For example, phytoplankton typically have a very large surface area relative to body size. They can be round and flat, have long spines or bristles, or join single-celled units in long chains. Many zooplankton need to stay in the same zone because the phytoplankton are their food source. Zooplankton are often wide and flat, and many have long spines and bristles, but they have an advantage over phytoplankton - they can swim. Tiny movements of their appendages can propel them and keep them in the food-filled zone of the ocean.
Some zooplankton also rely on accumulating tiny amounts of oils, whch help serve as floatation.
Have a variety of materials on hand. Students can bring in materials to use and share. The marbles and size restrictions help standardize the races. This activity is likely to produce splashes and spills, so it should be done in a location where this is not a problem. Start with observations of zooplankton and phytoplankton.