HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



eluded that the ctenophore population was releas- 

 ing eggs into the water but that the larvae were 

 eaten or died from other causes as fast as they 

 were hatching from eggs. A feedback control 

 mechanism which can account for the presumed 

 high mortality of -larvae, high abundance of eggs, 

 and rapid growth of postlarvae is discussed below. 



The calculated development rate from 1.5 to 6.5 

 mm in 30 days from field sample data is about 10 

 days slower than the growth rates in laboratory 

 cultures at both 15° and 20°C. 



Patterns in the seasonal distribution of stand- 

 ing stocks of postlarvae, larvae, and eggs at sta- 

 tion 5 (Figure 17) are similar to the respective 

 seasonal variations in numerical abundance. 

 Postlarval values in 1970 increased from March to 

 a seasonal maximum of 1,500 mg organic 

 matter/m^ in August, then decreased to a 

 minimum in December. The range over the year 

 for standing stock of postlarvae was about four 

 orders of magnitude. Note that the mean standing 

 stock of larvae was high relative to that of postlar- 

 vae in April and November 1970 and the following 

 winter months. Except for a few instances in Au- 

 gust and September, the mean standing stock of 

 larvae was about twofold to tenfold greater than 

 that for eggs. The crops for postlarvae were about 

 equal to those of the larvae, except from May to 

 October when they were much greater. 



Seasonal Variations in Parasites, Predators, 

 and Prey 



Coincident with seasonal variations in the 

 abundance and size frequency distribution of post- 

 larval P. bachei are variations in abundance of the 

 hyperiid amphipod, H. mediterranea (Figure 18). 

 The data shown are from station 5 off Scripps 

 Institution, but patterns in the seasonal distribu- 

 tion 16 and 32 km to the north (stations 3 and 1, 

 respectively) are essentially the same. Plots of 

 abundance per square meter on a semilogarithmic 

 scale are for postlarval ctenophores, attached en- 

 doparasitic larvae and early juveniles of H. 

 mediterranea, and unattached free-living late 

 juvenile and adult H. mediterranea. The appear- 

 ance of Hyperoche in the plankton is associated 

 with P. bachei when the abundance of hosts ex- 

 ceeded about 100 ctenophores/m^, which was 

 May-June to November 1970. Prior to June 1970 

 and after January 1971 H. mediterranea was 

 sparse enough to be absent in 6 to 10 samples of 



1000 p-i — r 



p. bachei '__ 



o — o Attached Hyperoche _ 

 O — O Unattached Hyperoche- 



100 — 



cr 

 a. 

 or 



LjJ 

 GD 



Figure 18. — Seasonal variations in numerical abundance of 

 postlarval Pleurobrachia bachei and attached (endoparasitic) 

 and unattached (free-living) Hyperoche mediterranea at station 

 5. Values are expressed as number per square meter of sea 

 surface in logarithmic scale, and the lines connect respective 

 means at each sampling date. The vertical bar represents the 

 range of values for the replicate samples, and solid symbols 

 indicate that the range is less than the size of the symbol. 

 Hyperoche was absent after December 1970. 



15-20 m^ each. Maximal abundance of Hyperoche 

 occurred about a week after the ctenophore 

 maximum, and may represent an "overshoot" 

 phenomenon in a density-dependent, para- 

 site-host system. Note that the larger am- 

 phipods occurred in highest abundance when most 

 postlarval ctenophores were at 6-8 mm sizes. Lit- 

 tle concerning the dynamic aspect of this 

 parasite-host interaction can be deduced from the 

 data because of uncertainties in immigration and 

 emigration over time. In August and September 

 1969-72 the occurrence of Hyperoche in and onP. 

 bachei has been noted during plankton sampling. 

 The co-occurrence and relative abundance of 

 these two species is predictable and should follow 

 the same pattern from year to year, with temporal 

 shifts in the maxima and minima, depending on 

 the type of "meterological year" and the sequence 

 of events that occur in the plankton during the 

 increase and decline of the ctenophore 

 summer-fall maximum. The important problems 

 to resolve are where the amphipods occur in the 

 winter-spring months, and whether the observed 

 seasonal pattern of co-occurrence is determined 



313 



