HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



Pleurobrachia species and other ctenophores 

 have been described (Moser, 1909; Mayer, 1912), 

 but no attempt was made to relate abundance 

 quantitatively to geographic location. Only a few 

 workers have studied vertical distribution of 

 ctenophores (Esterly, 1914; Russell, 1927; 

 Alvarino, 1967; Rowe, 1971) and only the study of 

 P. pileus in Kaneohe Bay, Oahu (Rowe, 1971) 

 could show that diel vertical migration occurs. 

 Pleurobrachia pileus in Kaneohe Bay follow the 

 "normal" pattern for zooplankton with the 

 ctenophores living at depth during the day and 

 moving up near the surface at night. However, 

 the vertical displacement of the migrants was 

 only on the order of 10 m because the bay is very 

 shallow. More is known about quantitative sea- 

 sonal changes in numerical abundance of P. 

 pileus (Wear, 1965; Eraser, 1970; Greve, 1971) 

 and P. bachei (Esterly, 1914; Parsons, LeBras- 

 seur, and Barraclough, 1970). There are also 

 numerous qualitative reports of ctenophore 

 swarms in coastal waters (Chopra, 1960; Ra- 

 jagopal, 1963; Eraser, 1970). Eraser (1962, 1970) 

 reviewed the role of ctenophores and salps in 

 marine food webs and their natural history. 

 Greve (1970, 1972) provided laboratory studies of 

 the effects of temperature, salinity, and food on 

 growth of P. pileus and a field study (Greve, 

 1971) of variations in abundance of P. pileus and 

 two of their predators, Bero'e gracilis and B. 

 cucumis. These studies did not relate seasonal 

 variations in abundance to rates of population re- 

 cruitment, growth, mortality, net production, or 

 advection. 



In order to describe the quantitative natural 

 history of P. bachei as outlined above, it was 

 necessary to sample natural populations and to 

 carry out laboratory experiments. The field work 

 was needed for data on the food web and for de- 

 mographic purposes, and the laboratory data 

 were used to calculate metabolic rates which 

 could not be measured from field samples. 

 Metabolic rates measured or calculated from in- 

 dividuals reared from eggs to adults in the 

 laboratory were applied to field populations. Pre- 

 liminary field studies were then made of the var- 

 iations in abundance of P. bachei as a function of 

 distance from shore. The vertical distribution was 

 determined by sampling with opening-closing 

 bongo nets (McGowan and Brown, 1966^) while 



^McGowan, J. A., and D. M. Brown. 1966. A new opening- 

 closing paired zooplankton net. Scripps Inst. Oceanogr. Ref. 

 66-23. (Unpubl, Manuscr.) 



tracking parachute drogues in those locations 

 where ctenophores were most abundant. Erom 

 the data on water movement and the horizontal 

 and vertical distributions of P. bachei, sampling 

 stations and sample depths (the maximum depth 

 to which a net sample is taken) were allocated for 

 a study of spatial and seasonal variations in 

 numerical abundance, standing stocks and net 

 production. Size or stage-specific instantaneous 

 mortality rates were calculated from the ob- 

 served size-frequency distribution in field sam- 

 ples and development rates calculated from 

 laboratory growth data. Standing stocks per unit 

 area of sea surface were calculated as the sum- 

 mation of the organic weight (ash-free dry 

 weight) of all individuals in a sample multiplied 

 by the ratio of maximum sample depth to the vol- 

 ume of water filtered. The organic weights were 

 estimated from regression equations of bodily 

 weight on bodily diameter. Rates of net produc- 

 tion per 24-h day were calculated from the esti- 

 mated standing stocks of each stage and the 

 stage-specific instantaneous rates of mortality 

 and growth on a daily basis. Eor a given set of 

 stage-specific instantaneous mortality rates, and 

 using the mean schedule of live births derived 

 from laboratory data, the following population 

 parameters were calculated: T, r, d, b, Cx which 

 are the generation time, instantaneous rate of 

 natural increase, death and birth rates, and sta- 

 ble age distribution, respectively. More than 

 12,000 specimens of P. bachei were counted and 

 measured during the seasonal study, of which 

 1,352 postlarvae in 10 size classes contained par- 

 tially digested food organisms and 1,007 postlar- 

 vae contained internal parasites of the hyperiid 

 amphipod, Hyperoche mediterranea. Attempts 

 were made to quantify changes in the absolute 

 numbers and the proportions of various prey 

 categories with changes in bodily size of P. 

 bachei. A study of the seasonal variation in num- 

 bers of parasites, percent hosts parasitized, and 

 the frequency distribution of numbers of para- 

 sites per host and percent hosts parasitized at dif- 

 ferent host sizes is also presented. 



GROWTH IN CULTURE AND 

 METABOLIC RATES 



Methods 



Techniques for the laboratory culture of P. 

 bachei at 15°C have been described previously 



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