FLEMINGER and HULSEMANN: FOUR SIBLING SPECIES OF PONTELLINA 



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INDIAN OCEAN AUSTRAL- 

 ASIAN 

 SEAS 



PACIFIC OCEAN 



Figure 35. — Mean abundance (No. adults/m^) and per- 

 centage frequency of occurrence in sets of samples shown 

 in Figures lb and 38 and listed in Table 14. Confidence 

 limits of the means shown in Table 14. Further discussion 

 in text. 



The appearance of sobrina is restricted to the 

 eastern tropical Pacific where its abundance 

 and occurrence resemble the values for »i(>rii 

 and plaiitata in their regions of dominance, 

 respectively. 



Quantitative data on Poiitellhto in the 

 Atlantic Ocean are few. In six quantitative 

 samples from the western Atlantic phiniata 

 abundance ranged from 0.01 to 0.46 adults per 

 m-'. Two samples containing platycliela provided 

 estimates of 0.001 and 0.08 adults per m''. 



Extremely high values of phunato s.l., how- 

 ever, have been reported from the Atlantic. 

 Judging from their geographical origin, the 

 northeastern Gulf of Guinea, these abundance 

 estimates (Mahnken, Jossi. and McCabe, 1968) 

 are probably referrable to platychela. Mahnken 

 and his co-workers record the species at 18 of 

 63 sampled localities scattered offshore from 

 the Bight of Benin west to Cape Palmas. They 

 indicate areal abundance of the species by 

 contouring selected class intervals of number 

 per 1.000 m-' water strained. In lieu of the actual 

 estimates per sample we used midpoints of each 

 contoured interval to calculate the mean abun- 

 dance. The yield is a surprisingly high mean 

 of 1.01 individuals per m^, an order of magni- 

 tude higher than our highest mean values from 

 the Pacific and Indian Oceans. Aside from 

 possible bias introduced by our extrapolations 



several factors may be responsible for these 

 unusually high values: e.g., count of immature 

 as well as adult specimens, use of nets with 

 smaller mesh width (0.281 mm), use of surface 

 tows in a region relatively rich in zooplankton 

 presumably concentrated in the very shallow 

 layer of tropical surface water above the 

 permanent thermocline, etc. 



Summing our mean values of pin )nata, niorii, 

 and sob7'iiia in each meridional set of samples, 

 we find remarkably good agreement between 

 our abundance estimates and those derived by 

 previous studies of pluniata s.l. in the Pacific 

 Ocean (Table 18). We normalized the published 

 data to conform to the units employed in the 

 present study. Normalization was simplified 

 by the following assumptions: 



a. we assumed 100% filtration efficiency; 



b. we assumed that PonteUina occurs only 



above 200 m and, in calculating volume 

 of water strained by the net, omitted 

 segments extending below 200 m; 



c. in sets of vertically stratified tows we con- 



sidered the overall estimate of abun- 

 dance as if it were from a continuous 

 tow sampling between 200 m and the 

 surface ; 



d. we assumed that previous studies on 



PaiitelUna failed to discriminate among 

 the different species; the published 

 values were regarded as representing a 

 combined estimate of the abundance of 

 all species of the genus found in the 

 region. 



Estimates obtained from Heinrich (1968) and 

 Vinogradov and Voronina (1963) are about one 

 order of magnitude higher than other middle 

 and west Pacific estimates. These higher values 

 may be accounted for by two factors, namely 

 that the counts include immature copepodids 

 and that the samples were taken with nets of 

 0.18-mm mesh, small enough to retain Pontel- 

 U)ia copepodids of stage II and possibly of 

 stage I as well (Table 15). Sherman's (1963, 

 1964) counts appear to have been derived from 

 adult specimens, partly by inference from his 

 text and partly from the relatively wide mesh 

 comprising most of the filtering cone in the 

 POFI (Pacific Oceanic Fisheries Investigation) 

 Standard Net (0.66 mm). 



109 



