Images captured in the laboratory of J. Rudi Strickler, except where noted.
 ore than 70 percent of our Earth is covered by water. Oceans, lakes, rivers, ponds, and streams, whether freshwater or salty, are filled from top to bottom with swarms of shrimp-like creatures smaller than the thickness of your fingernail. These tiny animals are zooplankters, crustaceans that measure only 1 to 2 millimeters in length. A conservative estimate of just the number of copepods, the largest division of zooplankton, stands at 1.347 x 10²¹. To maintain such a huge population, zooplankters must be efficient breeders.
The old theory held that zooplankters mated by chance, with the males simply drifting through their environment until a serendipitous encounter with a female allowed the animals to procreate and continue their species. In fact, this lackadaisical appearance has influenced theories on all aspects of a zooplankter’s existence: mating, feeding, escaping predators, and so forth. Their whole lives were believed to be riding on Lady Luck. But the research of Professor J. Rudi Strickler has changed the way some people look at zooplankton.
Fig. T. Male Cyclops scutifer swimming with its typical hop-and-sink pattern. The animal executes about one hop per second using the time between hops to perceive hydro-dynamical and chemical signals. Fig. U. Female Cyclops scutifer during the sink (and listen) phase. This female carries its approximately 12 eggs in two sacs until the young naupli become “airborne” themselves.
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Strickler, who was born in Switzerland, says he was always interested in aquatic sciences. He chose to study zooplankton (the community in which zooplankters live) because it was common to all aqueous systems, giving him a chance to travel for work and “see the world.” And see the world he has. Prior to becoming the Shaw Distinguished Professor of Biological Sciences at UWM in 1990, Strickler taught at such varied and illustrious schools as Johns Hopkins, Yale, the University of Ottawa, the University of Southern California, and Boston University. This past summer he taught in France as a visiting professor and then spent two weeks in Taiwan to attend a conference. His scholarly collaborations have taken him to laboratories around the world.
Even though Strickler has made many breakthroughs in the study of zooplankton, it is not the creatures that surprise him. “They are my true friends,” he says. They reveal their secrets to him when he creates the right conditions for them in the laboratory. His real surprise, he says, “is how the scientists react to the news.” Scientists are not unlike the rest of us: “They hate to change their perspective.”
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Fig. V. Zooplankters swimming through a light-density gradient. A special optical pathway (Schlieren system) allows us to see the hydro-dynamical disturbances produced by the swimming animals. These “footprints” are used by the animals to distinguish friends from foes and mates from all others.
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One of the secrets that Strickler has uncovered is the mating habits of zooplankters. Contrary to the old paradigm, he has found that zooplankters actively seek out their mates. Graduate students Mike Doall and Sean Colin from the State University of New York at Stony Brook visited Strickler at his lab in Milwaukee for two months. Using the zooplankters they had brought, the students set up their experiment in a tank of water hooked up to Strickler’s monitoring devices. It was not long before they noticed the males pursuing the females. So with their preliminary observations they had already accumulated evidence against the old notion of chance dominating the mating habits of the animals.
Strickler and the students, who came from the laboratory of Professor Jeannette Yen, wanted to detect whether the zooplankters found the object of their intentions through sight or some other mechanism. They discovered that both the male and the female swim with a normal speed of about 10 millimeters a second. But when the male crosses the track where a female has just swum, he changes course to follow her track, accelerating to speeds of 30 or 40 millimeters a second in order to catch up with her.
Fig. W. Head region of a typical zooplankter. Note the arrays of sensors on the antennules, which allow the animals to sense hydro-dynamical disturbances and their structures. Photo by Paul Dixon, Australian Institute of Marine Science.
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To make this discovery, the researchers had to simulate the natural environment of the zooplankton as closely as possible. When the zooplankters are placed in a tank, they tend to do one of two things. If the lights in the room are on, they head to the bottom of the tank. If the lights are off, they swim into one of the dark corners of the vessel. This is not conducive to studying zooplankters’ behavior, because, as Strickler explains, “Mating under the stress of swimming around in corners does not happen.”
To get the creatures out into the tank and swimming freely, Strickler uses “a light in the dark.” At the beginning of the experiment, the lights in the room are on; therefore, the animals swim around at the bottom of the tank. When the lights in the room are turned out, the zooplankters begin to swim upward toward positions in the corners. But this time Strickler has placed a blue laser beam at the center of the bottom of the tank, pointing vertically upward through the water and out the top. As the zooplankters swim through the tank seeking out its corners, some of them swim into the laser beam and begin to glow like fireflies. The other animals see this and swim toward them, which results in most of the animals swimming around the light and in the middle of the tank, where they have the freedom they need to feel as if they were in an ocean rather than constricted by the walls of a vessel. According to Strickler, with the blue-laser-beam method, the zooplankters act as they would in their native environment.
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Fig. X. Zooplankters swimming through a light-density gradient. On the right, Cyclops female leaves a trace males will recognize as generated by a potential mate and will track it. On the left, Daphnia are parthenogenetic and minimize traces to avoid predators finding them.
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One important piece of the mating puzzle remains to be solved: What signal is the male receiving from the female when he crosses her track? Is it related to the eddies in the water, to a chemical she is emitting, or something else? These are questions that Strickler hopes to answer soon, “before industry releases chemicals in tons into the oceans and interrupts the mating process.” Not only would it be disastrous for the food chain and our ecosystem, but, as Strickler puts it humorously, “there will be no next generation of zooplankton, the young fish will starve to death, and cans of sardines will be very expensive.”
The reason that research focusing on these questions has not yet begun is an old one: money. “Scientists do not give the go-ahead in the reviewing process of grant proposals,” Strickler laments. In scientific research, someone else is always controlling the purse strings. As far as finding funding for research is concerned, Strickler says, “It is no fun to be on the forefront.” The epiphanies he brings to his field may bring him fame but they have yet to bring fortune in the way of guaranteed funds for future research.
Fig. Y. Large hydrodynamic disturbances trigger an escape reaction. (a) The female Cyclops passes after three swimming strokes (S) a pipette (D), which will generate a disturbance similar to an attacking invertebrate predator. (b) Position of the disturbance (2) 0.021 sec, (3) 0.042 sec, (4) 0.063 sec, (4) 0.084 sec after (1). Positions of animal at the same time. (P) position of power strokes to escape from simulated predator. Escape speeds of the animals can be up to 1,000 body lengths per second for the duration of a few seconds.
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All the breakthroughs that Strickler has made in the study of zooplankton are the result of applying novel equipment and methods to test and challenge long-held beliefs. He realized that research methods useful for investigating the human world were woefully inadequate for seeing into the tiny fluid lives of zooplankters. These creatures, he explains, live in a world of “high speed, high frequency, three dimensions, and darkness.” Therefore, “you need special high-speed, infrared optical systems; tracers to see the flow of water; high-quality lenses to see the food, three-dimensional resolution, and so on. The earlier scientists in zooplankton research did not make this analysis.”
The issue of filming the animals in order to get an accurate visual on how they act is crucial. “Highspeed video was only possible in the last 10 years and until of late the resolution was sub-optimal. We used high-speed films (16mm) where we could change the speed according to topic,” Strickler explains. It was these films that allowed him to view the mating game of the zooplankters, as well as feeding habits and predator survival. Feeding experiments required adding a dye to the water in order to see the flow of food in and through the little animals. “To use dye to see flow is a common experimental tool in fluid mechanics. To use both methods [the dye and high-speed video] at the same time was, I think, my lab. If the topic is how zooplankters capture and ingest food, one has to see the path of the water and the paths of the food.” Most of the methods that led to the breakthroughs were devised and invented by Strickler himself.
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Fig. Z. Damsel fish (feeds exclusively on zooplankton) attacking four zooplankters.  Click on image for complete process.
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To examine the feeding traits of zooplankton required keeping the creatures still so that their internal movements could be videotaped. One creative method of Strickler’s was to use the hair of a dog to which he would glue a zooplankter. “The choice of species to glue is important. I take animals, which, in the ocean, do not move much or not at all. Therefore, gluing is no problem for them. Like putting a plant on a leash, it does not worry the plant very much.” To simulate the home oceanic environment for such a tethered zooplankter requires just 125 milli- liters of water, or about a half a cup.
The unique set-up Strickler has at his lab, part of the Great Lakes WATER Institute at UWM, piqued the interest of television producers at the British Broadcasting Corporation (BBC). They visited Strickler’s lab in the spring of 2000 to tape footage of zooplankton for part of their documentary series Blue Planet. A sweeping account of the history and biology of the seas, Blue Planet aired in the United States on the Discovery Channel over the course of eight segments and four months in early 2002. Not only have others learned about Strickler’s work from watching the show, but he has learned from viewing it as well.
“The best idea I got from watching the series, the results from other researchers, was when a killer whale wanted to capture a sea lion. The sea lion tried very hard to stay close to the tail fin of the whale because that is the least dangerous place for the moment. This strategy was new to me, and, sure enough, we found it in our animals as well.”
The study of the minuscule world of zooplankton has larger impacts on our world than we might imagine. One is the issue of the food chain, and the web of life in the seas. No zooplankton means no fish, which could have devastating and unforeseen effects on all earthly life. Strickler’s zooplankton research could impact the world in less obvious ways, as well. For example, zooplankton could help people better appreciate their role as stewards of the earth and its creatures: “the more you know about the intricacies of a system, the more you are willing to protect it,” Strickler says.
When Strickler was a student, his own professor studied algae, which is what feeds and sustains zooplankton. “He supported my drive to know more about zooplankton, the devils of his friends,” Strickler recalls fondly. He believes that “students tend to be carbon copies of their professors.” If that’s true, then we need not worry about the future of marine biology. Today’s students in Strickler’s classes will be tomorrow’s champions of the seas. 
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