I like to read science fiction. I particularly enjoy a good alien world, like Neal Asher’s world of hyper-aggressive invertebrates or Lois Bujold’s planet of radially symmetric beasts. Still better than these, however, are the alien worlds right here on Earth, hidden in plain sight. (Well… if “on the same planet” counts as “in plain sight.”) The hidden world I’d like to talk about today is that of whale falls.
A “whale fall” is just a nice way of saying a dead whale: when a whale dies in the ocean, it sinks—falls—to the bottom, and you have a whale fall. Whale falls are different from other dead animals in two big ways. First, they are, well, big. No other living animal gets as big as our biggest whales. When one of those dies, that’s a lot of dead whale.
Second, when they fall to the ocean floor, they change the environment on the ocean floor dramatically. A dead animal in a forest or jungle or lake is a piece of dead meat in a habitat already full of other kinds of food: leaves, berries, insects, fish, etc. Some animals in these habitats will scavenge on the dead meat, but many other animals will ignore it. The deep ocean floor is not like a jungle. It is barren, with no sunlight to support plants or plankton, which are generally the food sources that the rest of a food chain depends on. The only organic food sources near the deep sea floor are the dead things that fall down from the water column above, picturesquely named “marine snow;” and that water column is filled with creatures trying to eat anything they can find, so not a lot makes it all the way to the bottom. When a dead whale lands on the ocean floor, it is the equivalent of an enormous banquet being dropped into the middle of a desert.
When a whale falls, it gives rise to an entire ecosystem by itself.
The first thing that happens when a whale falls is that everyone flocks to it to eat it. Hundreds of hagfishes (whoever named those creatures was not in a generous mood) and sleeper sharks cover the carcass, eating for all they are worth: who knows when another such windfall will come their way? Tiny amphipods and large crabs also come to the party.
These animals strip the whale carcass to the bone, removing tissue at a rate of 40-60 kg per day. Whale falls that scientists have been able to observe—which are few: these are very deep under water, and very hard to observe—have been reduced to skeletons in less than two years. An adult blue whale carcass weighing 100,000 kg (!) might take five years to be stripped to the bone. Imagine gorging yourself on Thanksgiving dinner for five years straight.
Toward the end of this stage, as the pieces of tissue remaining on the bones get smaller and smaller, the size of the scavengers decreases too. The sleeper sharks and hagfishes are replaced by more little amphipods, which are finally replaced by minute copepods.
Next, the bones and the sediments around the bones (which are full of organic debris, since all those larger scavengers have been eating messily for months or years) are colonized by enormous numbers of polychaete worms, tiny snails, and juvenile bivalves. Around one skeleton, “a bed of free-living, centimeter-long polychaetes… undulated in the near-bottom flow, resembling a field of white grass.” (Smith & Baco 2003, p.319)
What’s that? You want a better look at its cute head? Here you go:
Once the polychaetes and friends have devoured what they can, they are replaced by a diverse community of animals—including limpets, snails, and more amphipods and polychaetes—that live off of sulphur emitted by the decomposing bones. This is more like the sort of food that is usually available at the ocean floor: namely, food that does not sounds like food at all. How do you live off of sulphur? In the nutritional desert of the ocean floor, some creatures have evolved to exploit energy from non-organic matter in a process called chemosynthesis. (Compare this to photosynthesis, where plants and some animals take advantage of the energy from sunlight.) When sulphur is what is available, someone on the ocean floor will figure out how to live off of it. The whale bones can support these sulphophilic (sulphur-loving) communities for decades.
407 species have been found on whale falls; 21 species have only been found on whale falls, meaning they may be whale-fall specialists. Species that can only live on whale falls must have to travel from one to the next, like birds flying from island to island, as each whale fall is eaten.
Other species found at whale falls can also live in other types of specialized ocean-floor habitat, such as cold seeps and hydrothermal vents, where hydrocarbons and minerals seep into the water and support chemosynthetic communities. It is thought that whale falls may be the “stepping stones” that allow these species to travel from cold seep to cold seep or from one hydrothermal vent to the next. Some scientists think that whale falls, and before them, falls of huge marine reptiles, may have been the evolutionary stepping stones that allowed animals to colonize the strange, extreme undersea environments of cold seeps and hydrothermal vents in the first place (Dominici et al. 2009).
Isn’t it amazing to think that, indirectly, this guy:
might have led, way down at the sea floor, to the diversification of guys that look like this:
Dominici S, et al. 2009. Mediterranean fossil whale falls and the adaptation of mollusks to extreme habitats. Geology 37(9):815-818.
Smith CR, Baco AR. 2003. Ecology of whale falls at the deep-sea floor. Oceanography and Marine Biology: an Annual Review 41:311-354.
*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!