The Great Migration of Miniscule Things
Time was running short. Dave Neuswanger, a retired fisheries biologist, had led such a fascinating discussion about winter limnology (limnology is the study of lakes), that the afternoon program of our Master Naturalist workshop was behind schedule. As we gathered boots, coats, and a digital meter that measures both temperature and dissolved oxygen, I suggested we bring the plankton net, too, and combine trips to the hole through the ice on Lake Namakagon. Dave hesitated, and gently reminded me why we hadn’t combined these data collection forays on the schedule in the first place: the critters we wanted to find just wouldn’t be near the surface in broad daylight!
Many types of zooplankton (which are mostly microscopic animals in lakes and oceans) participate in a cycle where they swim up to the surface at dusk and descend to the dark depths again at dawn. Scientists call this diel vertical migration (DVM). The word diel comes from the Latin word for day, and refers to a 24-hour period.
Such a great number of tiny organisms exhibit this behavior that Professor Hays of Deakin University in Australia wrote “DVM in ocean zooplankton is likely to represent the largest daily migration of animals on Earth, in terms of biomass.” The DVM was discovered during World War II, when the U.S. Navy was using sonar to watch for enemy submarines. A dense group of zooplankton, moving in sync through the water column, scattered their sonar so much that it looked like a false bottom in the sea.
On our second, later, visit to the lake, I wasn’t sure what we’d find as Dave pulled our fine-mesh plankton net up out of the hole. We all cringed as he used bare hands to coil the net’s wet line. A pink sunset was just fading over the hills around Lake Namakagon when we trudged back to shore though soggy snow.
While the Master Naturalists munched on appetizers, Dave concentrated the sample and I set up the Museum’s new digital microscope. Just as we were getting antsy and students started to fill their plates with baked ravioli for dinner, Dave brought over a small porcelain cup filled with lake water. Careful not to spill, I set it under the microscope and fiddled with the focus wheels. Onto my computer monitor jumped a school of darting critters. Excited now, and wanting to share, I plugged the projector cord into my computer. A cheer of excitement rippled through the group as larger-than-life aliens scurried across the screen at the front of the room.
Almost all of the swimmers were daphnia. You may have seen these mini-crustaceans before, in a high school or college biology lab. Through their translucent carapace, it is easy to view the daphnia’s heart beating and blood cells flowing. They react quickly to alcohol and caffeine, so students can easily quantify the effects these substances have on a daphnia’s heart rate.
We didn’t want to torture these critters, though, just admire them. Roughly oval in shape, daphnia have a single large eye at the front of their body, along with two pairs of delicate, branched antennae. It is the second, larger pair of antennae that drives their movement, especially the sideways jerky hops that earn them the nickname “water fleas.” Despite their ability to move short distances, on a larger scale daphnia are at the mercy of strong currents, and tend to avoid them. Daphnia’s many legs are used to create their own, personal current that drives food through their carapace so they can filter feed.
This ability to convert bacteria, algae, and fine detritus into fish food makes daphnia an essential part of many aquatic food chains. (The mass exodus of zooplankton from surface waters during the day must be a huge boon to their phytoplankton prey who can then photosynthesize in relative safety.) Although any organism’s role in the ecosystem includes eating and being eaten, it’s also their evolutionary prerogative to maximize the first and delay the second. That’s where the diel vertical migration (DVM) comes in. Daphnia (and other zooplankton) are highly visible to fish in daylight, and sunrise triggers them to escape by fast downward swimming, not simply by sinking. Fast-swimming daphnia are more conspicuous, but for a shorter time, and escape the brightly lit, risky, upper waters faster.
Once they’re at depth, beyond the reach of sunlight, though, how would the daphnia know that dusk is falling? Scientists hypothesize that they use an internal clock, run by their single eye. Pigment in their eye changes over time, so their migration is synchronized by light at dawn and triggered by biology at dusk. While light is the main factor in their movements, DVM is expedited when the critters can smell fish nearby. The fish smell is a result of chemical substances called kairomones. This fancy word just means that what the fish give off is detected by a different species, who gains some sort of advantage from it. (In contrast, pheromones are detected by an animal of the same species.) In murky water, these chemicals may actually be a more important trigger of DVM than light.
Several Master Naturalists agreed that watching the daphnia swim under our microscope was better than TV. In the end, we were glad we’d gone back out at dusk to sample plankton, even though it delayed our dinner. Happy and exhausted from our long day of discovery, we participated in our own diel migration—going home to bed.
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