Zooplankton

by Prentice K. Stout

Zooplankton (zoon - Greek for animals and plankton, a word derived from Greek meaning wanders) are divided into two types: those that spend their whole lives as small plankton and those that drift as plankton only in their young stages. This second type will mature into various larger life forms.

Some of the permanent members of the zooplankton community are forminiferans, radiolarians, and copepods. The first two are roughly comparable to the diatoms and flagellates of the phytoplankton community. Forminiferans (hole-bearers) are organisms that live within small shells made of calcium carbonate, a substance similar to limestone, marble, and chalk. The protoplasm, a complex chemical "soup" found in living cells of all animals, flows out through the holes in their shells, forming a sticky, food-catching network.

As they outgrow their shells, the old abandoned ones are shed and the animal grows a new one taking the calcium carbonate from the surrounding water. Dead animals and discarded shells sink to the deep ocean bottom, there to be dissolved into calcium and carbon, available for reuse. In shallower waters, where this breakdown cannot take place, their remains built up thick layers called "globigerina ooze," named for the most common forminiferans, the globigerina. Over the 500 million years these animals have been in existence, and have been deposited on the ocean floor, forces have shifted the seabed and the presence of these new and ancient deposits of ooze has enabled scientists to estimate the age of such sedimentary deposits in different, separate parts of the world. This has given us a good fossil record. The chalk beds of Georgia and Mississippi as well as the white cliffs of Dover, England, attest to the vast quantities of these tiny animals.

Radiolarians are similar to their relatives, the forminiferans, both in skeletal shapes and protoplasmic food trapping ability. But their skeletons are made of a more resistant material called silica, a valuable filter and abrasive agent used in such products as toothpaste and for filtering certain liquids. Their bodies litter the ocean floor covering approximately 3 million square miles of tropical oceans. Copepods are so numerous that it is estimated that they compose seven out of every ten zooplankters. Their huge numbers have led some biologists to suggest that there are more of these animals in the world than all the multi-cellular animals combined.

Treading water by beating their limbs (from which they get their Greek name "oar-footed") as many as 600 times a minute, they use a vast amount of energy. this causes them to be ravenous eaters, having to consume their own weight in food each day. All of their food consists of phytoplankton. They are related to crustaceans, among whose members are crabs, lobsters, shrimp, and barnacles. For all their smallness, some large inhabitants of the oceans are dependent on them for food. The 45 foot long Basking Shark and the 60 foot long Whale Shark head the list of those in the oceanic food chain that feed on these copepods.

Dr. A.A. Benson, at the Scripps Institute of Oceanography, has stated that half of the world's photosynthetic product is converted, for a time, into wax by these tiny animals. Their fat is converted into a polyunsaturated liquid wax which is stored for the animal's use, but when the animal is eaten by sardines, herring, and anchovies, their wax is converted back into common fats by these large predators. Thus, we can see the importance of the copepod in the oceanic food chain.

Members of the zooplankton community migrate within the water column vertically each day. The phytoplankton, which do not migrate in this fashion are doomed to be wafted about the seas, by wind and waves. During the daylight hours, depending on the individual species preference, the zooplankters confine themselves to a narrow vertical range. As the fading light announces the onset of evening, the plankton begin a haphazard upward drift until, with night upon them, the entire zooplankton community is on the surface. Here they feed on phytoplankton and are in turn eaten by other larger species. As daylight approaches they drift down to their respective levels in the water column. The causes of this vertical migration have eluded scientists as to the precise answers.

We have mentioned zooplankton in the food chain. We can best illustrate this if we take 10,000 ponds of producers, the phytoplankton. This would support 1,000 pounds of first-order consumers or the zooplankton. IN turn this would support 100 pounds of second order consumers such as herring or anchovies. This would support 10 pounds of third order consumers such as the larger fish species, which would then support 1 pound of the fourth order consumers, for example, a seal. So we can see that for a seal to gain one pound, he would indirectly have to consume 10,000 pound of phytoplankton, the ultimate producers in the sea. Thus anything that interrupts this food chain can have serious consequences to the wellbeing of these consumers up the food chain ladder.

References

Hammer, William M. 1974. "Blue Water Plankton" National Geographic Magazine, October.

Gaskell, T.F. 1964. World Beneath the Oceans, The Story of Oceanography. The Natural History Press, Garden City, New York.