HIROTA: NATURAL HISTORY OF PLEVROBRACHIA BACHEI IN LA JOLLA BIGHT 



in the physical-chemical habitat of the coastal 

 waters prevents the formation of a stable, 

 time-independent assemblage of organisms, al- 

 though the system appears to be basically com- 

 posed of the same recurrent species in seasonally 

 varying proportions. Occasionally expatriates 

 from oceanic, southern waters, and northern wa- 

 ters appear (e.g. , Candacia, Eucalanus attenuatus, 

 Tortanus discaudatus, Velella, etc.). The tran- 

 sients are joined by some organisms which appear 

 seasonally in this area during spawning migra- 

 tions (e.g., gray whale, squid, grunion). 



The regulation of population size in Pleuro- 

 brachia is postulated to be through density- 

 dependent feedback meahanisms proposed 

 by Greve (1972), in which the prey of larger 

 ctenophores (e.g., adult stages of copepods) are 

 detrimental to survival of the small ctenophore 

 larvae. Balance in the abundance of predators and 

 prey is conferred by selection of larger copepods by 

 the larger ctenophores (Bishop, 1968), but with 

 dependence of ctenophore larvae on copepod naup- 

 lii for food supply and low abundance of adult 

 copepods for their survival. A high density of 

 copepod nauplii and low density of copepod adults 

 would favor occurrence of ctenophore larvae and 

 early postlarvae. As both prey and predators grow 

 the roles of predator and prey become reversed to 

 some extent. The large ctenophores may nearly 

 deplete the water of large copepods to satiate their 

 metabolic demands, but this condition is unstable, 

 because the larger ctenophores will become food 

 limited. The population size will not increase 

 greatly because few adult copepods are available 

 to produce eggs, and the nauplii which are hatched 

 from eggs are needed for food of larval 

 ctenophores. If the abundance of postlarvae 

 should increase and some threshold is exceeded, 

 the ctenophore population also becomes vulnera- 

 ble to density-dependent predation by Bero'e and 

 other predators and infection by parasites. 



All organisms in nature consume food, recycle 

 materials through excretion (and exuviation), and 

 are themselves consumed. In this regard the func- 

 tional role or ecological significance of a species 

 population is closely related to its relative abun- 

 dance and rates of turnover. Pleurobrachia bachei 

 is a dominant carnivorous zooplankter during 

 summer and fall in the coastal waters off San 

 Diego. Its functional role can be divided into three 

 parts: 1) a predator which regulates the abun- 

 dance of small crustaceans (copepods and cladoce- 

 rans) and removes least fit individuals, 2) a vehi- 



cle which provides shelter and nutrition for para- 

 sites such asHyperoche, and 3) an organism which 

 transfers and transforms material and potential 

 energy in the planktonic food web. As a predator, 

 the role of selective removal of prey is an impor- 

 tant factor for both the evolution of size, shape, 

 behavior, etc. in the coastal water species and for 

 regulating the abundance and species composition 

 of prey. Pleurobrachia bachei is not unique as a 

 planktonic form in providing shelter and nutrition 

 for co-occuring species; salps are exploited in a 

 similar manner by copepods, except that details of 

 the life histories differ (Heron, 1969). As a season- 

 ally dominant carnivore, P. bachei is also un- 

 doubtedly an important species which transfers 

 organic matter and potential energy to higher 

 trophic levels in the food web of La Jolla Bight. 



ACKNOWLEDGMENTS 



I am indebted to Michael M. Mullin for his gui- 

 dance and suggestions throughout my research, 

 especially in laboratory work and critical evalua- 

 tion of data. I received considerable help from E. 

 W. Fager and E. Stewart with computer programs 

 and other help in calculations. E. W. Fager also 

 made many suggestions regarding statistical 

 treatment of abundance and production data. T. E. 

 Bowman kindly identified the amphipod parasite 

 of Pleurobrachia bachei as Hyperoche mediter- 

 ranea. The drogue studies and many other aspects 

 of field work were done jointly with A. M. Barnett 

 and D. Kamykowski. Their help in preparation of 

 equipment and participation in cruises is greatly 

 appreciated. I also thank my wife Gail and parents 

 for their support and encouragement. This re- 

 search was supported by Marine Life Research 

 General Funds, U. S. Atomic Energy Commission 

 Contract No. AT(ll-l) GEN 10, P. A. 20, and 

 National Science Foundation Contract No. 

 GA-35507. 



LITERATURE CITED 



Allen, K.R. 



1971. Relation between production and biomass. J. Fish. 



Res. Board Can. 28:1573-1581. 

 Allen, W. E. 



1928. Review of five years of studies on phytoplankton at 



southern California piers, 1920-1924, inclusive. Bull. 



Scripps Inst. Oceanogr., Tech. Ser. 1:357-401. 

 1941. Twenty years' statistical studies of marine plankton 



dinoflagellates of southern California. Am. Midi. Nat. 



26:603-635. 



333 



