The Chaetognatha of the Monsoon Expedition in the Indian Ocean 1 
Angeles Alvarino 
This REPORT deals with the chaetognaths col- 
lected by the "R/V Argo” during the Monsoon 
Expedition in the Indian Ocean in I960 and 
1961. The Monsoon collections extended from 
about 8° S to 42° S ( Fig. 1 ) ; that is, the region 
roughly limited by the Equatorial Countercurrent 
and the Subantarctic West Wind Drift (the 
Indian Central waters extending to the Sub- 
tropical Convergence ) ; and also the Indonesian 
seas and the South Australian waters. This re- 
port includes only data from the Indian Ocean. 
Data from collections made by the same Ex- 
pedition in the Pacific have been added to the 
study of the chaetognaths of the Pacific. How- 
ever, data derived from the Pacific are used here 
also in discussing the distribution of the species. 
The Monsoon Expedition covered in part the 
regions surveyed for chaetognaths by the Ga- 
zelle, Gauss, Sealark, Siboga, and Snellius expe- 
ditions, with the following exceptions: the Bay 
of Bengal, west coast of Ceylon, and waters of 
Somalia and eastern Africa. 
The samples were taken with a 1-m open net, 
towed obliquely from a depth of 200 or 360 m 
to the surface; and several mid-water trawls are 
also included (see Table I). 
It is well known that the hydrographic changes 
taking place in the Indian Ocean are influenced 
by the monsoon regime. The sampling extended 
from November I960 to January 1961. This is 
the season of the northeast monsoon ( Mattheus, 
1926 ). 
Two well-defined zoogeographical boundaries 
were indicated in the Indian Ocean by the study 
of the Chaetognatha population: (1) the equa- 
torial boundary extending south of the equator 
(from about 5° S in the West to 15° S in the 
East), and (2) the Subtropical Convergence 
region (at about 40° S). These well-defined 
1 Contribution from the Scripps Institution of Ocean- 
ography, University of California, San Diego. 
Manuscript received May 13 , 1963 . 
boundaries frame latitudinally three main re- 
gions: Equatorial, Central, and Subantarctic. 
Typical equatorial species of the Pacific that 
occur also in the Indian Ocean appear to be 
restricted to the Equatorial waters. Warm-water 
species that are cosmopolitan in distribution, 
extend along the Equatorial and Central Indian 
waters, while cold-water species do not extend 
to the Central waters. The Subtropical Conver- 
gence appears to be the fluctuating northern 
boundary for the latter. 
The data obtained aid in filling the gap that 
existed in the zoogeography of the chaetognaths 
by adding information from the Indian Ocean. 
These observations make it easy to compare the 
relationships found in the distributional pattern 
shown by each of the species in the Indian 
Ocean and their respective allies in the Atlantic 
and Pacific oceans. 
The Indian Ocean collection contains 23 
oceanic species of chaetognaths, already known 
from the Atlantic and the Pacific oceans. 
The following species were observed in the 
samples from the Monsoon Expedition in the 
Indian Ocean: 
Eukrohnia bathypelagica Alvarino 
E. fowleri Ritter-Zahony 
E. hamata ( Mobius ) 
Krohnitta pacifica ( Aida ) 
K. subtilis ( Grassi ) 
Pterosagitta draco ( Krohn ) 
Sagitta bedoti Beraneck 
S. bipunctata d’Orbigny 
S. decipiens Fowler 
S. enflata Grassi 
S. ferox Doncaster 
S. gazellae Ritter-Zahony 
S. hexaptera Quoy and Gaimard 
S. lyra Krohn 
S. minima Grassi 
S. neglecta Aida 
S. pacifica Tokioka 
336 
