Photomicrograph of Calanus marshallae showing recently eaten algae in the gut and stored lipids in the oil sac.
Tiny crustaceans called copepods form an important part of the Bering Sea food web. Some species, like Calanus marshallae graze on rich blooms of phytoplankton and store the energy from this food in a special organ called an oil sac. During the winter when little food is available, the juvenile copepods swim deep down into the water column and enter into a dormant state. They are kind of like bears fattening up during the fall and then hibernating over the long winter months. Copepods provide an important source of food for many predators, ranging from juvenile fish to baleen whales.
The size of the copepod population changes from year to year and can be hard to predict. Because scientists don’t know exactly how copepod dormancy is regulated, it’s also hard to predict exactly when they will move into deep water, and when they will swim back to the surface, begin feeding, and produce the next generation of baby copepods. All this uncertainty makes it hard to know how many copepods will be available as food for fish. Learning more about these variable populations will help us to better understand and predict the behavior of the ecosystem.
I’m pretty interested in copepods, but I guess that to most people, one copepod pretty much looks like another. We need to be pretty creative if we want to understand what’s going on inside those tiny chitinous bodies. Over the past few years, I’ve been working to develop markers that indicate changes in copepod physiology—when they are building up their energy stores, when they are getting ready to molt, when they are entering into dormancy. I do this mainly by looking at which genes get “turned on” and “turned off” under different conditions.
This cruise is providing me an opportunity to learn about how copepods respond to variability in their own environment, particularly how they are affected by the availability or lack of food. I’ve been picking copepods out of the bongo nets throughout our cruise and examining them one by one under the microscope. I can see that some of them have been munching on algae and others don’t seem to have eaten for a while. I’m also testing how the copepods respond when I culture them aboard the ship—some with food and some without. This experiment will help me to better understand the nutritional state of animals I collect from the field: Have they been eating and storing energy all along? Are they burning through their energetic reserves? Are they rebuilding their energy stores after a period of starvation? I’m hoping to eventually learn how metabolic condition varies within the population, and whether feeding and food availability helps to signal when the copepods enter dormancy.
note: Dr. Ann Tarrant is a visiting scientist from Woods Hole Oceanographic Institution at sea on the Oscar Dyson.
