A New Species of Micronereis (Annelida, Polychaeta) 
from the Marshall Islands 1 
Donald J. Reish 2 
During the course of surveying Eniwetok 
Atoll, Marshall Islands, for polychaetous an- 
nelids in the summers of 1957 and 1958, the 
author encountered from three localities four 
specimens of a new species of polychaete be- 
longing to the nereid genus Micronereis Clap- 
arede. Transects were made of both the ocean 
and lagoon sides of the islands shown in the 
map of Eniwetok Atoll (Fig. 1). One of the 
collecting methods employed was to preserve 
algae, coral, rocks, sand, etc., in formalin in the 
field and to sort out the polychaetes in the lab- 
oratory. The new species of Micronereis was 
collected in this manner. 
family NEREIDAE 
Micronereis Claparede 
Micronereis eniwetokensis n. sp. 
Figs. 2-6 
Four complete specimens, each with 17 seti- 
gerous segments measuring 1.5 to 2.0 mm. in 
length, came from three localities in Eniwetok 
Atoll. The prostomium (Fig. 2) is broadest 
posteriorly and is clearly separated from the first 
setigerous segment. The prostomium is weakly 
indented at its anterior margin. Four pairs of 
tentacular cirri are broadest near the point of 
attachment and tapered distally. The short palpi 
are viewed from the ventral side. There are no 
prostomial tentacles or peristomium as is char- 
acteristic for the genus. 
Four large eyes are in trapezoidal arrange- 
ment, with the anterior pair being the larger, 
1 The field work was made possible by the U. S. 
Atomic Energy Commission through its Eniwetok 
Marine Biological Laboratory. The author is indebted 
to Dr. Robert W. Hiatt for his assistance during the 
course of the field investigations. 
2 Department of Biological Sciences, Long Beach 
State College, Long Beach, California. Manuscript re- 
ceived February 5, I960. 
farther apart, and provided with a clear lens. 
A variation was noted in the eyes of one speci- 
men ( Fig. 3 ) . The anterior pair was elongated 
and extended posteriorly and laterally to the 
second pair. Each anterior eye was provided with 
a large clear lens. 
The yellow jaws (Fig. 4) were seen by dis- 
section or by mounting the worm in glycerine. 
They were broadest at their base and terminated 
in one large tooth and three smaller teeth. The 
apical tooth was serrated along its inner margin. 
The first two setigerous segments are uniram- 
ous with homogomph spinigers. Each has a fili- 
form ventral cirrus. 
All parapodia (Fig. 5) are biramous from 
the third setigerous segment to the posterior 
end. The rami are widely separated and each 
has a single black aciculum. The inner margin 
between the notopodium and neuropodium is 
ciliated. Each ramus has a cirrus; the dorsal 
one is longer than the ventral one. 
All setae (Fig. 6) are homogomph spinigers 
with the appendages lacking spines along their 
margins. The setae number 12-20 in each lobe 
of the parapodium. 
The pygidium is characterized by having a 
bilobed ventral fleshy membrane and two blunt 
lateral lobes. 
DISCUSSION 
Three species are previously known for the 
genus Micronereis , M. variegata Claparede 1863, 
M. nanaimoensis Berkeley and Berkeley 1953, 
and M. halei Hartman 1954. Sexual dimorphism 
has been described for the former two species. 
Specialized setae in the third setigerous segment 
of the male, and a difference in the number of 
segments between the two sexes, occur in both 
M. variegata and M. nanaimoensis. The jaws vary 
according to sex in M. variegata . Additional 
dimorphic characters in M. nanaimoensis include 
the pygidium, coloration, and capillary setae in 
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