low, from I to 10 sp/m 1 ; normal, from k) to 100 sp/m'; and 

 massive , > 100 sp/m 3 (Table 1 ). As a rule, determined forms 

 of mesozooplankton were found at the same frequency. 

 However, it is evident from Table 1 that, in many cases, the 

 frequency of species occurrence increased in the western 

 portion of the section. The number of massive species at an 

 individual level ranged from 4 to 14. The maximum number of 

 massive species ( 1 3-14) was recorded in the western portion of 

 the section at the equator; the minimum number was recorded 

 in the eastern portion. We note that the populations of only 2 1 

 species of zooplankton reached numbers above 100 sp/m 1 ; 18 

 consisted of the order Copepoda. Among other taxonomic 

 groups, massive concentrations were formed by Flassisagitta 

 enflata, Oikopleura sp. I, Euphausia similis var. "armata." 



Some massive species reached a high density at all stations 

 and levels. They included Clausocalanus porgens, Oithona 

 spp., Oncaea venusta, Corycaeus gibbulus, Microsetella rosea, 

 and Oikopleura sp. I. Of ihese, the most numerous species was 

 the cyclopoid. O. venusta. On the other hand, 60 rare species 

 occurring in 5-15% of the samples were counted. On the 

 whole, the lists, of planktonic organisms observed in equatorial 

 waters of the Pacific Ocean (Geinrikh, 1960; Vinogradov & 

 Voronina, 1963; Arashkevich, 1972; Stepan'yants, 1977) are 

 similar to those that we obtained (Table 1 ). 



Mesozooplankton species composition in the section from 

 Caroline Atoll to the equator was approximately uniform, from 

 east to west for species of Cyclopoida, Harpacticoida, 

 Appendicularia, and Siphonophora, as well as for the minimal 

 content of Euphausiacea and Chaetognatha(Fig. 3, Table 1). In 

 the eastern portion of the region, such groups as Ostracoda, 

 Mysidae, and Salpidae were practically absent. Of the 13 

 determined Polychaeta, only two species were detected in the 

 region near Caroline Atoll. Some species of Chaetognatha 

 (i.e., Ferosagitta ferox. Pterosagitta draco, Parasagitta 

 speticoela, Sagitta sp., Sagitta pulchra) found in the western 

 portion of the region were absent from Station 1 14. Although 

 species composition at the eastern stations was poor, relative to 

 those to the west, some species were found there that were not 

 found at any of the other stations (i.e., the Far-neritic 

 [Vinogradov & Voronina. 1963] copepod Undinula vulgaris 

 and certain siphonophores). 



In the western section of the region, the number of species 

 of Cyclopoida, Harpacticoida, Euphausiacea. Amphiphoda, 

 Calanoida, and Chaetognatha increased significantly, 

 sometimes severalfold, in comparison with the eastern section 

 (Fig. 3). As one moved toward the equator, from Station 1 17. 

 the number of previously undetected deep-sea species of 

 Calanoida (Neocalanus gracilis, N. robustion, Bradycalanus 

 sp., Rhincalanus cornutus, Bradyidus armatus, Euchirella 

 amoena, Pleuromamma abdominalis, P. gracilis, Haloptilis 

 acutifrons, H. longicornis, Candacia longimana, and 

 Labidocera detruncata) increased in the samples. This may 

 have been due to a more intense mixing of equatorial waters in 

 the region of their increase or to more active vertical daily 

 migrations of mesozooplankton (Vinogradov & Voronina, 

 1963). The western section was also much richer in the species 

 composition of Cyclopoida: previously undetected species of 



Numbers, sp./m 

 2000 ■ 



1500 - 



a) 



1000 - 



500- 



120 119 118 117 116 115 114 Stations 



b) 



400 



300 



200- 



100- 







— i 1 1 1 i 



120 119 118 117 116 115 114 Stations 



Fig. 3. Horizontal distribution of mesozooplankton in the 200 m surface 

 layer: (a) distribution of numbers; and (b) distribution of biomass. 



Corycaeus lautus, C. robustus, and six species of the genus 

 Sapphirina, Copilia longistylis, and Pachysoma dentatum 

 were present in the samples. The Harpacticoida group in the 

 samples near Caroline Atoll was represented by the single 

 species, Microsetella rosea. The new species of Clytemnestra 

 scutellata and Miracia sp., from deeper waters, appeared in 

 samples from the western stations. In contrast to the eastern 

 section, where Amphipoda were represented by species in the 

 family Hyperiidae, to the west, there were more species of 

 Platyscelidae and Pronoidae. Isolated specimens of Ramosia 

 sp., Vogtia serrata, and Maresearsia sphaera; representatives 

 of the Gerionidae family; larvae of cephalopod mollusks; and 

 large forms of Appendicularia ( Oikopleura sp. II) were observed 

 only in the western stations. Thus, moving from Caroline Atoll 

 to the equator, the diversity and abundance of mesozooplankton 

 increased. 



Differences in species composition were characteristic not 

 only of the extreme western and eastern points of the region: a 

 number of interesting characteristics were also discovered at 

 Station 117. located at the center of the region. All the 

 taxonomic groups were represented most completely in the 

 community of zooplankton found at this station. For example, 

 of 30 species of Amphipoda, 22 were found at Station 117. and 

 of 13 species of Polychaeta, 1 1 were found. Large forms of 

 Tomopteris sp.. Oikoleura sp. II, a single specimen of 

 Nematoscelis gracilis, and concentrations of large Euphausia 

 similis var. "armata" and tomopterids were also found in the 

 waters of Station 1 1 7. 



215 



