QUANTITATIVE NATURAL HISTORY OF 

 PLEUROBRACHIA BACH EI IN LA JOLLA BIGHT 



Jed Hirota' 



ABSTRACT 



An assessment of the quantitative natural history oi Pleurobrachia bachei A. Agassiz was made by 

 estimating growth rates, metabolic rates, distribution, abundance, occurrence of prey, predators and 

 parasites, population parameters, and net production. These were then integrated to give an indica- 

 tion of the ecological significance of this animal in the plankton. 



Rates of somatic growth and digestion of prey were observed in laboratory experiments and applied 

 to field data. A comparison of growth curves of P. bachei at 20° and 15°C showed development rates 

 from hatching to the same diameter which were 10-15 days faster at 20°C. In addition, a much higher 

 mortality of the ctenophores was observed at 20°C. Maximum growth rate coefficients on a daily basis 

 were 0.21-0.47 and were for 2.5- to 6.5-mm ctenophores (0.1-2 mg bodily organic weight). Studies 

 on the rates of digestion of six frequently ingested prey species by various sizes of P. bachei showed 

 marked differences between species. Although Labidocera was the largest prey offered, it was di- 

 gested the fastest per unit weight. 



The horizontal, offshore distribution of P. bachei postlarvae often showed maxima within 5 km 

 from the shore and decreased about tenfold by 10 km. Patterns of water movement in La Jolla Bight 

 were described as a prerequisite to the distributional studies. The near-surface current velocities 

 showed counterclockwise rotational motion over the submarine canyon complex of La Jolla Bay; 

 otherwise the water generally moved onshore and towards the north at speeds of about 5 km/ day. The 

 high abundances of the animal nearshore are believed to be caused in part by these water move- 

 ments. The ctenophores occurred in the upper 50-60 m, living mostly in the upi>er 15 m in the day 

 and at about 30 m at night. The range of average abundances of postlarvae was from 1,000/m^ and 

 1,000 mg organic matter/m^ in August to being nearly absent in December. 



Hyperoche mediterranea, a parasitic amphifwd, and Beroe sp., a potential predator, showed pat- 

 terns in seasonal abundance similar to that of P. bachei postlarvae, except that H. mediterranea was 

 absent in winter and spring. The higher frequency of occurrence of endoparasites with larger sized 

 hosts and few multiple infections suggests that the parasites are adapted to prevent overexploitation 

 of hosts. The stomach contents of postlarvae showed a pattern of larger prey in larger ctenophores, 

 and within some prey sp>ecies increasing frequency of occurrence in larger ctenophores was observed, 

 e.g., Acartia tonsa. The diel emd seasonal variations in stomach contents were also considered. Prey 

 selection by P. bachei may be determined by the following attributes of prey: density, size, avoidance 

 and escapement behavior, strength and protective spination. 



Size-specific instantaneous mortality rates, the mean schedule of live births, and somatic growth 

 rates were used to estimate population parameters and compute rates of net production. The highest 

 rate of population growth was 0.02 on a per day basis, which would enable a population doubling in 

 about 35 days. The first 50-100 eggs laid by young postlarvae are most important to replace the 

 population. The mean and range of annual net production by postlarval P. bachei are 5.24 and 

 2.32-7.65 g organic matter per square meter; mean values for eggs and larvae are 0.08 and 0. 10 g/m*, 

 respectively. The mean annual net production of all stages is 5.4 g/m^, with 95% confidence limits for 

 the mean being 4.4-6.5 g/m^. 



The ecological significance and functional role of P. bachei are as: Da seasonally dominant 

 carnivorous zooplankter which preys selectively on small crustaceans and may regulate their abun- 

 dances; 2) a vehicle which provides shelter and nutrition for parasites and; 3) an organism which 

 transfers a substantial amount of organic matter and potential energy in the food web of La Jolla 

 Bight. 



Ecological studies may be grouped into four species populations, 3) communities, and 4) 

 categories, depending on the level of complexity ecosystems. The long-term objective in ecology is 

 being considered: 1) single individuals, 2) single the description of ecosystems. More specifically, 



two important objectives in studies of ecosystems 



are: the elucidation of complex interactions be- 



'Scripps Insti^tution of Oceanography Institute of Marine Re- tween species in a food web and the understanding 



sources. La Jolla, CA 92037; present address University of ^ 



Hawaii, Institute of Marine Biology, Kaneohe, HI 96744. and prediction of the dynamic processes that OCCUr 



Manuscript accepted September 1973. 

 FISHERY BULLETIN: VOL. 72. NO. 2. 1974. 



295 



