FISHFR^ Bl I I FTIN: \ Ol ':. NO. 2 



"T — I — I — r 



O STA. I 



0STA.2 _ 



D STA. 3 



ASTA.4 



O STA. 5 



• ALL STATIONS"! 



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V' A M 



1971 



Figure 19. — Seasonal variation in the percentage of postlarval 

 Pleurvbrachia bachei parasitized hy Hyperoche mediterranea at 

 five stations located 1.6 km from shore. The line connects the 

 mean value at each sampling date, and the various types of ojjen 

 symbols represent different stations. 



suit is caused hy the seasonal distrihution of para- 

 sites in relation to the seasonal distribution of 

 length frequency of postlarval P/t'j/ro6rac/j;a (see 

 Figures 16 and 18). Note that the 6-8 mm sizes 

 with highest frequency of infection are at the size 

 range for beginning reproduction as adults indi- 

 cated by results from laboratory cultures. Also 

 notice that only about I'l of all postlarvae in the 

 1- to 2-mm size classes were parasitized. These 

 are the sizes of ctenophores which reproduce at an 

 early age with small numbers of eggs. Secondly, 

 the distribution of percentages of total occur- 

 rences and total numbers of parasites for single 

 infection and multiple infection show a decreasing 

 occurrence of multiple infection, such that over 

 9(y~c of the occurrences and numbers of parasites 

 are as one. two, or three parasites per host. 



Seasonal variations in abundance of Bfrot' sp., 

 a known predator of other ctenophores, show a 

 pattern very similar to that of P. bachei (Figure 

 20). The data plotted are numbers per 2 m^ (the 

 sum of numbers per square meter of each repli- 

 cate sample I at station 5. The distributions show 

 seasonal maximum values in July- October with 

 secondary high abundances in winter months. 

 This pattern of seasonal co-occurrence is similar 



by a periodic convergence of w-ater types contain- 

 ing //vperoc /it' and Pleurobrachia. 



Data from all stations during the seasonal study 

 were plotted as the percentage of postlarval 

 Pleurobrachia containing one or move Hyperoche. 

 The mean percentage over time shows that Au- 

 gust w'as the month of highest percentage hosts 

 parasitized (Figure 19); at this time over one in 

 three postlarvae were infected. The rate of in- 

 crease of percentage infection appears to be faster 

 than the decrease, although the range of 3 mo 

 time around the maximum was the same for both. 

 The very large variability on some sample dates 

 was more a result of differences in percentages 

 between stations than an artifact of sample size, 

 since several hundred ctenophores were ex- 

 amined per sample date. 



The frequency distributions of the percentages 

 of total occurrences and of total numbers of para- 

 sites for single and multiple infection and for dif- 

 ferent sizes of hosts show two interesting results 

 (Table 4). First, there is a central tendency in the 

 percentages of total occurrences of parasites, and 

 in the total number of parasites, with 6-8 mm 

 sizes being the most frequently infected. This re- 



10000 



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a. 



bJ 



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JJASONDJF 

 1971 



Figure 20. — Seasonal variation in the numerical abundance of 

 p)ostlarvalP/ewro6racAia bachei and postlarval Beroe sp. at sta- 

 tion 5. Values are expressed for simplicity as the numbers per 2 

 m^ (the sum of numbers i>er square meter in the replicate sam- 

 ples*. The range for the mean of replicate samples has been 

 indicated previously (e.g.. Figures 13 and 18). 



314 



