Using a combination of field samples from the Norwegian Sea and a new method for analyzing sea life populations, researchers have shown that tiny marine crustaceans called copepods use cannibalism as a mechanism to limit their population.
Traditional notions have assumed that fluctuations in marine populations can be explained mainly by physical processes. That is, if you understand ocean circulation and other physical functions affecting marine life, then you can understand how and why populations fluctuate. Other theories of marine populations are based on birth rates, including the processes of feeding and growth that lead to the production of new offspring.
Mark Ohman of Scripps Institution of Oceanography at the University of California, San Diego, and colleague Hans-Jürgen Hirche of the Alfred Wegener Institute for Polar and Marine Research in Germany, have provided evidence that there is much more to the story. Instead of birth rates, they flipped conventional notions and analyzed the death rates of copepods.
In their study, published in the August 9 edition of the journal Nature, Ohman and Hirche found that egg mortality in the copepod species Calanus finmarchicus is directly related to the abundance of females in the population.
"We found that a tenfold increase in adult egg-producing female copepods will not result in a similar increase in the number of surviving eggs," said Ohman, a professor in the Integrative Oceanography Division at Scripps. "Rather, half as many eggs will survive. We believe the reason is cannibalism and we think it's a self-regulating mechanism that no one has found before in the open sea."
Copepods, tiny marine crustaceans (about four Calanus finmarchicus fit end to end on a dime), are the most numerous multicellular animals in the oceans, and possibly the most numerous on Earth. They figure prominently in marine food webs as grazers of phytoplankton and serve as prey for a variety of fishes, marine mammals, and marine birds. They also play a key role in the ocean's "biological pump," a process that transfers carbon from the atmosphere to the deep ocean.
Data for the Nature study were derived from an intensive field study conducted in the center of a gyre, a closed ocean circulation system, in the Norwegian Sea north of the British Isles and east of Iceland at a destination called "Ocean Station M." Battling rough seas and waves that sometimes surpassed 50 feet, Hirche's group and other marine researchers performed experiments and collected zooplankton samples for 80 days.
The sample data were then transferred to Ohman, who analyzed them using a new numerical method he devised for copepod populations.
"In the past, zooplankton loss rates were treated as constant and linear," said Ohman. "People didn't know enough about them so the simplest assumption was made, which was to pick a value that sounded reasonable out of the hat and keep it constant. We found a population-limiting mechanism that you couldn't predict using that approach."
Ohman believes the new study will point future research toward the importance of death rates in addition to birth rates.
"The implication of this study is if you want to predict how marine ecosystems might respond to climate change, or how the ocean food web is going to influence the ocean's capacity to take up carbon dioxide, you have to understand the biological dynamics as well as physical processes," said Ohman. "If you want to understand how variability in zooplankton populations influences variability in fish populations, then you have to understand and correctly model the rate of growth of these copepod populations. That's essentially what we're after–trying to develop better predictive models for the future."
A copepod specialist, Ohman stresses the importance of the tiny crustaceans in the marine environment, but believes they are not widely known because of their microscopic size.
"This group is my specialty because I'm fascinated with their highly diverse forms, coloration patterns, and appendages that can change remarkably from one species to another," he said. "Their life histories and the way they feed, grow, and interact with predators are intrinsically interesting.
"I think if children took a moment to look at these intriguing animals they would forget all about dinosaurs."
The Nature paper is dedicated to the late Scripps Professor Michael Mullin, a pioneer in experimental zooplankton studies.
The study was supported by the European Commission through the Trans-Atlantic Study of Calanus finmarchicus (TASC) project and by the National Science Foundation and National Oceanic and Atmospheric Administration through the U.S. Global Ocean Ecosystem Dynamics (GLOBEC) program. The goal of GLOBEC is to understand the ways global climate change may affect the abundance and production of animal populations in the sea.