How Dead Whales Form Unique Ecosystems
A young octopus (Graneledone verrucosa) moves across the seafloor. Observed during the Okeanos Explorer Northeast U.S. Canyons 2013 expedition. (Image Credit: NOAA Ocean Exploration & Research, CC BY-SA 2.0, Image cropped)
In nature every death brings new life. A fascinating example are whale-falls: when a whale dies, its carcass will sink down to the ocean floor where it creates a unique ecosystem for bottom-dwelling organisms. Whales’ bodies can weigh up to 200 tons and contain massive amounts of fat and proteins. When a dead whale reaches the ocean floor it brings a lot of resources to an environment which is usually limited by food availability. The fortunate creatures experiencing the whale-fall welcome such a great source of nutrition, and use up everything they can, until the last vertebra is decomposed.
The first time I became aware of the whale-fall phenomenon was last year while watching the live-stream of the Nautilus deep-sea expedition with some fellow students. It took us by surprise: after hours watching nothing but starfish or sea cucumbers crawling on a seabed desert, the remnants of a baleen whale carcass appeared on screen. We heard the expedition crew members’ jubilation, and their cries of, ‘Wow, phenomenal!’, watching the camera moving closer to the bones which were inhabited by octopuses, grenadier fish, crabs, eelpouts, worms … All of us edged closer to the screen, captured by the fascinating finding. I was awed by the flourishing life that was crawling, swimming and lurking there, deep down on the ocean floor.
Exploring Bones on the Sea-Floor
Whale-falls are quite a new subject, even to marine science. Before the era of live-streams and underwater robots, only individual whale-bones or skeleton parts have been brought to the surface by trawling through fishermen. In 1900 such a bone find led to the discovery of the saltwater clam species Idas simpsoni, described then as Myrina simpsoni from James T. Marshall. The clam was found on a dead whales’ skull, anchored to it with their strong byssus filaments. Nowadays we know that the bones of dead whales are their preferred habitat.
The first direct observation of a whale-fall happened in 1987. A submersible was exploring the sea-floor when the crew found something spectacular. Since no one had seen anything like it, their first thought was: dinosaur skeleton! Actually they had come across a 21m long blue whale carcass lying on the deep sea-floor – which eventually was named a ‘whale-fall’. What they discovered exceeded all expectations: the remnants of the dead whale were full of life.
This spectacular finding initiated the research of whale-falls. This and the intensified use of remotely operated vehicles (ROVs), allowed for the discovery and observation of more whale carcasses, ranging from falls in the deep-sea to shallow-water environments. Scientists have even conducted whale-fall experiments by using stranded whale carcasses to simulate a whale-fall, letting it sink to the ocean-floor and observing what happens to the body over time. All over the world such experiments have been done, revealing new insights into the ecology of whale-falls.
Islands of Diversity
After the first observations and experiments it quickly became clear that dead whales represented a true biodiversity hotspot: over 400 species have been found associated with whale-falls including more than 30 species which occur exclusively on whale carcasses. In 2016, Sumida et al. described 15 new species found in one whale carcass alone. Much more is still to be discovered, with many species descriptions doubtless waiting to be published.
Various Forms of Animal-Falls
Beside huge whales, we also find other animals that fall down to the ocean floor: ranging from the smallest plankton to pelagic tunicates. These tiny corpses are commonly known as marine snow, and comprise all kinds of dead organic material including zoo- or phytoplankton that sink down into the deeper layers of the ocean and eventually rain down to the sea floor. There, they provide benthic organisms with food, tiny piece by tiny piece.
The Scyphozoan medusa Periphylla periphylla aggregates in huge amounts in Norwegian fjords and as they die off the dead jellies sink down and bring a deposit of carbon and nitrogen to the seabed. Scientists call this event jelly-falls. Who would have thought so: even the tiniest carcasses, as well as gelatinous ones, are a great resource for benthic communities down on the seafloor.
If you want to read more, have a look at these studies:Sweetman, K & Chapmann A (2011). First observations of jelly-falls at the seafloor in a deep-sea fjord. doi:10.1016/j.dsr.2011.08.006
Henschke, N. et al. (2013). Salp-falls in the Tasman Sea: a major food input to deep-sea benthos. doi: 10.3354/meps10450
The 4 Stages of a Whale-Fall Ecosystem
The decomposition of a whale happens in successive, overlapping stages which can be characterized by different communities living on the carcass and creating a diverse whale-fall ecosystem. Whales consist of different organic material. At first the whale arrives as we know it: fat, muscles, blubber attached to a strong skeleton.
The fleshy parts are delicacies for some invertebrates and a variety of fish, such as eels, sharks or hagfish. Removing the soft tissue is the first step of decomposition: the mobile-scavenger stage. Depending on where the whale sunk and the whales’ size, this stage can last several months up to some years.
During that stage the sediment around the whale carcass becomes enriched in nutrition due to the left-overs of the scavengers which lose fleshy bits while feasting on the whales’ tissue. The enriched sediment attracts omnivorous organisms such as polychaete worms, crustaceans, cnidarians and many more. This period is called the enrichment-opportunist-stage.
When the soft tissue is removed, the bones are exposed and only specific bacteria are able to decompose the bare bones. This stage is called sulphophilic or ‘sulphur-loving’ stage because sulfide is produced through bacterial breakdown of organic matter, such as in this case the lipid-rich bones and the carbon-enriched sediment around the carcass. The produced sulfide is nutrition for another microorganism community, made up of so-called chemoautotrophic bacteria. These can be free-living, mat-forming bacteria that cover the bones and sediment surfaces. They can also act as endosymbionts, living inside other organisms where they are responsible for their nutrition and survival. The diversity found during this sulphophilic stage is the highest: Smith and Baco in 2013 calculated a mean of 185 species on a large whale skeleton. This diversity exceeds any other hard-substrate in the deep sea observed to this point.
After munching away all the fat in the bones, the remaining substrate are the nutrition-depleted bones which can act as scaffold for filter feeders. This stage is called reef-stage and becomes the home of a diverse community of bivalves, barnacles, tunicates, sponges, as well as potentially a variety of cnidarians, such as anemones, hydrozoans and corals.
Munching on Whale Bones
The most famous example of whale-fall specialists is the bone-eating polychaete worm Osedax. Since the first description of Osedax rubiplumus in 2004, 37 species of this genus were found related to whale-falls. They live in symbiosis with the aforementioned chemoautotrophic bacteria. The symbiont bacteria live inside the worms’ root-like anchoring structure, which they use to attach to the whale bones and reach the contents inside. Osedax not only looks stunning, its symbiont-bearing root system is also unique in the animal kingdom, and has never been found before. However, it remains unknown how Osedax feeds on the bones. Newest studies suggest that the worms digest the collagen using specific enzymes, but how the diverse endosymbiont community, the bacteria inside their roots, help with that is unclear.
Scientists initially wondered why they only ever found female Osedax individuals. They found that the females contain tiny male harems inside the tubes which surround the females’ body. This strategy probably ensures that they are able to reproduce in an environment where the next whale carcass, and thus potential sexy partners, could be hundreds or thousands of meters away.
Why Whaling Harms More Than Whales
Whales have been hunted over centuries, reaching its peak with a depletion of whales by 66-90% until the first protections and commercial whaling regulations were introduced only 50 years ago. This exploitation of whales led to an enormous decline in whale populations, pushing some great-whale species like the blue whale or the North Atlantic right whale to the brink of extinction.
Craig R. Smith and his colleagues wanted to know how this reduced abundance of whales, and thus whale-fall habitats, affected whale-fall communities and specifically its endemic species. Are whale-fall specialists also threatened with extinction due to a reduced number of whale carcasses on the seafloor? To tackle this question the scientists tried to model a scenario prior to whaling and a ‘now’-scenario with depleted whale abundances.
The model showed that the whale-fall ecosystem may be threatened. Especially if large-sized whale species don’t increase in abundance in the near future, the specialized whale-fall species might go extinct soon. In order to successfully reproduce and distribute themselves these species need large-sized carcasses to survive and sustain connectivity between populations. Since we don’t know how these communities looked like prior to whaling it’s hard to say how whale-fall ecology has changed since then. It can be assumed that we have already lost species in these communities that we’ll never be able to recover. The reality is that we’ve already diminished whales to such an extent that we destroyed an ecosystem without even knowing it.
Even dead you can combat climate change
Besides creating islands of diversity and resources, dead whales store massive amounts of CO2 inside their carcasses. A great-whale adds up to 33 tons of CO2 on average – approximately the emissions of 4 households in a year. By removing whales from the ocean, we are also transferring this stored carbon into the terrestrial world, eventually ending up in the atmosphere. If a whale sinks down to the ocean floor it not only creates an oasis for many bottom-dwelling organisms but also combats climate change. That adds even more reasons to protect our fascinating ocean – dead or alive all marine creatures play their role in the complex web of life.
And if you ever wanted to know how to not to get rid of a dead whale:
Lara Beckmann is a master’s student at the University of Stockholm in the program ‘Biodiversity & Systematics’ and is currently working on her thesis project at the University Museum of Bergen focusing on the diversity of marine polyps and jellyfish in the group Hydrozoa. Follow Lara on Twitter here.
References and Further Reading
Bastian, M 2020, ‘Whale falls, suspended ground, and extinctions never known’, Environmental Humanities.
Treude , T. et al. 2009, Biogeochemistry of a deep-sea whale fall: sulfate reduction, sulfide efflux and methanogenesis’, Mar Ecol Prog Ser 382: 1–21, doi: 10.3354/meps07972
Rouse G.W, Goffredi S.K, Vrijenhoek R.C. 2004, ‘Osedax: Bone-Eating Marine Worms with Dwarf Males’, Science 305, 668, doi: 10.1126/science.1098650
Smith G.R. & Baco A.R. 2003, ‘Ecology of Whale falls at the deep-sea floor’, Oceanography and Marine Biology: an Annual Review 2003, 41, 311–354
Smith G.R., Roman J. & Nation J.B. 2019, ‘A metapopulation model for whale-fall specialists: The largest whales are essential to prevent species extinctions’, Journal of Marine Research, 77, Supplement, 283–302