214 



GERBER 



Parascelus edimrdsi, Phronimopsis spini/era, Hyperioides 

 sibaginis. H[;pena sp., and Anchfjiomera blossevillii. For the 

 taxonomy and identification of the amphipods from 

 Enewetak, see Barnard (1965), Bowman and Gruner 

 (1973), and Dakin and Colefax (1940). 



Euphausiacea 



A single species of euphausild has been reported from 

 Enewetak Lagoon where it occurs regularly at both shallow 

 and deep stations (Gerber, 1981). This species was also 

 the dominant euphausiid recorded inside the Great Barrier 

 Reef Lagoon (Russell, 1934). Gilmartin (1958) reported 

 euphausiids in his plankton hauls at Enewetak Lagoon but 

 did not identify them. In the surrounding oceanic waters of 

 Enewetak and Bikini Atolls, seven species of typically 

 oceanic euphausiids were reported by Chiba et al. (1955), 

 and none of these occurs in the atoll lagoons. They are 

 Stijiocheiron carinatum, S. submii, Euphausia gracilis, 

 E. gibba. E. krohnii, Th\;sanopoda obtusifrons, and 

 T. (ricuspidafa. Useful works concerning the identification 

 and taxonomy of euphausiids include Dakin and Colefax 

 (1940), Delsman (1939), Hansen (1910), Mauchline and 

 Fisher (1969), and Mauchline (1980). 



CHAETOGNATHA 



At least nine species of chaetognaths have been 

 reported and identified from Enewetak Lagoon, in addition 

 to seven species from outside the lagoon or in the nearby 

 ocean waters. They are an imp)ortant planktonic group and 

 appear in most net samplings. Johnson (1949, 1954) 

 reported three species from net tows taken during 1946 at 

 Enewetak Lagoon (June), and the lagoons at Rongelap 

 (June), Rongcrik (June), and Bikini (March to May) and 

 from outside Bikini Atoll (March to May). Distribution of 

 chaetognaths within Bikini Atoll was also studied. A 

 translagoon study of plankton at Enewetak from April 

 through December 1955 by Gilmartin (1958) revealed four 

 species, most of which were abundant in the net samples. 

 Gerber (1981) reported at least six species from a mid- 

 lagoon station in the winter and summer (1972, 1974) 

 with increased abundances in summer. Collections made in 

 1946 and later revealed six species from the lagoon and 

 several more from outside, east of the lagoon (Alvarino, 

 personal communication). In the neighboring waters around 

 Bikini Atoll, extensive plankton collections were made in 

 May and June 1954 by Chiba et al. (1955) and later by 

 Tsuruta (1963). These researchers identified at least 16 

 species; the eight not found within the lagoons include 

 Sagitta decipiens. S. hexaptera, S. Ii/ra, S. minima, S. pul- 

 chre, S. pseudoserratodentata, Krohnitta pacifica, and 

 K. subtilis. Three species of chaetognaths were reported 

 from the gut contents of lagoon fish at Majuro Atoll (Hob- 

 son and Chess, 1973) and at Enewetak Atoll (Gerber and 

 Marshall, 1974). Important sources for the identification of 

 chaetognaths include Alvarino, (1967), Sund (1959), and 

 Thomson (1947). 



CHORDATA 

 Thaliacea 



This group is much less abundant than the Larvacca in 

 the Enewetak Lagoon and from nearby atolls. They were 

 first reported from Enewetak by Gilmartin (1958) who 

 identified two species from April through December in his 

 translagoon plankton study of 1955. An additional species 

 has been collected from the lagoon in 12 m of water and 

 identified by R. A. Kinzie (MPML reference collection). 



Appendicularia (Larvacea) 



This is an abundant zooplankton group in the atoll 

 lagoons and surrounding waters of the Marshall Islands. At 

 least seven species have been reported from Enewetak 

 Lagoon, five of which have been collected from outside the 

 atoll. Johnson (1954) refxjrted densities of appendiculari- 

 ans which were considerably higher for Enewetak and 

 nearby atoll lagoons than in the nearby open ocean; he did 

 not identify genera or species. At Enewetak Lagoon, Gil- 

 martin (1958) also reported high densities of this group 

 especially during April and in early November and identi- 

 fied them as Oikopleura sp. Gerber similarly (1981) found 

 high densities of larvaceans in the lagoon especially in the 

 summer; at least six species are recorded belonging to two 

 families. In the waters near Bikini Atoll, Chiba et al. 

 (1955) identified 16 species, five of which are common to 

 Enewetak Lagoon. These additional species include 

 Fritilhria borealis forma sargassi, F. formica, F. hap- 

 hstoma, F. pellucida, F. venusta, Oikopleura cophocerca, 

 Oikopleura sp., Kowalevskaia tenuis, Megalocercus hux/eyi, 

 Stegosoma magnum, and Pelagopleura sp. Appendicularia 

 have been reported at Majuro Atoll Lagoon from the gut 

 contents of plankton feeding fishes (Hobson and Chess, 

 1973); the species were not identified. The identification 

 and taxonomy of the Thaliacea and Larvacea are included 

 in the works of Bjornberg and Forneris (1955), Delsman 

 (1939), and Fraser (1947). 



CONCLUSIONS 



There appears to be little difference of the holoplank- 

 tonic zooplankton species composition between Enewetak 

 and nearby atoll lagoons such as Rongelap. This observa- 

 tion is largely based on the copefxxi data which have been 

 most extensively studied at these two lagoons. Clearly, this 

 faunal similarity reflects the rather similar habitats of the 

 two lagoons and their similar locations with respect to the 

 prevailing ocean currents. 



The information available comparing the copepod 

 species assemblege inside the lagoon with that of the sur- 

 rounding ocean outside the lagoon demonstrates that they 

 are different. The lagoon zooplankton populations are par- 

 tially composed of oceanic species known to occur in the 

 neighboring waters outside the atoll, in addition to neritic 

 species which occur only in shallow protected environ- 

 ments. It has also been found that the density of zooplank- 



