HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEl IN LA JOLLA BIGHT 



California (Brusca, 1970). 



Off southern California, the strongest equator- 

 ward surface flow occurs during spring and sum- 

 mer, and south of Point Conception the semiper- 

 manent cyclonic eddy produces a northward in- 

 shore circulation (Wyley, 1966). Beneath the 

 California Current, the undercurrent is a sub- 

 thermocline poleward flow of water of relatively 

 high temperature and salinity; for example in Au- 

 gust 1966 the undercurrent at lat. 31°N, long. 

 177°W was close to the continental slope, being 

 about 20 km wide and 300 m thick (Wooster and 

 Jones, 1970). This undercurrent below 200 m sur- 

 faces well inshore of the main stream in late fall 

 and early winter when northerly winds are weak 

 or absent (Reid et al., 1958). 



The seasonal distributions of P. bachei in La 

 Jolla Bight showed some features which are con- 

 sistent with seasonal changes in the vertical 

 movement of the California undercurrent (also 

 called the Davidson Current). Postlarvae de- 

 creased by over two orders of magnitude from the 

 end of October through December, and reappeared 

 at moderate abundance in late January and Feb- 

 ruary. For larvae and eggs, the timing and mag- 

 nitude of the winter decrease were about the same, 

 but the frequency of absences was less than for 

 postlarvae (see Figures 13 and 14). Another vari- 

 able associated with the presumed winter shoal- 

 ing of the undercurrent is the fivefold decrease in 

 prey standing stock from early November to the 

 middle of December (Table 10). A subsurface cur- 

 rent which rises to the surface in winter is ex- 

 pected to contain relatively low crops of animals 

 and plants, and poleward advection of water from 

 the south should cause decreases in abundance of 

 Pleurobrachia. 



Studies of stomach contents of P. pileus in the 

 Scottish North Sea (Fraser, 1970) and in Kaneohe 

 Bay, Oahu (Rowe, 1971) indicate that this 

 ctenophore is predominantly a crustacean feeder, 

 especially of copepods, cladocerans, and cirriped 

 nauplii. In the Scottish North Sea, P. pileus fed 

 about 809c of the time on Acartia, Calanus, 

 Evadne, invertebrate eggs, Temora longicornis, 

 Oithona, unidentified copepods, cirriped larvae, 

 Spiratella, andPodon. In Kaneohe Bay 75% of the 

 prey were nauplii of barnacles and copepods and 

 the appendicularian Oikopleura longicauda 

 (Rowe, 1971). The evidence indicated that 

 Pleurobrachia very rarely fed on fish eggs and 

 larvae. The North Sea study included seasonal 

 and annual data, the differences between which 



Fraser attributed to differences in the composition 

 of the plankton rather than prey selectivity by 

 Pleurobrachia. 



The gut contents of P. bachei in La Jolla Bight 

 generally agree with the results for P. pileus in 

 that they fed: 1) predominantly on crustaceans, 

 especially copepods; 2) very rarely on fish eggs and 

 larvae; and 3) on a broad spectrum of organisms 

 some of which appear seasonally for only limited 

 periods. The major difference between the results 

 from the three study areas is that in Kaneohe Bay 

 Pleurobrachia fed on relatively few prey 

 categories, the number being about one-fifth that 

 in my study and the North Sea study. The three 

 most frequent foods on a numerical basis were: 1) 

 barnacle and copepod nauplii, Oikopleura and 

 other copepods in Kaneohe Bay; 2) Acartia, 

 Calanus, and Evadne in the Scottish North Sea, 

 and 3) Acartia, Euterpina, and Corycaeus in La 

 Jolla Bight. Both studies of P. pileus gut contents 

 considered the postlarvae as a homeogeneous 

 group. I have treated the postlarvae of P. bachei 

 as being made up of 10 separate size classes to 

 show that some changes do occur in prey fre- 

 quency during ontogeny (Table 7). All studies of 

 ctenophore gut contents have been inadequate to 

 describe quantitatively the developmental stages 

 of prey species eaten by different life history 

 stages of ctenophores, including the larvae. Great 

 difficulties and amounts of work would be re- 

 quired for such a study (each copepod species has 

 13 developmental stages counting the eggs). Many 

 important biological interactions probably occur 

 during different developmental stages during on- 

 togeny, yet we know very little about them. 



Feeding rate experiments with 9- to 10-mm 

 diameter P. bachei (Bishop, 1968) have shown 

 differences between mean ingestion rates of 

 copepodids of Epilabidocera amphitrites and 

 Pseudocalanus minutus; P. bachei also fed at a 

 faster rate on copepodids and adults of P. minutus 

 than on their nauplii. These results showed that 

 rates of feeding depend on prey size and other 

 differences between the same stages of different 

 prey species and between different developmental 

 stages of one species. The study of feeding be- 

 havior of P. pileus indicated that this ctenophore 

 regulates its feeding rate by changing the average 

 size of the tentacles in response to different con- 

 centrations of Artemia nauplii (Rowe, 1971). 



During laboratory culturing and rate of diges- 

 tion of prey experiments, differences were ob- 

 served in: 1) the avoidance and escape behavior of 



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