Life After Death - The Shipwrecks that Sustain the Abyssopelagic

Deep in the abyssal zone, far below the reach of the sun’s rays, the hull of a different kind of ship settles in its final resting place. A rich oasis in the desert of the ocean floor, the shell of a leviathan. The skeleton of a sunken cetacean has the propensity to sustain the creatures of the deep for decades at a time. The delayed decomposition of its giant carcass creates a localised and unique ecosystem, attracting a variety of intriguing marine organisms to feed on the remains and seek shelter within the cage of bones.

This phenomenon is commonly referred to as “whale fall”. When a whale reaches the end of its life far out at sea, it’s body will sink like a stone to the bathyal or abyssal zone. These zones extend deeper than 1000m below the surface, and the chilling temperatures combined with high hydrostatic pressures slows the decomposition rates of organic matter so that a falling carcass stays intact during its descent (Allison, Smith, Kukert, Deming, & Bennett, 1991). This cannot happen in shallow waters, where the carcass of such a large animal would be quickly stripped of flesh by scavengers within weeks. The extension of the decomposition process at depths of thousands of metres is key in allowing the formation of a rich and local ecosystem.

Many species of deep-sea marine organisms have been observed to visit these carcasses, which go through three definite successional stages (Amano & Little, 2005). Mobile-scavengers such as sleeper sharks and hagfish feed on the soft tissue for as long as two years (Little, 2010), which defines the first stage of a whale’s afterlife. After the consumption of the soft tissue, an enrichment opportunist stage follows, defined by colonisation of the newly exposed bones. Polychaetes, marine annelid worms, are a large proportion of colonisers. They strip the carcass clean of any remaining tissue and blubber, until all that’s left are the bones. For mysterious reasons, whale bones are a rich source of lipids, which makes them particularly attractive for anaerobic microbial decomposition.

At this stage, the sulphophilic stage, hydrogen sulphide as waste is produced by anaerobic bacteria, and oxidised by chemosynthetic bacteria often found living as symbionts with other organisms, such as mussels. The body of a whale at the bottom of the ocean benefits organisms across a range of taxa: cnidarians, polychaetes, sipunculids, crustaceans, and echinoderms (Goffredi, Paull, Fulton-Bennett, Hurtado, & Vrijenhoek, 2004) as well as the microbial communities that are supported.

Whale carcasses not only provide a large meal for the ocean’s residents, but also an incredible amount of carbon. A typical carcass contains about two metric tons of carbon, equal to the amount exported in 100 – 200 years to a hectare of the seabed. Immediately beneath the whale fall, a pulse of carbon equivalent to around 2000 years of background carbon flux erupts, creating an area of high primary productivity where the whale lands and immediately surrounding it. The exact implications of this carbon flux are not entirely understood, but it’s possible the carbon contained in whale fall has certain implications for the biological pump – the heart of the ocean. This is the sequestration of carbon by the ocean through biological factors, from the atmosphere. It is an important part of the oceanic carbon cycle.

While many of us would think of the death of a whale as a tragedy, it’s contribution to life on the seafloor is outstanding. Whale falls are difficult to observe naturally, but with improving technologies such as deep-sea robotics, we may finally be able to gather more data of these events. Death is always a part of life, and life after death for whales and similar marine organisms is just as important to life itself.