Our second blog today is from Jesse Powell.
Pop quiz! What’s the largest migration on Earth? Is it the wildebeest of the African plains, or perhaps a marine bird like the sooty shearwater seasonally migrating from New Zealand to North America, or maybe the migration of the gray whales from Baja California to Alaska? In fact it is none of these. The largest migration on Earth, in terms of total biomass migrating, or number of individuals migrating, is the daily migration of plankton and small fishes from depth to the surface and back again. This phenomenon is called diel vertical migration, or DVM for short, and is by far the largest migration on Earth. Each night, plankton and small fishes (aka micro-nekton) come to the surface from a depth of a few hundred meters. They come to the surface to feed on phytoplankton and on other zooplankton. Before day breaks these same zooplankton will descend again to the depths, usually 200 to 800 meters (656 to 2625 feet) below the surface.
Let’s consider a specific case of a copepod 2.5 millimeters (0.1 inches)in length that migrates 350 meters (1148 feet). Three-hundred and fifty meters? I hear you scoffing! Granted it doesn’t sound like a long distance. However, when you realize that 350 meters equals about 140,000 body lengths for a migrating copepod, then you realize that this is equivalent to me walking from downtown San Diego to Santa Barbara every evening for dinner, and then walking back. And migrating plankton do this every day of the year–roughly equivalent to me walking around the world, more than twice, every year! Literally trillions of individual animals participate in this nightly mass migration. Also consider that a significant portion of the ocean’s biomass participates in this nightly migration, so we’re talking about hundreds of millions of tons of biomass. It quickly becomes clear that DVM is larger than all of the other great migrations combined.
Why do they do it? Well, it’s pretty clear that zooplankton and micro-nekton come to the surface layer (e.g. top 100 meters (328 feet) to feed. That’s where the phytoplankton are, and therefore, that’s where most of the food is. But this doesn’t explain why they descend with the coming day. Why spend all that energy moving up and down vast distances when you could simply stay at the surface and gorge on the
all-you-can-eat-buffet? The best answer is predators. During the day, light renders zooplankton and small fishes much more visible to their predators. DVM is an adaptive behavior that probably evolved independently in many taxa, or groups of zooplankton and fishes, over many, many generations. Several interesting corroborating observations support the light/predator hypothesis. First, during a solar eclipse, migrating zooplankton and fishes will start to migrate to the surface, only to turn around when the solar eclipse ends. Tricky! Second, when a parcel of water containing migrating zooplankton passes over a shallow seamount during the day, the zooplankton are unable to descend to darkness, and are consequently massacred by planktivorous fish. A total bloodbath. Finally, and most convincingly, is the observation that in those rare areas where predators are found predominantly at depth during the day, and at the surface at night, the local zooplankton prey will reverse their migration (e.g. reverse-DVM) in reaction to the new predation pressures. So, ultimately, we believe that DVM is driven by predation pressures. I guess I’d walk back to San Diego, too, if there was a strong possibility of me being eaten in Santa Barbara.
I hope you have all found this little diversion into the secret lives of critters interesting! I’ll leave you with a couple pictures from a recent manta net tow. This manta tow was remarkable in that we collected a huge number of zooplankters, and that the sample was composed mainly of only two species of small crustaceans. You can see in the pictures a clear, smaller copepod species, and also a beautiful iridescent blue amphipod. I’ve never seen this amphipod species before. When the cod-end from the manta net came up filled with what looked like blue paste, I was sure that we had captured a big blob of grease or waste. Instead, imagine our surprise when we saw these!
Contents from the cod end of one of today’s manta tows filled with blue amphipods.