Bathymetric Distribution of Chaetognaths 1 * 
Angeles Alvarino 
The present report on the vertical distribu- 
tion of the Chaetognatha is based on a study of 
the collections of plankton made by the Scripps 
Institution of Oceanography expeditions in the 
Pacific (Fig. 1) and Indian oceans. Therefore, 
all the data included and discussed in this paper 
have been obtained by the author from studies 
and analysis of thousands of plankton samples 
from those oceans; and, when other sources of 
information are used in the discussion, the name 
and date of the corresponding authority and 
publication are given. The samples studied here 
that cover the Pacific and Indian oceans were 
taken at 140 m depth (oblique hauls), and at 
other various depths: 270, 300, 363, 600, 700 or 
868 m ( closing nets or vertical tows ) , down to 
3000 m deep (mid-water trawls). The results 
obtained from these expeditions and from the 
material examined while studying the seasonal 
distribution of chaetognaths in the California 
waters, and also from previous work in the 
Atlantic, has made it possible to group the 
species of this phylum into several categories 
based on their distribution in depth. 
The correlation between the distribution in 
depth of the species and the conditions affecting 
this distribution can only be reached through a 
knowledge of the two-dimensional distribution 
of each of the species, the factors involved in 
their distributional fluctuations, and the extent 
to which changes in those factors may affect the 
distributional pattern. 
It was found, from these observations, that 
each individual species of chaetognaths occurs 
generally at about the same levels throughout 
the oceans, with the particular exception of Eu- 
krohnia hamata (Mobius), as will be shown 
later in this paper. Therefore, these species can 
1 Contributions from Scripps Institution of Ocean- 
ography, New Series. Manuscript received July 5, 1962. 
be grouped in three categories according to the 
strata they populate: 
Epiplanktonic (upper 150-200 m) 
Mesoplanktonic (200-1000 m) 
Bathyplanktonic (below 1000 m level) 
In each of these categories several series of 
stratification, or zonations, can be established. In 
the upper layers there is more zonation of the 
species than in the levels below 200 m. Because 
of the large number of species involved and the 
great variability of the conditions in the upper 
layers, the picture is a complex one. Migrations 
related to food, light, and seasonal changes may 
take place in the upper strata. In these upper 
layers competition is more active than in the 
deeper strata, and this is conditioned by the 
large number of species involved. 
In general, the Chaetognatha decrease in 
number of species and in number of individuals 
per species with increase in depth, that is, the 
number of individuals per volumetric unit of 
water is larger in the upper layers than in deep 
waters. The species typical of waters below the 
200 m level are one-fifth of the total of the 
phylum. Differences are found regarding this 
point, depending on the geographical localities 
under study. Thiel (1938) found more species 
and greater biomass of chaetognaths in the 
upper 50 m. Hida (1957) stated: "regardless 
of time of day of hauling, oblique hauls between 
the surface and 40m. yield larger numbers of 
chaetognaths and pteropods for m 3 of water 
strained than deeper hauls between the surface 
and 140m.” A similar relation was found in the 
tropical Pacific (Hida and King, 1955). How- 
ever, in both north and south high latitudes 
the layers below 200 m sustain a larger number 
of species of chaetognaths than do the upper 
strata. Whereas, as noted above, in other oceanic 
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