The Long Afterlife Of A Whale

AWhen a large whale dies at sea its body sinks, and on the deep sea floor it becomes something no other event supplies: several tonnes of concentrated food delivered in a single parcel to a habitat that ordinarily receives a thin drizzle of particles from above. Since the first such carcass was examined from a submersible in 1987, the fate of these whale falls has been described by a three-stage model. Mobile scavengers such as sleeper sharks, hagfish and amphipods strip the soft tissue within months. Worms and crustaceans then work the enriched sediment around the skeleton for a few years. Finally the bones, which are unusually rich in lipids, are broken down by bacteria that release hydrogen sulphide, and animals dependent on that chemistry settle upon them. The third stage is the one that has excited most argument, because the animals arriving during it resemble those found at hydrothermal vents hundreds of kilometres away.

BHugo Bergstrom, a deep-sea ecologist at the Tarrant Institute of Oceanography, has built the strongest version of the argument that follows from that resemblance. Between 2003 and 2015 her group towed fourteen stranded carcasses out to the Vega Basin and sank them at depths between 1,150 and 1,400 metres, revisiting each with a remotely operated vehicle at intervals. The fourteen skeletons yielded 178 invertebrate species. Thirty-one of these had not, at the time of publication, been recorded anywhere except at whale falls or at vents. From this Bergstrom argued that carcasses act as stepping stones: larvae released at one vent field cannot cross the intervening plain within a single generation, but they can settle on a whale fall, breed there, and send a further generation onward. The claim is not that whale falls resemble vents, which nobody denies, but that they make the crossing between vents possible.

CThe timetable of the third stage was recalculated after the discovery of Osedax, a genus of mouthless worm that mines lipids from bone with the help of symbiotic bacteria housed in root-like tissue. Colonies are dense: one team counted 4,300 individuals on a single vertebra. In an experiment at 1,200 metres reported in 2014, ribs left open to Osedax lost mass roughly six times faster than ribs enclosed in cages that excluded the worms, and the skeleton of a medium-sized whale was reduced to fragments within eight years rather than the several decades that earlier estimates had assumed. The result cuts both ways. It shows the deep sea disposing of a windfall with unexpected efficiency, but it also shortens the window during which any single carcass can house a breeding population, and a stepping stone is of no use if it crumbles before the traveller has bred.

DHendrik Vasseur, a benthic biologist at the Marrow Institute for Marine Geochemistry, accepts the species lists and disputes what may be read from them. His first objection concerns how the carcasses came to be there. Implanted whales are placed where a ship can reach them, in soft sediment at moderate depth, so the resulting inventory describes a set of chosen sites rather than the sea floor at large. His second concerns the endemics. After a wider survey of cold seeps published in 2020, twenty-seven of Bergstrom's thirty-one species were found elsewhere, leaving four. His third is the one he presses hardest: among the species genuinely shared with vents, most already produce larvae capable of remaining in the water column for many weeks, which is long enough to cross the plain without stopping. A stepping stone that serves only travellers who could have made the journey anyway, he writes, explains nothing. He grants that his own sampling was concentrated in one ocean basin.

EBergstrom replies that the objection about placement applies to any deep-sea experiment that is not an accident, and that four confirmed endemics from fourteen skeletons is a high yield for a habitat of that size. She also questions the larval argument: endurance in the laboratory is measured in still, cool, well-fed water, and says nothing about whether a larva arrives anywhere alive. Her group's estimate of spacing, a carcass every twelve kilometres along the migration corridors of the eastern Pacific, rests on population figures and stranding records rather than on any survey of the sea floor, a limitation she states in the paper itself. She concedes further that no experiment of hers has followed a larva from one carcass to another, that her counts were made from video and therefore favour animals larger than about five millimetres, and that the figure of thirty-one should never have been quoted without its date.

FThe dispute has since acquired an audience outside the journals. Commercial whaling reduced some populations by more than ninety per cent, and one reading of the stepping-stone hypothesis holds that the sea floor thereby lost habitat as surely as a forest loses it to felling; the argument has been cited in submissions opposing the licensing of nodule mining in the same basins. Vasseur regards the reasoning as sound in principle and unsupported in detail, since nobody has measured what the sparser supply of carcasses did to any population. Bergstrom, whose work is quoted in those submissions, has said that mining should be opposed on evidence that does not depend on her hypothesis being right. Neither has yet done the one thing that would settle the matter: to find a whale that has died unobserved and to watch what arrives, in the order in which it arrives, without having put it there.