Bathymetric Distribution of Chaetognaths — AlvARINO 
69 
with oxygen values of more than 6 ml/1. The 
presence of S. elegans was expected in the rest 
of the localities of this region (because of the 
characteristics of the waters); however, it was 
noteworthy that every negative station and sam- 
ple for S. elegans corresponded to oxygen values 
below 6 ml per liter. Consequently it was found 
in these studies that S’, elegans does not occur in 
the Pacific in waters with less concentration of 
oxygen than the amount stated above; and this 
could also be the limiting factor in the spread- 
ing of this chaetognath to deeper levels. S. ele- 
gans is present in the Oyashio waters, but when 
these waters collide with the Kuroshio, its south- 
ern distribution is abruptly interrupted, even at 
deep levels, in the region where the two bodies 
of water meet. 
The conditions are also more or less uniform 
in antarctic waters in the west-east direction 
(Baker, 1954) , while the gradients change from 
south to north. It is well known that the plank- 
ton ( and this is true for the Chaetognatha popu- 
lation according to our data) follows a rather 
homogeneous circumpolar distribution in the 
antarctic belt. S. gazellae is the typical chaetog- 
nath in the upper layers of these waters. It 
extends along the antarctic and subantarctic 
regions spreading northward from the subtropi- 
cal convergence in deep waters. 
5". gazellae was observed in the present study 
at MacMurdo Sound (samples were kindly sent 
by Mr. J. L. Littlepage ) , in a current from ice- 
water interface to 7, 10, and 72 m depth; ex- 
tending along the upper 200 m in the south 
Pacific to 30 °S. and to 21°S. at about 400 m 
depth. S. gazellae was also observed in the upper 
200 m, in the samples taken by the Drake 
Expedition (material kindly sent from Argen- 
tina by Dr. E. Balech) at 55° S to 60° S and 
60° W to 62° W to 63° 33' W. 
The typical mesoplanktonic Chaetognatha oc- 
cupy different layers. Thus S. decipiens inhabits 
the upper part of the mesoplanktonic region, 
while S. zetesios and S. planctonis occupy the 
core, and S. macrocephala and S. marri extend 
from the lower levels of the mesoplankton to 
the upper part of the bathyplanktonic domain, 
and S. maxima changes its depth distribution 
with latitude. The fact that David (1956), 
Moore (1949), and Thomson (1947) reported 
S. planctonis in surface waters could be due to 
upwelling processes. S. planktonis Steinhaus 
and S. zetesios Fowler are easily distinguished 
(although many authors failed to separate them), 
by using morphological characteristics that are 
constant, instead of the meristic characteristics 
related to hooks, teeth, etc. For example, reliable 
morphological characteristics for this identifica- 
tion are: length of tail segment in relation to 
the total length, and the shape and position of 
the lateral fins. In S. planctonis the tail segment 
is shorter than in S. zetesios. The posterior fins 
in S. planctonis are triangular in shape, while 
those in S. zetesios are roundish. In S. planctonis 
the anterior end of the anterior fins reach up 
to the level of the ventral ganglion,whereas in 
S. zetesios they reach only to the level of the 
posterior end of the ventral ganglion. In order 
to verify this, here are the quotations from the 
respective species’ authors. Steinhaus (1896): 
"Vorderflossen sehr sehmal bis zur mitte des 
Bauchganglion”; and Fowler (1905): "anterior 
fin well separated from the posterior, longer and 
narrower than posterior, wider behind than in 
front, not quite or only reaching the hinder end 
of the ventral ganglion.” 
In the samples studied here, S. marri was 
found only at levels below 200 m, south of 
parallel 50° S, in 59% of the samples from 
stations located between 50° S and 65° S, in 
the south Australian waters and the south 
Pacific. 
S. planctonis was observed in the present 
studies distributed along the subantarctic waters, 
in a region limited by the subtropical converg- 
ence and the antarctic convergence. 
S. hexaptera, S. regularis, and K. pacifica are 
also found very often extending (present data) 
to the lowest strata of the epiplankton and even 
to the upper strata of the mesoplankton. 
5’. bipunctata is an oceanic cosmopolitan spe- 
cies, which inhabits the epiplanktonic region, 
extending in the Pacific from the 40° N to the 
35° S (Alvarino, in press a). Bieri (1959) sug- 
gested it is an antiequatorial species, as he did 
not find it within 5° to 10° on either side of 
the Equator. S. bipunctata was observed in the 
present studies in the upper 150 m in 46 sta- 
tions along the Pacific equatorial belt, in a band 
of a width extending from 10° N to 15° S. 
(The stations correspond to the following ex- 
peditions: Equapac Horizon, Equapac Stranger, 
