Last week, writing about copepods, I mentioned that they make up what is probably the most massive group of animals on earth. I also mentioned the likely runner up: krill. In particular, the Antarctic krill, Euphausia superba.
Photo by Uwe Kils, via Wikipedia
The Euphausiids are a major group of small, shrimp-like crustaceans found worldwide in the marine plankton. Euphausia superba is probably the best-studied, and certainly the most abundant, of these species. They live all around Antarctica in the Southern Ocean, and are usually the dominant macrozooplankton grazer, occurring in vast, patchy swarms. But they are not the only one out there.
Their main competitor for the king of Antarctic plankton is a particular salp species, Salpa thompsoni. Salps are pelagic tunicates, barrel-shaped gelatinous animals that move and feed using a kind of lethargic jet propulsion, drawing water in one end of their bodies and filtering it for food particles before pushing it out the other. They also reproduce incredibly quickly, thanks to two features:
- Their bodies are mostly water, so they do not need to actually grow that much material on their way to full size, and
- they alternate sexual and asexual generations, with the asexual generation budding off long chains of self-feeding clones.The sexual clones then release sperm and eggs into the water, where they fertilize and grow into the next generation of asexual salps.
Together, these two facts enable salps populations to respond explosively within a few days of phytoplankton blooms. Krill, on the other hand, live several years, and spawn all at once in the early Austral spring. Their larvae will not reproduce until the next year.
Krill and salps do not tend to do well at the same time. Good years for salps tend to be bad ones for krill, and vice versa. What’s more, abundances of both are related to the extent of winter sea ice—positively for krill, and negatively for salps. This is due to a combination of their different reproductive strategies, as well as different food needs.
During the winter, krill live underneath the sea ice, and feed on ice algae, which grows on the underside of the ice pack. Well-fed krill are better prepared to produce eggs and spawn early in the season. Salps, on the other hand, can’t scrape the algae of the ice, but can respond quickly to open-water phytoplankton blooms. The upshot is that following cold winters with more sea ice, krill spawn more successfully, leading to a bigger year-class the next season. In winters with little sea ice, krill do not spawn as successfully, but salps explode as soon as the ice retreats and the spring phytoplankton bloom begins.
Over the last half-century, temperatures have been trending upwards in the Antarctic, and sea ice extent has been trending down. Along with these changes, “krill years” appear to have become less frequent. This does not augur well for the parts of the Antarctic food web that feed on krill—whales, seabirds, penguins, fish, and many others. Salps, though totally cool and more reproductively energetic than rabbits, just don’t have the crunch, tang, and oily, protein-ey goodness of Euphausia superba, the superb Antarctic Krill.
V Loeb, V Siegel, O Holm-Hansen, R Hewitt, W Fraser, W Trivelpiece, S Trivelpiece (1997). Effects of sea-ice extent and krill or salp dominance on the Antarctic food web Nature, 387, 897-900