Whale falls: when a dead whale feeds the ocean for 100 years

When a great whale dies in the open sea, its carcass sinks slowly towards the abyss, sometimes to 4,000 metres of depth, where it sets off one of the most extraordinary biological phenomena on the planet: a whale fall (“chute de baleine” in French). This sudden windfall of several dozen tonnes of organic matter in a food-poor abyssal desert sets off a genuine feast for hundreds of species, some of which are entirely specialised in exploiting these carcasses and exist nowhere else. A single blue whale can in this way feed an entire ecosystem for 50 to 100 years, passing through 4 ecological stages that oceanographers have clearly identified.

The 4 stages of a whale fall

1. The “mobile scavenger” stage (months to 2 years)

As soon as the carcass reaches the bottom, mobile scavengers arrive quickly, drawn by the organic molecules diffusing through the deep water: Greenland sleeper sharks, hagfish, crabs and giant amphipods. These animals consume all the soft flesh (muscles, viscera, skin) at an impressive rate, about 40 to 60 kg a day documented on the whale falls followed by autonomous cameras. Within a few months to 2 years, nothing is left but the skeleton and a few stubborn tissues.

2. The “enrichment opportunist” stage (2 to 4 years)

Once the flesh is exhausted, a new wave of opportunists colonises the enriched sediments around the carcass: polychaete worms, crustaceans, molluscs. The bones are then covered by a thick white bacterial mat. The excess organic matter temporarily alters the chemistry of the surrounding sediment over several square metres around the skeleton.

3. The “sulfophilic” stage (up to 100 years)

This is where things become genuinely strange. Anaerobic bacteria break down the residual lipids in the bones (which contain up to 60% fat in whales), producing hydrogen sulphide (H₂S). This compound, toxic to most organisms, attracts a specialised fauna usually found at hydrothermal vents and cold seeps: Idas mussels, tubeworms, Vesicomyidae clams. These animals live in symbiosis with chemosynthetic bacteria that turn the H₂S into energy. This phase can last several decades.

4. The “reef” stage (indefinite duration)

Once the organic matter has been entirely consumed, the bare skeleton serves as a physical support for filter feeders (sponges, sea fans, anemones) that attach themselves to the bones as they would to a reef. The skeleton becomes a permanent island of life in the abyssal immensity. Some skeletons documented off California are still colonised more than a century after they reached the bottom.

The zombie worm that eats bone: Osedax

The most spectacular discovery associated with whale falls is that of the worms of the genus Osedax (“bone eater” in Latin), described for the first time in 2004 by Robert Vrijenhoek and his team off Monterey, California. These polychaete worms have no mouth, no anus and no digestive tract. They anchor themselves in the bone by bacterial “roots” that break down the bone lipids and feed the worm through symbiosis. To date, more than 30 species of Osedax have been described, some of them entirely endemic to a single host cetacean species. A surprising detail: only the females are visible; the males are minute (a few hundred microns) and live inside the females, which may host up to several dozen of them.

Why this matters ecologically

The abyssal floor is a nutritional desert: only 1% to 5% of surface production reaches it (in the form of “marine snow”). The carcass of a great whale delivers at a stroke the equivalent of 2,000 years of the normal nutrient rain. On the scale of the oceans, researchers estimate that before industrial whaling, 500,000 to 1 million whale falls were permanently present on the ocean floor. Today that number has fallen by 80 to 95% because of the collapse of the great whale populations in the 20^th century. The recovery of those populations is therefore as crucial for abyssal biodiversity as it is for the whales themselves.

How scientists study whale falls

Natural whale falls are almost impossible to find blind (the deep ocean covers 360 million km²). Oceanographers therefore use two approaches: opportunistic observation during ROV (Remotely Operated Vehicles) campaigns that come across a carcass by chance, and experimental artificial whale falls, in which carcasses of whales that died naturally (strandings) are deliberately sunk at chosen depths, with cameras and sensors installed. The flagship programme is that of MBARI (Monterey Bay Aquarium Research Institute), which since 2002 has been monitoring several experimental whale falls in Monterey Canyon, at a depth of 2,893 m.