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the most highly beloved, nor the most feared. They are not the whales, the sharks, or the sea stars. They are not the familiar animals we eat, such as clams or oysters. They are not the organisms we mostly study, like porpoises, or the animals we capture to exhibit in public aquariums. They are usually not octopuses, although some octopuses are part of plankton as juveniles and a few as adults. In The Log from the Sea of Cortez (1951), writer John Steinbeck said that the disappearance of plankton, although the components are microscopic, would in a short time probably eliminate every living thing in the sea and ultimately the whole of human life. Since he wrote that, we have learned of the ecological communities surrounding the deep-sea hydrothermal vents that are not dependent on plankton for their survival, but instead depend on the hydrogen sulfide there, and they use bacteria to break it down and gain energy. But everything else in the ocean depends on plankton.
    Plankton can be divided into plants (phytoplankton) and animals (zooplankton). Most animal phyla have representatives in the plankton, at least at some stage of their lives, since many animals such as fish and octopuses spend their juvenile stages there. Those creatures that are only in plankton temporarily are known as meroplankton, such as octopus paralarvae, while those that spend their whole lives there are called holoplankton, such as small single-celled plants, or diatoms.
    The animals and plants of the plankton are extremely important to the ecology of the oceans. First, the plankton provides food for most of the ocean’s other animals. Phytoplankton provide food for zooplankton, which in turn are eaten by fish and by other tiny organisms like paralarval octopuses. Little fish are eaten by big fish, and fish are eaten by sea birds, squid, marine mammals, and humans. The world’s largest fish, the whale shark, and the world’s largest animal, the blue whale, both live on plankton. Dead plankton and their waste products drift down to provide food for mid-water animals and those living on the abyssal bottom. The numbers of animals and plants in the plankton are tremendous. Terrestrial insects represent the most species (over a million have been described), but there are more copepods, small weak-swimming crustaceans, in the ocean than any other animal on earth.
    Counting Copepods
    I got an idea of how many plankton there are when I was an undergraduate at the University of Washington in Seattle. As part of the laboratory experience in a biological oceanography class, we had to separate a plankton sample into its constituent parts, to species level. The sample came from the Chukchi Sea, northwest of Alaska. It was almost all copepods, collected with a large plankton net towed behind the oceanographic research vessel
Thomas G. Thompson
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    The sample consisted of a quart-sized jar filled with microscopic organisms. I don’t remember how long a plankton tow this represented or how much water was strained to get this amount of plankton. The plankton was divided into equal portions for each member of the class. The class was not large, maybe a dozen students, and we each had to separate our sample into six species. My sample was in a specimen bottle about the size of my middle finger and was filled with copepods. After several full afternoons poring over a microscope and breathing formalin, we ended up with smaller specimen jars, each with a precise number of different species.
    This tedious chore gave us a valuable experience in the field of taxonomy, the science of zoology that divides animals into species. And the exercise gave the principal investigator on the staff an idea of water conditions where the sample was taken, since different species live in different conditions.
    Copepods are the most numerous animals on Earth, and they are very important. Large planktivores such as whales eat them, as do juvenile octopuses drifting in the plankton