Animal of the Week, August 2, 2017—could krill save the world?

In the Animal of the Week archives, charismatic birds and mammals far outweigh other creatures, and invertebrates especially are fairly lightly represented. This week let's go some way to reddress that imbalance. By weight, Antarctic krill (Euphausia superba), are the single most abundant animal on Earth.

The most recent AOTW, blue whales, might be the largest animal ever to have lived, but if you were collect all the blue whales together, they might just tip the (very large pair of) scales at approaching 1 million tonnes, all the Antarctic krill in the world would weigh almost 500 times as much—which is just as well, because they are the main source of food for blue whales and numerous other species.

The total numbers of krill in this half a billion tonnes are immense—as many as 500 trillion (that's half a quadrillion) might be alive at any one time. They form swarms of immeasurable number, although the density of these swarms can reach 30 000 individuals per cubic metre. As you might expect for something that exists in such a multitude the individuals are pretty small: the bioluminescent free-swimming shrimp like creatures might reach 6 cm in length and weigh up to 2 grammes.

The importance of Antarctic krill to the world probably cannot be overstated—as well as providing the main food source of a host of whales, seals, penguins, seabirds, fish, and squid, they are thought to be key players in the global carbon cycle and, as such, probably have an important role in moderating climate. Krill migrate up and down the water column, rising to the surface to feed on microscopic plants (phytoplankton) and some animals (zooplankton) but when they are full, they stop swimming and sink while they digest their food (same). The outcome of this is that they transfer carbon from the surface of the water (trapped in photosynthesising plants) to the lower levels (as faeces and their own bodies). The krill's waste sinks to the ocean floor where it is sequestered—were the krill not dragging the carbon down, the plankton would decay in the surface waters releasing greenhouse gases to the atmosphere.

As well as providing food for a multitude of sea creatures, krill are also popular with people—in Japan and Korea krill have been consumed as food for a while, but in the past couple of decades krill oil has become a popular dietary supplement and krill products are increasingly used in aquaculture as fish food and in other pet foods. But given their role in ocean food webs and the potential impact on climate change of depleting krill numbers, we must be careful not to overexploit the remarkable creatures.