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Whalefall after 18 months, Santa Cruz Basin, 1670m
© C. Smith & Mike Degruy, University of Hawaii

A whale fall is defined as a dead whale sinking and transporting nutrients to the sea floor. Whales are the only marine mammals that have so far been found to be associated with similar animals to those found on hydrothermal vents and cold seeps - i.e. have associations with chemosynthetic fauna.

Following the discoveries of hydrothermal vents and cold seeps, scientists have found that certain large falls of organic matter that land on the deep-sea floor can also be the basis for the development of animal communities related to vents and seeps. Researchers have suggested that these habitats could be used as stepping stones for vent and seep species to disperse between ocean basins.

The deep sea floor is somewhat lacking in food with only a very slow supply of nutrients raining down from surface waters. Therefore, the arrival of a massive input of organic matter in the form of a whale fall has a huge impact upon the seafloor, albeit extremely localised and highly episodic. A single whale carcass can supply up to 160 tons of organic matter. This is equivalent to several thousand years worth of the normal supply of nutrients expected in the deep sea! Just a short time after arriving at it\'s final resting place, the whale carcass will be surrounded by a succession of animals, eager to begin feeding on the unexpected food source.

When you consider the fact that there are in the region of tens of thousands of marine mammals annually transporting food to the sea floor when they die, the impact on the sea floor is likely to be significant in some regions. For example, dead whales are predicted to occur along their migratory routes and around feeding grounds - hence along continental shelf areas and the deep ocean. In those regions where whales hang out, there could be a vast number of whale falls impacting the seafloor.

The amount of time a whale fall has been on the seafloor will determine the animals that you are likely to find there. When the whale first arrives at the bottom, animals known as scavengers move in rapidly to take advantage of all the soft tissue available for food. These animals include hagfish, squat lobsters, lithodid crabs (a new species of crab found at whale falls) lysianassid amphipods, rattail fish and sleeper sharks. They remove most of the soft tissue within a period of a few months to a couple of years, depending upon the size of the whale. This first of three stages of community structure surrounding a whale fall (identified by Professor Craig Smith, University of Hawaii), is termed the "mobile scavenger stage".

The second stage is characterised by infestation of the surrounding few meters of sediments by dense communities of chrysopetalid and dorvilleid polychaetes, cumaceans and sometimes juvenile gastropods and bivalves. These animals feed directly upon organic material in the whale bones and the surrounding sediment. They also make use of the bone as somewhere to settle. Many of these animals have so far only been found on whale falls and many are new to science. This is called the "enrichment-opportunist stage".

The next set of animals you find associated with the whale carcass are animals that not only make use of bone as a substrate, but also of the sulphides that begin to leak out from the whale bones. As the bones start to be decomposed by bacteria feeding on the fatty marrow deep inside the bones, hydrogen sulphide is gradually leaked out. These chemicals provide the base for the final stage - known as the "sulphophilic stage". Specialised bacteria known as "chemosynthetic bacteria" are able to use the chemicals to produce organic matter. These bacteria are a food source for other animals to graze upon. In addition, the bacteria may form a special "symbiotic" relationship with some animals, whereby those animals are able to survive by gaining the nutrition from the bacteria living within their body cavities. A huge number of species, including mussels, vesicomyid clams and vestimentiferan worms, have been found to be associated with this final stage of the whale fall decomposition, many of which have very specialised adaptations to use the whale falls as a food source and substrate. In fact, scientists found more than 40,000 animals on a single whale skeleton! It seems that these animals can hang around and use the sulfides for tens of years and in some cases up to a century.

Lithodid crab on mandible of 60 ton blue whale on seafloor for ~54 years © C. Smith, University of Hawaii

A most bizzare creature was recently discovered to be associated with whale falls. This strange worm, named Osedax (Latin for "bone devourer"), is a deep-sea polychaete commonly called the zombie or bone-eating worm. It was first identified from collections in Monterey Bay, California in 2002 living on the decaying carcass of a gray whale in the Monterey Canyon at a depth of 2,800m. Scientists now have recognised that related species of Osedax occur widely on whale falls off California, Sweden and Japan - these appear to be worldwide whale worms! Why are they more bizzare than any other deep-sea worm you may meet? Well, they have no eyes, legs, stomach or mouth and they digest the fat and oils in whale bones using root-like structures which bore into the bones and with the help of symbiotic bacteria. This is the first time that a fat-degrading bacteria has been observed in such a relationship. Another curiosity - the females have very small males living inside them - between 50 to 100 of them! The males do not develop further than the larval stage and they contain huge numbers of sperm, which is rather convenient as it seems that the female worms of all sizes are constantly full of eggs - a good strategy it would appear!

Osedax are really rather beautiful to look at. They are very colourful and have feathery plumes (gills). Similar types of worm may be found at hydrothermal vents and cold seeps, as has been shown by examinatin of their DNA showing the close relationship. Both the large tubeworms found at vents and seeps, and Osedax, obtain their nutrition with the help of symbiotic bacteria.

Professor Craig Smith from the University of Hawaii suggests that about half the Osedax species died out when 90% of the whale population was depleted by hunting at the end of the 19th century.

Professor Craig Smith from the University of Hawaii has been studying whale falls for almost twenty years! He first stumbled upon a whale carcass in 1987 when he and his co-workers discovered a giant skeleton at the bottom of Santa Catalina Basin off the coast of Los Angeles during a dive in the submersible Alvin. They were surprised to find that the bones were covered with bacterial mats and there were clam shells surrounding them. Similar communities had already been seen at hydrothermal vents. Animals from vents have the capability to use energy from toxic hydrogen sulphide in the vent fluids to produce organic matter. Such communities were subsequently found at sites where cold water seepage occurs, where the fuel is sulphide or methane. And now they have been found here... on the bones of a dead whale! The question was, what chemicals in the bones were supporting this life?

The first research was conducted by Craig and his team on a whale skeleton 1240 meters deep in Santa Catalina Basin. They found that the whale bones contained up to 60% fats by weight, and hence provided a substantial food source. Bacteria were found to decompose the fat anaerobically (without oxygen), which results in the release of hydrogen sulphide. Specialised bacteria that are able to live off the sulphide soon colonise the site and these bacteria become the base of a food chain which supports a host of other animals such as worms, molluscs and crustaceans, to name a few, similar to those found on vents and seeps.

Hagfish swimming over ALVIN's instrument basket at the floor of the Santa Cruz Basin. The skeleton of a gray whale whale is visible in the background. The instruments in the basket are used for collecting samples of seafloor sediments and associated animals.

Since these initial investigations, research has continued and much has been learned regarding the relationships between species found at whale fall sites and those found in other chemosynthetic environments. A detailed understanding of the different phases of feeding and colonisation by animals at the whale fall sties has also been achieved by Craig's team, by using both naturally found whale carcasses and implanted carcasses. It is now known that whale falls pass through three successional stages [see the "What animals are attracted to whale falls?" content above for more details]. The diversity of species on large whale skeletons during the final stage is higher than any other deep-sea hard substratum community.

Craig's work led to his theory that whale falls could act as "stepping stones" that enable hydrothermal vent and cold-seep communities to disperse across the depths and to colonise different sites. If, as he suggests, whale falls are relatively common on the deep-sea floor, it is quite possible that, as whale falls share over 10 species with hydrothermal vents and over 20 species with cold seeps, the whale fall communities could serve as a conduit for dispersal for animals requiring these chemosynthetic conditions for survival.

Many more questions remain to be answered concerning the communities and processes relating to whale falls. Studying these processes, although logistically tricky in terms of first finding a dead whale and then towing and sinking decomposing whale carcasses, are essential to gain a comprehensive understanding of the role of whale falls in deep-sea colonsation processes. Biotechnological applications for some of the animals found at these sites are also being considered. For example, whale-fall bacteria are a novel source of cold-adapted enzymes of potential use in cold-water detergents.