FISHERY BULLETIN: VOL. 72, NO. 2 



in this web. To obtain this information, one ap- 

 proach is the investigation of basic trophic rela- 

 tionships among the various developmental 

 stages of different species in the food web (e.g. for 

 herring, Hardy, 1924) and the quantitative mea- 

 surement of matter or energy transferred along 

 these paths (e.g., for a lake, Lindeman, 1942). 

 From the four categories of complexity I chose to 

 study Pleurobrachia bachei A. Agassiz at the 

 single species population level. I have attempted 

 to integrate three basic aspects of the population 

 ecology oi Pleurobrachia into a study of its quan- 

 titative natural history: 1) the structure of its food 

 web, 2) the population parameters and attributes 

 which most affect the population growth rate, and 

 3) the trophic-dynamic aspect of the quantitative 

 transfer of organic matter. In previous work 

 (Hirota, 1972) the culture and metabolism of P. 

 bachei have been described. 



Studies on the trophic-dynamics of marine 

 planktonic food webs have concentrated on the 

 measurement of primary production and the fac- 

 tors which influence its level. MuUin (1969) sug- 

 gested that similar production studies of total zoo- 

 plankton or of single species are few, because no 

 simple, direct methods exist for the measurement 

 of secondary production in situ. He stated that two 

 basic approaches exist for these studies: the 

 laboratory "carbon balance" study and the popula- 

 tion dynamics approach. A somewhat more direct 

 measurement could be made as a modification of 

 the approach used in lakes by Haney (1971). In 

 situ population feeding rates (measured using 

 food particles labeled with isotopes) multiplied by 

 the population gross growth efficiency is the rate 

 of net production. This method has the advantages 

 of being more direct and made in nature, but it is 

 impractical for complex marine plankton com- 

 munities with their numerous and relatively 

 large, mobile species. It also requires detailed 

 knowledge of factors which affect gross growth 

 efficiency. 



Most marine planktonic species are not amena- 

 ble to culture in the laboratory for entire life cy- 

 cles, and results of laboratory experiments may 

 fail to represent accurately activity in nature. 

 Present field sampling techniques and variability 

 in plankton studies are often such that it is neither 

 possible to obtain sequential samples from the 

 same target population nor calculate the rates of 

 biological activity. In spite of these difficulties and 

 such severe limitations (Hall, 1964), more and 

 better data are needed in different ecosystems 



from their functionally distinct component species 

 before a clear understanding of the structure and 

 dynamics of food webs is obtained and generaliza- 

 tions of predictive nature concerning the systems 

 can be made. 



Studies on the population dynamics and produc- 

 tion of marine zooplankton (reviews by Mann, 

 1969 and Mullin, 1969) almost exclusively pertain 

 to "herbivorous" calanoid copepods. At present lit- 

 tle information exists on the production rates of 

 carnivorous marine zooplankters (McLaren, 1969; 

 Petipa, Pavlova, and Mironov, 1970; Sameoto, 

 1971), and the study of Sergestes lucens (Omori, 

 1969) is one with the supportive catch data of a 

 commercial fishery. The lack of data for higher 

 trophic levels is in part the result of an inability to 

 culture and maintain delicate or large, mobile 

 forms. Nearly all laboratory data on the 

 long-term metabolism and life cycles of zooplank- 

 ton species come from successful rearing of one or 

 more generations of facultative herbivorous 

 copepods (see Hirota, 1972 for references). Hamil- 

 ton and Preslan (1970) and Gold (1971) have cul- 

 tured ciliate protozoans. 



The genus Pleurobrachia (Tentaculata, Cydip- 

 pida) includes about 12 species (see Ralph and 

 Kaberry, 1950 for the most recent summary of the 

 species), some of which may be synonymous. The 

 current taxonomic status of the synonymies in 

 this genus is uncertain, because there are few sets 

 of general characteristics which have been set up 

 as important for the separation of species. In par- 

 ticular, some possibly distinct species have been 

 grouped with the boreal species P. pileus O. 

 Miiller of the North Atlantic. One of these, P. 

 bachei, is the boreal form which inhabits the 

 Pacific coast of North America from Puget Sound 

 to San Diego. This species is believed synonymous 

 with P. pileus, based on the works of Moser (1909) 

 and Mayer (1912). However, I agree with Torrey 

 (1904), Bigelow (1912), and Esterly (1914) thatP. 

 bachei is a distinct and separate species. This dis- 

 tinction is supported by work in progress on the 

 differences between these two forms in both 

 meristic and metric characters (Hirota and Greve, 

 unpubl. data). 



Studies of spatial distribution, vertical migra- 

 tion, seasonal variations in numerical abun- 

 dance, and natural history in the planktonic 

 ctenophores have provided some data on natural 

 populations, but information on population 

 dynamics and rates of production are especially 

 needed. Patterns of the geographic distribution of 



296 



