Eating bones and building habitats: the life of an ecosystem engineer

Source: Alfaro-Lucas JM, Shimabukuro M, Ferreira GD, Kitazato H, Fujiwara Y, Sumida PYG (2017) Bone-eating Osedax worms (Annelida: Siboglinidae) regulate biodiversity of deep-sea whale-fall communities. Deep Res Part II Top Stud Oceanogr

Whales, as the biggest animals in the ocean, leave quite a lot of biomass behind when they die. That biomass largely doesn’t stay in the upper reaches of the ocean, where the whale lived while it was alive – instead, the whale falls deeper and deeper into the ocean’s abyss. When the carcass finally hits the ocean floor, there are a host of animals and microbes ready to welcome it to the next phase of its life.

Whale falls, as these events are called, are crucial to the habitat of the deep sea. In an area without a lot of light or food to eat, a massive dead whale is an oasis in the deep dark desert of the featureless abyssal plain. Many animals flock to it and savor the final gift that the whale was able to give the ocean: its tissues.

Animals eat the whale in stages. The first morsels to go are the soft tissues of the whale – the skin, the muscle, the fat, and the blubber. Animals of all types are able to consume those tissues. However, once that resource is depleted, the whale continues to provide nutrients – animals just have to be a bit more creative about how to harness them. The most well known animal that is able to make use of the whale’s bones is the Osedax worm.

Known by various other horror-movie inspired names (the bone-eating worm, the zombie worm), Osedax isn’t just one kind of worm – there are many species under the umbrella of the Osedax genus. Like many other deep-sea dwellers, they exhibit sexual dimorphism, with the macroscopic females doing all the heavy lifting while the microscopic males live inside their bodies. The females bore into the whale bones with a root like structure that uses local acidification to erode the bone (Figure 1).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Osedax’s hard work accomplishes two tasks. First, the worm is able to make a meal out of the bone’s collagen, and eats the material with the help of its symbiotic bacteria. Secondly, the worms also create burrows as they go through the bone, maximizing the surface area of the bones available to other animals. Other animals that aren’t able to burrow like Osedax are then able to get at the inside of the bone.

The authors of this paper were interested in figuring out how the presence of these Osedax worms affects biodiversity. They hypothesized that having these worms present would increase the number of different species and the abundance of animals at a particular bone, but no study had ever been done to see what that difference actually was.

Methods

To investigate this question, the researchers located a whale fall near Rio de Janeiro, Brazil. They found nine whale vertebrae all from the same whale, with 5 of those vertebrae untouched by Osedax and the other four already colonized (Figure 2). They did video surveys to determine abundances of animals, and then collected the vertebrae and brought them to the lab to sort through and find the smaller animals.

 

 

 

 

 

 

 

 

Results and Importance

The first five vertebrae, the ones that were untouched by Osedax, had thick, white mats of bacteria all over the superior side (the side exposed to the seawater). On the interior side (the side laying on the sediment) there were black, dark stains on the bone where bacteria had previously been. These vertebrae were intact with no heavily degraded areas or burrows. Not many animals were found on top but there were some found in the inside. There were 3239 animals from 14 species found within the first centimeter of the bone (many of which were new species!). Nematodes were found the most often, representing 77.4% of the animals, and polychaete worms had the most species with 14 different ones represented.

The second four vertebrae, the ones that supported colonies of Osedax, were very different. There were a lot of holes in those vertebrae, exposing more surface area of the bone to the marine environment. The bones were anywhere from 19% to 45% degraded due to the worms’ activity. There were many more animals on these bones – 10080 individuals (a 300% increase!) representing 23 different species (a 140% increase!).   Those animals inhabited far more than just the top one centimeter of the bone and were able to access the nutrients deeper in the core of the vertebrae (Figure 3).

 

 

 

 

 

 

 

 

 

 

The researchers didn’t expect these bones to be so different from each other since they were all from the same whale fall and in the same area as one another. Why did the Osedax worms colonize the latter four vertebrae and not the previous five? The researchers aren’t sure, but they think it has something to do with the timeline of when the other soft tissues were eaten. The bacterial mats that were found on the first five vertebrae mean that the bones were still in an earlier stage of the whale fall called the sulfophilic stage. In this stage, sulfur-reducing bacteria dominate the ecosystem and create a chemosynthetic (chemical based) food web.

Even so, these findings confirmed that these Osedax worms are ecosystem engineers of their habitat. Ecosystem engineers are animals that are affecting the availability of resources to others. In this case, the worms are changing the whale bones physically – the burrows – to allow other animals access to the resources within. They enhance biodiversity by creating more structural complexity.

However, Osedax worms are hardly queens of the whale fall. These worms are not the dominant species at the whale fall, nor are they the longest living. In fact, their presence will accelerate the bio-erosion of the whale carcass – the more they eat and create burrows, the faster the whale will degrade. Much like an oasis, each animal will be able to take a drink, but they have to act fast before the resource dries up.

Engage: Can you think of another example of an ecosystem engineer in the terrestrial habitat?