A Key to the Chaetognatha of the Tropical Eastern Pacific Ocean 
Paul N. Sund^ 
The Inter-American Tropical Tuna Com- 
mission recently commenced a study of zoo- 
plankton collections from the tropical eastern 
Pacific Ocean to investigate the occurrence of 
planktonic organisms that can be used as bi- 
ological indicators of water masses. This 
study is a part of the Commission’s broader 
investigations of the relationship of the yel- 
lowfin and skipjack tunas and their food 
supply to the hydrographic regime within the 
eastern Pacific area. 
The eastern tropical Pacific Ocean has been 
recently defined by Wooster and Cromwell 
(1958) as ”the region lying between the 
Tropic of Cancer (23°27'N.) and the Tropic 
of Capricorn (23°27^S,) and extending west- 
ward from the coast of Central and South 
America to 130° W.” 
Two groups of animals which include spe- 
cies of known indicator value were selected for 
particular attention. These are the Euphausi- 
acea and the Chaetognatha. The identities of 
the euphausiids were readily determined us- 
ing the keys in the recent work of Boden, 
Johnson, and Brinton (1955). At the outset 
of our studies, the Chaetognatha proved to 
be quite difficult to identify because the pub- 
lished keys (e.g., Ritter-Zahony, 1911; Mi- 
chael, 1908, 1911; Thomson, 1947) used 
characters that are often indistinguishable or 
whose determination is so time consuming 
that their use in studies of this sort was 
found impractical. 
For these reasons it was decided to formu- 
late a new key (of the type published by 
Fraser, 1952, for the chaetognaths of northern 
waters) that would allow rapid, but accurate, 
identification of a given specimen by using 
^ Inter-American Tropical Tuna Commission, Scripps 
Institution of Oceanography, La Jolla, California. Man- 
uscript received January 15, 1958. 
characters that are readily distinguishable in 
preserved animals of good-to-fair condition. 
General discussions of the characters and 
their use for purposes of identification are 
presented by Fowler (1904, 1906) and Mi- 
chael (1908). 
ACKNOWLEDGMENTS 
The author wishes to express his sincere 
thanks to Dr. Robert Bieri for reading the 
manuscript and offering valuable comments 
leading to its improvement. Thanks are also 
due to Forraine Pilon Sund for her excellent 
technical assistance with the drawings. 
METHODS AND MATERIALS 
The material was selected from the zoo- 
plankton collections made by University of 
California Scripps Institution of Oceanogra- 
phy research vessels during Eastropic Expedi- 
tion, 1955. Figure 1 shows the locations of 
zooplankton stations from which the material 
for this study was taken. Holmes, Schaefer, 
and Shimada (1957) have outlined the meth- 
ods and equipment employed in making 
these collections. 
Specimens in good condition were identi- 
fied and isolated from the Eastropic zoo- 
plankton. Upon accumulation of a number of 
these, the measurements and counts neces- 
sary to complete the armature formulae were 
made. All measurements were made with a 
calibrated ocular micrometer. These are pre- 
sented in tabular form in the text, together 
with similar data obtained from specimens of 
known identity from eastern and central Pa- 
cific waters. The latter specimens were kindly 
donated to the writer by Dr. T. Tokioka of 
the Seto Marine Biological Laboratory, Japan; 
and Mr, T. Hida of Pacific Oceanic Fishery 
Investigations, U. S. Fish and Wildlife Serv- 
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