FISHERY BULLETIN: VOL. 72, NO. 2 



(Hirota, 1972); culturing has also been done at 

 20°C to examine the effect of temperature on 

 growth rates. The rates at which different prey 

 species were digested were measured in the 

 laboratory in order to make corrections for preda- 

 tion by P. bachei on the most abundant crusta- 

 ceans during field sampling with nets (Judkins 

 and Fleminger, 1972, discuss feeding by Sergestes 

 in nets). 



Six ctenophores were cultured from eggs at 20'^C 

 and about 20 /.(g C/liter as prey for the adult 

 ctenophores in order to evaluate the effect of 

 temperature on growth rate (the surface tempera- 

 ture in summer is about 20°C). The basic tech- 

 niques were the same as described previously 

 (Hirota, 1972, Table 1), except that adult 

 Paracalanus parvus replaced Artemia nauplii as 

 food for 2- to 3-mm ctenophores. In this manner, 

 it was possible to raise ctenophores without "arti- 

 ficial" foods of any kind and instead raise them on 

 prey species which they utilize in nature. In addi- 

 tion, antibiotics (streptomycin sulfate and penicil- 

 lin G each at concentrations of 50 mg/liter) were 

 added after 4 wk of culturing at 20°C when several 

 specimens appeared very weak or moribund. 

 When changes in bodily diameter indicated that 

 the last two specimens might also die, the experi- 

 mental temperature was changed back to 15°C to 

 determine whether or not recovery might occur 

 and whether the mortality effect was due to lethal 

 temperature. 



In order to determine whether or not a prey spe- 

 cies found in the gut of Pleurobrachia sampled 

 with nets was eaten prior to or during capture, a 

 number of observations were made of the rate at 

 which five prey species were ingested and digested 

 after initial entanglement with the tentacles. The 

 time elapsed to achieve one of four scores was 

 recorded during observations with a dissecting 

 microscope through the transparent bodily wall of 

 the ctenophore. These scores are: (4) the prey en- 

 ters the mouth and is in the distal half of the 

 stomach; (3) the prey is moved into the proximal 

 half of the stomach but no digestion of the prey is 

 indicated; (2) the prey is being digested and as- 

 similated, as indicated by less than 10% of the 

 bodily tissues clearing and the occurrence of prey 

 tissues in the aboral, transverse, and pharyngeal 

 canals; (1) the prey is almost fully digested and 

 assimilated, as indicated by transparent skeletal 

 remains (crustacean exoskeletons are not di- 

 gested) and the presence of digested tissues 

 throughout the canal network. The elapsed time 



to achieve a given score was then compared to the 

 maximum time period a prey organism was at risk 

 in the net sample with the ctenophores. This time 

 period is the elapsed time from the start of the tow 

 until preservative was added to the sample jar. 

 Those prey found in the ctenophore stomachs 

 whose score required more time than the max- 

 imum period at risk are presumed to have been 

 eaten in nature prior to capture by the net. 



Results and Discussion 



The growth curve of bodily diameter up to 6 mm 

 at 20°C indicates similar patterns as is the case for 

 15°C (Hirota, 1972, Figure 1), except that the de- 

 velopment rates to the same bodily diameter are 

 10-15 days faster at 20°C (Figure 1). The other 

 important differences are: 1) very much higher 

 mortality rates at 20°C than at 15°C, 2) 60% mor- 

 tality despite the addition of antibiotics on day 29, 

 and 3) the recovery and prolonged growth and sur- 

 vival of two specimens after the temperature was 

 lowered to 15°C from 20°C when growth had 

 ceased at 20°C. Note that there is a lag of over a 

 week before the apparent effect of lowered tem- 

 perature is indicated by a response in bodily diam- 

 eter. The significance of the effect of temperature 

 on growth rate will be discussed below. in relation 

 to the stratification of water temperature in na- 

 ture, the diel vertical distribution of the 

 ctenophores, and the effect of these distributions 

 on seasonality in the standing stocks and net pro- 

 duction of the ctenophores. 



Using the data for growth in bodily diameter at 

 15°C (Hirota, 1972, Figure 1), it is possible to cal- 

 culate rates of growth in bodily organic weight 

 from regressions of organic weight on bodily 

 diameter (Figure 2). A curve for the mean growth 

 in organic weight and the ranges for weight at a 

 given age and age at a given weight are shown in 

 Figure 3. The mean growth rates are highest from 

 0.1 to 2 mg (2.5 and 6.5 mm diameter, respec- 

 tively); the exponential growth rate coefficients on 

 a daily basis are 0.21-0.47. Below 0.1 mg and over 

 2 mg the exponential growth rates are slower, the 

 values being 0.12-0.17 and 0.04-0.17, respec- 

 tively. The range for weight at a given age is about 

 tenfold and for age at a given weight about 15 

 days. 



The rates of digestion of five prey species are 

 shown in Table 1. These data show that undi- 

 gested prey present in ctenophore stomachs 

 (scores 4 and 3) can be ingested during a 0.3- to 



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