128 
PACIFIC SCIENCE, Vol. XII, April, 1958 
food of the pelagic turtle, Thalassochelys caretta. 
I have recently identified five specimens of 
Thalia democratica agg. from the gut contents 
of a fish, Chaetodon unimacidata , taken by 
rotenone poison off Honolulu at a depth of 
about 25 feet. This fish contained many ap- 
pendicularians also, probably of the genus 
Oikopleura. Other than this, no information 
is available as to the enemies of salps. 
Because there is no apparent selection of 
food, and because the food, feeding methods, 
and mechanisms are similar in all species of 
salps, and because enemies are evidently simi- 
lar, all salp species appear to occupy similar 
niches (with some slight differences between 
salps of different sizes in their ability to handle 
the larger food organisms). Inasmuch as many 
species of salps have been found together in 
one plankton tow (up to 9 in the POFI col- 
lections studied), and all species in a plankton 
tow can be assumed to be subject to at least 
approximately the same enemies and to the 
same environmental conditions, it can be stated 
that all salps apparently occupy similar niches 
simultaneously, and are, for all practical pur- 
poses, ecological equivalents. This statement 
doubtless applies to many other plankters as 
well as to salps: Any one niche in the plank- 
ton community of the epipelagic zone of 
tropic oceanic waters may be occupied simul- 
taneously by many species. To illustrate, a 
few specimens of doliolids and pyrosomids 
have been subjected to a gut content analysis, 
and these animals also contained generally the 
same food organisms as the salps. Appendi- 
cularians, although to some extent feeding on 
similar food as other planktonic tunicates, are 
probably more restricted in the type of food 
they can handle as a result of their highly 
modified food-catching mechanisms, and are 
thus probably limited in their equivalence 
with other pelagic tunicates. It is probable 
that predaceous plankters such as chaeto- 
gnaths, heteropods, coelenterates, and some 
Crustacea found in tropic epipelagic waters 
also occupy similar niches simultaneously. 
Marshall and Orr (1953) briefly discussed 
the concept of the niche (including the con- 
cept of habitat in the same term) in regard to 
the plankton community and pointed out 
that . . it is at first sight hard to understand 
how different ecological niches can be avail- 
able in a medium so constant as sea water.” 
They suggested that differences in niches 
among plankters may be found in the different 
foods on which the plankters depend. Such 
a suggestion may be applicable to plankters 
found in coastal and temperate waters, but 
for tropic oceanic waters it seems scarcely 
applicable, at least to the salps and undoubt- 
edly also to many other plankton animals. 
To be certain of this niche equivalence, it 
would be desirable to know the vertical dis- 
tribution of the captured animals, but the 
tows studied cannot furnish such information, 
as they were made with an open net. Con- 
jectures have been made as to the causes and 
results of this apparent niche equivalence of 
many species, but they are not included in 
this report. They have resulted in a study of 
species variety in Silver Springs, Florida, in 
which factors controlling the numbers of 
species are discussed (Yount, 1956). 
REFERENCES 
Apstein, C. 1894. Die Thaliacea der Plank- 
ton-Expedition. B. Vertheilung der Salpen. 
Ergeb . der Plankton-Exped . 2(E.a.B.) : 1-68. 
- 1906. Salpen der deutschen Tiefsee- 
Expedition. Wiss. Ergeb . der Dent . Tiefsee 
Exped. 1898-99 12(3): 245-290. 
Berner, L. D. 1955. Two new pelagic tuni- 
cates from the eastern Pacific Ocean. Pacific 
Sci. 9(2): 247-253. 
Bigelow, H. B. 1926. Plankton of the off- 
shore waters of the Gulf of Maine. U. S. 
Bur. Fisheries, Bui. 40(2): 1-509. 
Cromwell, T. 1953. Circulation in a meri- 
dional plane in the central equatorial Pacific. 
Jour. Mar. Res. 12(2): 196-213. 
- 1954. Mid-Pacific Oceanography II. 
Transequatorial waters, June-August 1950 
and Jan. -March, 1951. U. S. Fish and Wild- 
life Ser., Spec. Sci. Rpt.: Fisheries. 131. 
