FLEMINGER and HULSEMANN: FOUR SIBLING SPECIES OF PONTELLINA 



fragments (Table 19). Though all of the species 

 appear to be similarly predaceous within their 

 respective epiplanktonic communities, we must 

 conclude that appreciable differences in the 

 abundance and occurrence of the sibling species 

 are indicative of real changes in habitat con- 

 ditions and real differences in the adaptive 

 optima for each species. 



Remarks on Geographical Distribution 



This discussion hinges upon inferences drawn 

 from the evidence presented in the preceding 

 sections. Chief among them are the validity of 

 the four sibling species of Po)itelUua as separate 

 noninterbreeding populations. Based on mor- 

 phological homogeneity each population appears 

 to be closely adapted genetically to prevailing 

 environmental conditions in the geographically 

 limited hydrographic systems comprising its 

 particular habitat. Except for differences be- 

 tween Atlantic and Indian-Pacific populations 

 of pluniata morphological indications are that 

 panmixis prevails in each species. 



The three tropical species, moiii, sobrina, 

 and platychela, occupy eutrophic waters 

 characterized by equatorial upwelling and a 



shallow, steeply graded, permanent thermo- 

 cline. The mixed layer overlying the thermocline 

 is relatively homogeneous in temperature and 

 has been referred to as Tropical Surface Waters 

 (Wyrtki, 1966, 1967). In our use of this term, 

 Tropical Surface Waters are restricted to the 

 surface layer in regions where the permanent 

 thermocline has a temperature gradient of 

 ^0.1°C per m and encompasses an overall de- 

 crease in temperature from about 24° ± 1°C 

 at the top to about 15° ± 1°C at the bottom. 

 These pools of warm water are subjected to 

 seasonally repetitive changes in the strength of 

 the equatorial Trade Winds (Wyrtki, 1966, 

 1967; Taft, 1971). The seasonal changes pro- 

 duce monsoonlike reverses in the circulation of 

 the equatorial segment inhabited by each species. 

 This phenomenon apparently provides a suf- 

 ficiently closed hydrographic circulation to 

 maintain breeding stocks in proximity to suit- 

 able nursery grounds and thus ensures contin- 

 ual success of each species. 



The equatorial distributions of the tropical 

 species of PoiiteUina are not without prece- 

 dence. The tropical Atlantic has previously been 

 characterized in faunistic terms, for example, 

 by a number of mesopelagic fishes (Backus et 

 al., 1970) as well as by a sergestid shrimp 



Table 19. — List of identified particles from microscopic analysis of stomach con- 

 tents in adult female Pontellimi. 



Species 



Speci- Cope- "Para- Crust- 



men pod "Oncaea"calanus" ocean Algol 

 number ports ports ports parts ports 



Source of specimen: 

 Oceon Station 



plumatii 1 X X 



2 X 



3 X X Pocific 



4 X X Indian 



5 X X Pocific 

 Percentoge no. with ingested particles in midgut: 71 °o 



Atlantic la Creuse 5 

 Indion DodoVI-81 



Scorpio II -146 

 Lusiad V-45 

 Jordan 57-076 



111 



