The aquatic and marine animals we are interested in are small. They are about 1 millimeter in length, similar to a pinhead. If we assume that every liter of water contains one animal on the average, we get a population of 1021 animals. If we took all these animals together and formed a cube with their bodies, the cube would have a dimension of 6 by 6 by 6 miles. All mankind would only result in a cube of 2,000 by 2,000 by 2,000 feet.
Fish, either throughout their lives or at their early stages, feed on these animals. We call the community of them zooplankton, which in turn feed on the phytoplankton, the algae, bacteria, and other small food items suspended in the water. Their generation time is about three to six months. Their swimming speed is about 1 to 10 millimeters per second, but when they escape danger they run much faster.
One feature makes research on these animals challenging: The world at the dimensions of zooplankton is counterintuitive. We have no connection to their world. Water at their levels is very viscous. The volume of water they need for life is in the order of a few liters, but we see them as the animals of the seas, oceans, lakes, and ponds. What are their rules of survival? How could they hang on for more than 500 million years? How many species of zooplankters share the same way of life? How many ways are there?
The use of special optics in the laboratory allowed us to see what humans cannot perceive. The world of zooplankters is not only a small one, it is also a viscous one where signals stick around for some time, and where finding food and mates is always overshadowed by the danger of meeting a predator.
Zooplankters live in a three-dimensional space. Their species-specific densities in lakes and oceans are so low that the distances between two animals of the same species can be up to 10,000 body lengths. Most crustacean zooplankters are either females or males. They have to meet for mating. Whatever signaling they use to get together in this vast three-dimensional space, they have to avoid attracting predators. Some, like Daphnia, avoid the problem altogether by being parthenogenetic: Females need male fertilization only every 10 generations or so.
How do zooplankters do it? Lets see some examples: The female in Fig. U met a male (Fig. T) some days earlier. In order to attract him, she produced a mating call (Fig. X). This hydro-dynamical disturbance is species-specific and different from disturbances generated by other zooplankters (Fig. V).
In order to avoid predators, mechanoreceptors on the antennules (Fig. W) perceive all water motions. Should a signal represent something big, like a predator, the animals execute an escape reaction, swimming at high speed away from the source of the signal (Fig. Y).
Fish have evolved to overcome the high sensitivity of the zooplankters, their prey, by using a special tactic. Damselfish line up very accurately and carefully; when pushing their mouth over the target prey, they also produce a signal mimicking an attack from the opposite side. The fooled prey will jump into the mouth of the fish (Fig. Z). Besides lining up accurately, the fish also has to avoid producing a signal for nearby prey. In this way they can line up with the next one and again get their food (Fig. Z).
To learn more, visit the Strickler Web site: www.uwm.edu/~jrs.