebrae with Osedax worms had higher number of species and a larger number of individuals overall compared to the vertebrae without Osedax worms. They also found that the bones without Osedax worms only had organisms living within the outer-most layers of the bone, while the vertebrae with Osedax worms had life throughout the entire bone.

Given these results, the scientists conclude that by burrowing into the bone, the Osedax worms alter the habitat, allowing more organisms representing different species to colonize the whale skeleton. By making holes in the bone, the structural complexity of the bone is increased and these ecosystem engineers enhance the biodiversity of these important deep-sea communities. While scientists still have much to learn about these bizarre zombie worms, it is clear that they are important components of whale fall communities.

 

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