The deep ocean is one of the most impoverished biomes on the planet. It encompasses more area than all of the world’s land biomes combined but exists hundreds or even thousands of kilometers away from the nearest solar ray, a lightless void punctuated almost entirely by the wispy phosphorescence of the creatures within it. Such light cannot sustain an ecosystem, for the energy that powers it comes from within that very ecosystem. Nearly all of the resources available to the creatures that call the deep ocean home fall from above, nutritious plankton remains forming the dense sludge called “marine snow” that coats much of the seafloor. There is productivity at the seafloor, involving chemical reactions at geologic sites called hydrothermal vents, and these locations occupy an outsized portion of the public imagination. But there is another deep-ocean ecosystem that is no less fascinating for its obscurity, and is proving to be instrumental to the persistence of hydrothermal vent life: the whalefall.
When large animals, most conspicuously whales, die at sea, their remains make a long, slow descent to the seafloor. Along the way, various scavengers take their fill, but much of each carcass still reaches the bottom, alighting on the layer of marine snow. When this happens, a characteristic series of scavengers finds them in relatively short order. Sharks, crabs, giant isopods, echinoderms, various fish species, and the primitive hagfish descend on the carcass from kilometers around, feeding for months or even years. After the soft tissues are exhausted, specialized organisms such as the Osedax worm feed on the lipid-rich bones and marrow, a process that itself can last years or even decades. At the end of the cycle, anaerobic bacteria decay the last of the non-mineralized material in the skeleton using hydrogen sulfide instead of oxygen, becoming a food source for filter-feeding organisms growing on the whale bones in the process. What remains after this can anchor pelagic larvae and become a substrate for deep-water reefs. Any large enough animal, from a whale to a shark to a sea turtle to an alligator dumped by curious researchers, can initiate this cycle, but whales’ status as enormous air-breathing animals whose bodies are denser than water and whose bones are unusually lipid-rich makes whales more likely to reach the seafloor mostly intact and to host the complete cycle when they do.
The whalefall cycle can take up to a century to complete. Throughout that time, the whale carcass is effectively an oasis in a largely featureless desert, temporary but nevertheless enduring enough to become a landmark for the creatures that depend on it. On land, scavengers are usually considered a niche within a larger ecosystem, but the ocean’s vastness and the long duration of a whalefall makes whalefalls different. Many of the species that populate the whalefall biota are rarely found anywhere else, spending their entire lives either on a whalefall or traveling to the next. This makes whalefalls a distinct ecosystem in their own right, one that rarely gets the same attention and fascination that the hydrothermal vent biota rightly receives.
Scientists are coming to appreciate the major role that whalefalls play in the overall oceanic biome. Whales move huge amounts of nutrition around the world by feeding in upwelling zones and defecating elsewhere, and their carcasses represent the return of this nutrition to the deep ocean. The near-extinction of numerous whale species during the heyday of whaling continues to have a sizable impact on the prevalence of whalefalls. The lingering effect of this loss on deep-sea nutrient flows, in turn, is only beginning to be understood. Similarly, because deep-sea nutrients take an enormous amount of time to return to the surface once deposited there, each whalefall represents the sequestering of thousands of kilograms of carbon where it will not affect the atmosphere. Understanding what the depletion of whale populations means for atmospheric carbon levels is one of the keys for a truly complete model of anthropogenic climate change. At a more local level, research is beginning to show that whalefalls have another role in oceanic life: as way stations for hydrothermal vent specialists migrating to new hydrothermal vents when their previous homes move away from volcanic hotspots and begin to go quiet. Just as whalefalls sustain their characteristic predators, they can also sustain vent specialists on the move.
Whalefalls are about as glamorous as roadkill and as scenic as a pack of hyenas dismembering a zebra. They lack the raw, colorful charisma of coral reefs or the sheer scale of marlins feeding on sardines at the surface. But in the cold depths of the ocean floor, where light is a distant memory and food is what one makes it, there is a stark, alien beauty to the churning swarm of whalefall eaters, and to the sheer scale of what they represent. Everything about whales is big, including what happens when they die.
