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Biology

Decomposition in the Deep Sea

Article:

Hilario, A., Cunha, M. R., Génio, L., Marçal, A. R., Ravara, A., Rodrigues, C. F., & Wiklund, H. (2015). First clues on the ecology of whale falls in the deep Atlantic Ocean: results from an experiment using cow carcasses. Marine Ecology36(S1), 82-90.DOI: 10.1111/maec.12246

Introduction:

Conditions in the deep oceans are harsh. Without sunlight, the water is much colder and food more scarce than at the surface. Any source of food or nutrients that reach the bottom are welcomed by the animals that live there. When whales die, they sink to the seafloor and provide an enormous pulse of food to an otherwise food-limited environment; these events are known as whale falls. Different stages in whale carcass decomposition support a succession of animal communities, ranging from large sharks to microscopic bacteria.

The first stage is known as the “mobile-scavenger” stage, when soft tissue is removed from the whale by scavengers like sharks and crabs (Fig. 1). Next, during the “opportunist” stage, other heterotrophic animals (i.e. snails and worms) take advantage of the remaining food made available by larger scavengers. They colonize the surrounding sediments and pick off any leftover scraps on or inside the whale bones. The final “sulfophilic” stage, is when whale bones decay, resulting in sulfide production (a form of sulfur). Certain bacteria can directly obtain energy from sulfur, while larger organisms rely on sulfur-reducing bacteria living inside them (called bacterial symbiosis). A single whale fall can support surrounding benthic life for an astonishing 50 years! Clearly, these incidents are important and provide a long-lasting wealth of food to many animals on the seafloor.

Whale fall

Figure 1: Diagram of the three different stages of whale decomposition on the seafloor.

However, all of this information comes from whale carcass falls in the Pacific – what is community succession like in the Atlantic? Whale fall events are random, non-uniform and impossible to predict, making them hard to study. The authors of this study came up with a unique solution that allowed them to document whale fall communities in the Atlantic Ocean.

Methods:

The authors deployed a single bundle of five cow carcasses (570 kg total weight) off the coast of Portugal. Why use cows in their experiment? Cows were chosen because they are easy to obtain and have high lipid content within their bones. Cow bones also produce high amounts of sulfide, which would allow ample time to reach the “sulfophilic” stage in this model system. After 18 months of deployment, the team retrieved the cow bones and described the colonizing animals they found.

Results: Who was there?

After 18 months, all of the tissue had been consumed and only bones remained (Fig. 2a). A total of 33 different animals were found on or inside the cow bones, representing a diverse community. The class Polychaeta, comprised of annelid worms, were most abundant in the samples (67%). In addition to worms, other animals like snails, limpets, clams, mussels and crustaceans were present (Fig. 2b-h). Researchers found several marine worms of the genus Osedax (Fig. 2c-d). Osedax are referred to as “bone-eating” worms and, as the name implies, they bore into the bones of whale carcasses to extract lipids.

Whale fall organisms

Figure 2: (A) Experimental site after 18 months. (B-H) Animals found on or within retrieved cow bones.

The number of species found in the cow bone samples was comparable to previously studied whale fall communities. There was even an example of symbiosis from the mussel, Idas simpsoni. These mussels gain their energy by harboring sulfur-loving bacteria that thrive off the decaying bones. In repayment, mussels give the bacteria a comfy and safe home. These types of symbiotic associations are found all throughout the ocean and add to animal complexity.

Conclusion and Significance:

It is hard to study whale falls because they occur thousands of meters below the ocean surface and are spread out geographically. This geographic barrier can be overcome by using alternative substrates like cow carcasses to assess whale fall communities. Terrestrial cows and marine whales differ in many ways, but this study shows that cows can be used to help model the different stages of whale fall decomposition in the deep sea. In this study, animal life found on or within cow bones was highly diverse and comparable to diversity normally seen at whale falls. After 18 months, all of the cow tissue had been removed, so researchers were unable to capture the first “mobile-scavenger” stage. They did retain organisms like worms, snails and mussels. Small-sized scavengers (mainly worms) picked off any remaining tissue, while suspension feeders (mussels) attached to bones and captured floating material during the “opportunist” stage. Evidence for the final “sulfophilic” stage was also apparent from the abundance of symbiotic mussels found in the samples.

For deep-sea animals living under harsh conditions, whale falls provide important sources of food. Whale falls may be equally important as stepping stones for hydrothermal vent species. Large areas can separate hydrothermal vents and thus whale falls may serve as temporary oases, providing food to organisms dispersing across barren sediment environments. The impact of whale falls on the deep sea will be better understood if we can increase our sample size and hit more locations throughout the global oceans. This study takes an important first step toward this goal by describing mammal fall communities in the Atlantic Ocean.

Discussion

One Response to “Decomposition in the Deep Sea”

  1. Inspired methodology and awesome results. Just reading “At the Fall” by Alec Nevala-Lee; SF short story set in the benthic. Great background info and v informative graphics

    Posted by SP | March 18, 2021, 6:28 pm

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