HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



10 



e 



E 



or 



UJ 



< 



10 



20 30 40 50 60 



DAYS AFTER HATCHING 



Figure 1. — Growth in bodily diameter o{ Pleurobrachia bachei 

 at 20°C, expressed as a function of age in days. Each symbol 

 represents measurements of a different individual. The point 

 indicated by AB refers to the starting date with antibiotic addi- 

 tions and the points indicated by 15°C refer to change of the 

 experimental temperature from a constant 20°C to a constant 

 15°C. All ctenophores died after the last observation shown for 

 each individual, except that one which was still alive after 80 

 days. 



5-min period at risk while both the predator and 

 prey are being sampled by nets. Partial or fully 

 digested states (scores 2 and 1), however, required 

 more than 9 and 15 min, respectively. Prey of 

 scores 2 and 1 in ctenophore stomachs are, there- 

 fore, very likely to have been ingested by 

 ctenophores prior to capture by nets in samples of 

 short duration (i.e., less than 5 min). Only prey of 

 these scores were used in the study of stomach 

 analyses presented below, unless the prey were 

 too small to be retained by the 0.363-mm meshes 

 of the net and, therefore, were not at risk to preda- 

 tion during the sampling. Examples of these smal- 

 ler prey species not at risk are nauplii of Acartia 

 and all stages of Euterpina acutifrons, a copepod 

 of 0.7 mm length. 



Measurements of the organic weight of six 

 species of "important" planktonic marine crusta- 

 ceans in La Jolla Bight are given in Table 2. The 

 first four species are copepods and the remainder 

 are cladocerans. Note that for adults, Labidocera 

 is tenfold larger than Acartia and Evadne and 

 about twentyfold larger than Paracalanus, 

 Corycaeus, and Penilia. 



A trend exists in the data for scores 2 and 1 when 

 the respective medians for the elapsed time to 

 achieve these scores are expressed per unit bodily 

 organic weight for each prey species (Table 1). 

 Labidocera trispinosa is the most easily digested 

 prey per unit bodily mass although it is the 

 largest. Acartia is digested slightly faster than 



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X 

 UJ 



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CI 

 (T 

 O 



0.0001 



J — I I I 1 1 III 1 I I I 1 1 III 



I I I I III 



0.1 0.5 I 5 10 



DIAMETER IN mm 



50 100 



Figure 2. — The relationship between bodily organic weight and 

 bodily diameter of P. bachei on a double logarithmic scale. The 

 open circles represent data on field-collected ctenophores from 

 La Jolla, Calif; the triangles represent data on laboratory cul- 

 tured ctenophores; and the diamonds and squares represent data 

 on ctenophores grown in the deep tank facility at Scripps Institu- 

 tion during experiments 1 and 2, respectively. The lowest four 

 values are calculated from determinations of organic carbon. In 

 the equations the upper line is for ctenophores larger than 3 mm 

 and the lower line for those smaller than 3 mm. In both equations 

 W = log 10 (bodily organic weight in milligrams) and£> = logio 

 (bodily diameter in millimeters). 



Paracalanus and each of these faster than either 

 Corycaeus or Penilia. Part of the cause for the 

 delay in digestion of Corycaeus and Penilia 

 relative to the above-mentioned calanoid 

 copepods is the protective spination. In addition to 

 stout furcal spines, caudal rami, and very sharp 

 projecting corners of the last thoracic joint, 

 Corycaeus has a large, smooth cephalothorax 

 which encloses much of the bodily tissues and may 

 retard penetration of digestive enzymes. The spi- 



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