On the basis of radiolarian data, 4 major areas can be recognized in the eastern equatorial Pacific Ocean (transected from .apprx. 9.degree.N, 80.degree.W to 2.degree.N, 140.degree.W to 17.degree.N, 155.degree.W). Between 5.degree.N, 80.degree.W and 2.degree.N, 95.degree. W, with high overall productivity and low radiolarian abundance and diversity, Spongodiscus sp. A, and to a lesser extent Didymocyrtis tetrathalamus, are the dominating taxa in this assemblage. Along the equator, between .apprx. 95.degree.W and 140.degree.W the productivity and planktonic standing stock decrease to the west; while radiolarian diversity and abundance increase, as well as the diversity of some other zooplankters. This area can be further subdivided into 2 sections at 124.degree.W, the western one being conspicuously richer both quanti-and qualitatively than the eastern section. Octopyle stenozona + Tetrapyle octacantha are characteristic of this area. From .apprx. 6.degree.N, 138.degree.W to 10.degree.W radiolarian abundance and diversity drop sharply, as well as overall planktonic productivity and standing stock. From .apprx. 10.degree.N, 142.degree.W to 17.degree.N, 153.degree.W there is a further decrease in radiolarian abundance and diversity. Radiolarian distribution shows good agreement with surface water-masses (South Equatorial Current, Equatorial Counter Current, North Equatorial Current), and with previous biogeographic zonations produced on the basis of several zooplanktonic groups. There is adequate coincidence with the distribution of radiolarian shells on the sea-floor of the same area. The distribution of specimens and species seems to be coincident with adverse near-shore conditions and zooplanktonic grazing pressure in the easternmost section, and with planktonic productivity in the western part of the transect. The uneven distribution of Nassellaria might be explained by their more oceanic (rather than neritic) preferences, and by the shallowness of the thermocline layer which supplies nasselarian specimens to surface waters. The similarity between the specific inventory recorded and those of previous works which studied radiolarians from the same and/or analogous areas is extremely low, due mainly to sytematic ambiguities as well as to very high specific diversities of radiolarian assemblages.
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