Whale Falls: Islands of Abundance and Diversity in the Deep Sea

Although bottom-dwelling animals are surprisingly abundant in Monterey Bay’s deep waters, they are often food-limited – most deep benthic food webs are supported by the slow drizzle of organic particles and detritus (“marine snow”) from sunlit waters far above. But every now and then, this slow, nutrient-limited world receives a really big food “particle,” such as a dead whale. One whale fall can deliver as much organic material as several thousand years worth of marine snow.

In February 2002 Robert Vrijenhoek and Shana Goffredi of Monterey Bay Aquarium Research Institute (MBARI) discovered a recent whale fall while exploring the outer portion of the Monterey Canyon with MBARI’s remotely operated vehicle, Tiburon. They returned to the site in October 2002 with Craig Smith, a University of Hawaii professor who has studied whale falls for nearly twenty years. Based on repeated observations of several whale falls off southern California, Smith believes that many whale falls develop similarly over time.

Photo montage of whale fall in Monterey Bay taken during the discovery dive in February 2002. photo 2002 MBARI

When a large whale dies, its body often sinks directly to the sea bottom, especially if the animal is undernourished. Within days, active scavengers, such as sleeper sharks, rattails, hagfish, and amphipods, converge on the new food source and voraciously remove the flesh from the bones. (Smith has estimated consumption rates of forty to sixty kilograms of flesh per day.) In many cases the whale is stripped to the bone in a matter of months.

Within a year after the whale fall, the whalebones and nearby organically enriched sediment typically become infested with huge populations of polychaete worms and unusual crustaceans, as well as mollusks and other invertebrates. Worms often carpet the seabed at densities of up to 45,000 animals per square meter – higher densities than in any other deep-sea environment. Animals in this “enrichment-opportunist” community feed directly on organic material in the whalebones and surrounding sediment. Many of the species are as yet undescribed and may be unique to deep-sea whale falls. Despite the high biomass, species diversity is relatively low at this stage, as it is near other concentrated sources of organic material in the marine environment, such as sewer outfalls and salmon pens.

About a year or two after the whale fall, most of the easily digestible organic material has been consumed. However, sulfur-reducing bacteria continue to feed on lipids deep within the whale bones, gradually releasing hydrogen sulfide. This hydrogen sulfide provides the basis for a third-stage, “sulfophilic” community. This community is remarkable for a number of reasons. First, it consists of a self-contained food web with up to five levels (e.g., bacterial producers, bacterial grazers, animals that obtain nutrients from sulfur-oxidizing bacteria within their bodies, scavengers, and primary and secondary predators). Second, this food web is based almost entirely on energy from chemosynthesis instead of photosynthesis. Third, it includes an extremely diverse group of animals (up to 190 different macrofaunal species have been found on a single whale skeleton), many of which are specifically adapted to utilize whale-falls as a food source and substrate. Fourth, the community can be amazingly persistent – at least one large whale-fall community has apparently lasted more than fifty years.

Octopus living in the remains of the whale’s skull. photo 2002 MBARI

Late-stage whale-fall communities resemble communities at deep-sea hydrothermal vents and cold seeps, where chemosynthetic bacteria also form the basis for unusual, self-contained food webs. About 10 to 20 percent of the roughly 200 sulfophilic species found at whale falls are also found at underwater hydrothermal vents and cold seeps, respectively. However, the majority of species found in each type of environment are unique. Since many of these organisms are difficult to identify based on appearance, researchers at MBARI are using genetic and molecular tools to understand the evolutionary patterns among whale-fall communities and to determine relationships among animals at whale falls, seeps, and hydrothermal vents.

Thirty-million-year-old assemblages of fossil clams and whale bones suggest that whale-fall communities have been around at least as long as whales themselves. This persistence is particularly impressive because each whale-fall community is based on a transitory food source. Sooner or later, planktonic larvae of invertebrates at one whale fall must somehow find and colonize a new whale in order to survive. But whale falls might be relatively common. Smith estimates that, based on current whale populations and whale-fall community persistence, dead whales may occur roughly every five to sixteen kilometers along the seafloor off the Pacific coast of North America. Distances such as these are easily traversed by planktonic larvae.

The recent whale fall in Monterey Canyon is particularly interesting because it lies in deep water (2,891 meters). At this depth mobile scavengers are probably less active, but sulfophilic organisms may appear sooner. Smith and the MBARI scientists are currently analyzing the results of their latest visit to learn more about the persistence and distribution of these unique benthic communities.

Kim Fulton-Bennett
Monterey Bay Aquarium Research Institute