FLEMINGER and HULSEMANN: FOUR SIBLING SPECIES OF POSTELLINA 



tribution of the four species of Pontellina. This 

 is apparent in the relationship between the 

 distribution of each species and the location of 

 prevailing near-surface isotherms that locate 

 the hydrographic limits of these bodies of water. 

 The localities for the tropical species are largely 

 enveloped by the mean winter season position 

 of the 24 °C isotherm at 10 m (Figure 36), 

 the lower thermal limit of Tropical Surface 

 Water. The 10-m depth was chosen to reduce 

 the influence of diurnal fluctuations. The local- 

 ities for plumata, however, vary broadly be- 

 tween the position of the 20° and 15° C mean 

 winter season isotherms (Figure 37). Factors 

 confining the distributions to the observed lim- 

 its, however, are not obvious; more data on 

 depth range^ vertical migratory behavior, and 

 depth of food organisms would probably be 

 enlightening. 



Notably, more than three-quarters of the 

 samples (77.5% ) containing Pontellina yielded 



specimens of only one species. To examine joint 

 occurrences of Po)itelli)ia congeners more close- 

 ly, all capture records of a species were tallied 

 by ocean and grouped with respect to the pres- 

 ence or absence of other congeners in the same 

 sample (Table 20). Comparison of singular and 

 joint occurrences for all possible pairings indi- 

 cates that the latter are relatively infrequent. 

 In no case of joint occurrences did the index of 

 affinity (Fager and McGowan, 1963) reach a 

 positive value. No two species within the genus 

 would appear to occupy the same spatial habi- 

 tat or, in other words, be members of the same 

 community. Thus, the extensive overlapping of 

 morii and plumata in the equatorial Indian 

 and Pacific Oceans may be viewed as a function 

 of intermingling due to the spatial proximity 

 of the two habitats and perhaps also due to a 

 greater number of similarities shared by these 

 two habitats than between those of the other 

 possible pairings within the genus. 



60° 



Figure 36. — Comparison of geographical area enveloping all capture records of tropical species of Pontellina with select- 

 ed mean isotherms at 10 ni for winter season of each hemisphere. Data from Muromtsev (1958, 1963) and Wyrtki 

 (1971). Dotted shading outlines capture records of P. platychela shown in Figure 8; horizontal shading outlines capture 

 records of P. sobrina shown in Figure 15; vertical shading encloses the capture records of P. morii shown in Figure 12. 



113 



