HIROTA; NATURAL HISTORY OF PLELROBRACHIA BACHEI IN LA JOLLA BIGHT 



Table 4. — The frequency distribution of the number of parasites per host for 14 size 

 classes of postlarval Pleurobrachia bachei, the percentage of total occurrences At 

 and of total numbers 'B* of parasites for each size class, and the percentages of total 

 occurrences <C) and of total numbers <Di of parasites for single and multiple 

 infections. The blank spoces indicate absences. 



to that of P. bachei and//, mediterranea, except 

 that Beroe persists through the year rather than 

 being absent for the winter and spring months. 

 The patterns in the seasonal distribution of Beroe 

 and P. bachei at station 3 were much the same as 

 at station 5, except that the abundance of Beroe 

 was fourfold lower at station 3, and the secondary- 

 high abundances of Beroe in the winter and 

 spring months at station 5 was not as well de- 

 fined at station 3. 



The partially digested stomach contents of P. 

 bachei captured at the surface in tows of short 

 duration '60 s duration, 95 s maximum period at 

 risk to feeding in the net) showed that the same 

 species groups occur as in tows of fivefold longer 

 duration. These species are : li copepods — L. tri- 

 spinosa, A. tonsa, P. parvus, C. anglicus, E. 

 acutifrons, and 2 ' cladocerans — Evadne nord- 

 manni, E. spinifera. E. tergestina, and P. avi- 

 rostris. The results provide evidence to support 

 the contention isee results of stomach contents 

 below) that the prey in stomach contents of P. 

 bachei captured in tows of short duration are 

 those which were ingested and digested in nature 

 prior to capture by the net. The same species 

 would probably occur in stomachs of P. bachei if 

 the ctenophores were pipetted from the sea sur- 

 'face and preser\'ed immediately. Seven species 

 of zooplankton. which were retained quantita- 

 tively in the 0.363-mm mesh net as adults, were 



considered in calculations of electivity indices. In 

 the Ivlev electivity index. E = r - pj ''r - pj, r 

 and p being the proportions of a food item in the 

 stomach and in the environment respectively. 

 Paracalanus parvus, C. anglicus, and E. acuti- 

 frons, which occurred frequently in the stomachs 

 but passed through the net, were not included in 

 the calculations. The results on the basis of num- 

 bers show moderate positive selection for Acartia, 

 Labidocera, andE. tergestina; high positive selec- 

 tion for E. nordmanni a.ndE. spinifera; and mod- 

 erate and strongly negative selection for Penilia 

 and Sagitta 'Table 5). The indices on the basis of 

 organic weight show the same trends, but the 

 values for both copepods and Evadne are in- 

 creased somewhat, and that for Penilia 

 decreased, due to differences in bodily weights. 

 Penilia has a negative electivity and was a slowly 

 digested prey species (Table 1), whereas 

 Labidocera and Acartia have positive electivity 

 indices and were more rapidly digested. Prey 

 selection by Pleurobrachia is more complex than 

 dependence on prey digestibility alone. Data on 

 the stomach contents of P. bachei during the sea- 

 sonal study and observations in the laboratorv* of 

 avoidance behavior and prey protective 

 mechanisms will be discussed below in the con- 

 text of prey selection. 



In the study of diel variation in feeding, differ- 

 ences in the percentage of ctenophores with 



315 



