Plankton' 



By 



LOUELLA E CABLE, Fishery Biologist 



Bureau of Commercial Fisheries Biological Laboratory 

 Ann Arbor, Mich. 



INTRODUCTION 



The word "plankton" was derived from the 

 Greek "planktos," which means "wandering." 

 It is a collective term applied chiefly to all 

 those small, extremely diverse forms of 

 plants (phytoplankton, fig. 1) and animals 

 (zooplankton, figs. 2 and 3) that drift aimlessly 

 with the currents in all natural waters and in 

 artificial impoundments. Most of the forms 

 swim feebly and are incapable of making long 

 horizontal journeys except as they are swept 

 along by persistent winds and currents. Some, 

 nevertheless, make extended vertical migra- 

 tions in response to stimuli not clearly under- 

 stood. These movennents have been partially 

 correlated with diurnal changes in the inten- 

 sity of light; the salinity, temperature, and 

 density of the water at different levels, and 

 community pressures may also be important 

 factors. Most planktonic organisms, especially 

 the one-celled plants and animals, are ex- 

 trennely snnall and were unknown prior to the 

 invention of the compound microscope late in 

 the I 7th century. 



Although most of the waters of the world 

 are inhabited principally by plankton, little is 

 known about these organisms compared with the 

 great store of knowledge available on land 

 plants and anin-ials. The plankton and their 

 environment have been difficult to study. For 

 the nnost part, knowledge has been gained 

 indirectly by the use of electronic and other 

 specialized instruments that record move- 

 ments of the animals and the depths, tem- 

 peratures, and chemical composition of the 

 aquatic environment, or directly by analyses 

 of samples of water, bottom sediments, and 

 living organisms brought to the surface and 

 preserved for later study in laboratories. 



Louella E. Cable. Fish. Market News 5(11):6-12. 



.Mthough the minute plants (diatoms and blue-green 

 algae) and very small animals (protozoa, entomostracans, 

 and larvae of various other forms) make up the great bulk 

 of the plankton, a few larger forms (Jellyfish and salpae) 

 that have only weak powers of locomotion are also in- 

 cluded. 



The future holds promise of more intimate 

 observations of the plankton in their under- 

 water habitats than were possible in the past. 

 Soon the observations will be made fronn ma- 

 neuverable submarines capable of sustained 

 operation at great depths and from permanent 

 underwater research stations. The research 

 stations are still in the planning stage and 

 may not become reality for some time, but the 

 deep-water submarine is already in use for 

 close scrutiny of the bottom and study of 

 plankton and other inhabitants in all strata of 

 the seas. The Woods Hole Oceanographic 

 Institution has operated the Alvin , one of the 

 first of a series of these specialized sub- 

 marine laboratories that are being developed. 

 The Deepster , with an outer hull of fiber 

 glass, and the Alumina ut , with a hull com- 

 posed of a number of thick aluminum rings 

 bolted and glued together, are joining the 

 deep-water fleet. Some of these vessels may 

 be equipped with blue-green lasers for under- 

 water communication and for improvement of 

 visibility for television and photography. 



CLASSIFICATIONS OF PLANKTON 



The major taxonomic units, or primary 

 divisions, of the plant and animal kingdoms 

 are called phyla. Each phylum is a unit com- 

 posed of organisms sharing one or more 

 fundamental characters that reflect a common 

 descent. Many levels of this systematic clas- 

 sification of life are represented in the plank- 

 ton, from the simplest one- celled plants 

 (Thallophyta) and animals (Protozoa) to very 

 complex little animals such as crustaceans 

 and fish larvae (fig. 4). Certain many-celled 

 animals are planktonic for a short period 

 during early development even though, as 

 adults, they become either sedentary (e.g., 

 oysters, clams, and sponges) or free swim- 

 ming (e.g., shrimp and fish). Plankton of less 

 than l/600-inch maximum diameter make up 

 the nannoplankton or microplankton. Forms 

 large enough to be seen with the naked eye are 

 macroplankton. 



