Biogeographical Change in the Latitudinal Boundary
403
species is subject to marked diurnal fluctuations in abundance, 3 graphs were prepared
(Fig. 2) based on day, night and total distribution to minimize any error in the estimation
of abundance.
Fig. 2 shows that the latitudinal variation in occurrence of S. subula is almost uniform.
This species is absent in the northern latitudes (except for a single record) between 25 0 N
and lOoN and sparse (10%) between lOON and SON, rising to 22% between SON and
the Equator. From the Equator to 150 S there is a gradual increase in percentage occurrence (22 to 35%) with the peak (40%) between 15 0 Sand 20 0 S and maintaining more
or less the same percentage up to 30 0 S. Between 30 0 Sand 35 0 S there is a sudden
decrease to 18% with a further reduction (5%) beyond 35 0 S and total disappearance
beyond 40 0 S. The peak percentage occurrence between 150 and 30 a S indicates clearly
that S. subula flourishes in the subtropical zone rather than in the tropical zone of the
Indian Ocean. The unique feature in the distribution of S. subula is the change in occurrence with latitude. The absence of this species in the northern latitudes and its continuous
occurrence from 100 N to 40 0 S suggests a possible change in the bisubtropical distribution
of a subtropical species in a unipolar ocean.
The diurnal variation of S. subula is clearly seen in Fig. 2. A distinct rise in percentage
occurrence is observed during the night, especially between 50 Nand 30 0 S. The largest
difference in the percentage between day and night is found between 15 a Sand 30
0
S.
This diurnal variation is attributed to the position of the sun at different latitudes in
different months of the year, the intensity of light, the transparency of water and other
local environmental conditions which determine the vertical division of the hydrosphere
into photic, dysphotic and aphotic zones.
From the present state of knowledge of the hydrography of the Indian Ocean it is
really difficult to define the exact barrier to the distribution of S. subula in the northern
part of the north Indian Ocean. It is probably due to the combination of several factors
caused by the land barrier in the north.
With regard to surface salinity, the waters of the Bay of Bengal are less saline than the
waters of the Arabian Sea. The average values are 30-33%0 for the Bay of Bengal and
34-370/00 for the Arabian Sea (PANIKKAR and jAYARAMAN, 1966). Salinity increases
from north to south in the Bay of Bengal and decreases from north to south in the
Arabian Sea (GALLAGHER, 1966). As RAMPAL (1966) has observed, this species as a
stenohaline form is avoiding the area of high salinity (380/00) in the Mediterranean, the
water mass with low salinity and high temperature in the northern Bay of Bengal and
the water mass of high salinity and high temperature in the northern Arabian Sea, might
possibly act as hydrological barrier for the distribution of S. subula. The unique subsurface O2 minimum (0.5 mil-I) found north of 8 0 N might also be a major reason for
the absence of this species in the northern part of the Indian Ocean.
In support of the relict theory of bipolarity, MEISENHEIMER (1905) traced the various
stages in the migration of pteropods. He believed that the pteropods originally spread
evenly from the Equator to the north and south without any gap. The next step was
moving away from the warmest water on either side of the Equator. He quotes S. subula
and Cavolinia gibbosa as examples of this migration from the Equator because they are
rare in the equatorial zone and abundant in the subtropical zone. The gradual increase
in the percentage occurrence of S. subula from equatorial to subtropical latitudes in the
Indian Ocean supports this hypothesis, even though this species still occurs in the
403
species is subject to marked diurnal fluctuations in abundance, 3 graphs were prepared
(Fig. 2) based on day, night and total distribution to minimize any error in the estimation
of abundance.
Fig. 2 shows that the latitudinal variation in occurrence of S. subula is almost uniform.
This species is absent in the northern latitudes (except for a single record) between 25 0 N
and lOoN and sparse (10%) between lOON and SON, rising to 22% between SON and
the Equator. From the Equator to 150 S there is a gradual increase in percentage occurrence (22 to 35%) with the peak (40%) between 15 0 Sand 20 0 S and maintaining more
or less the same percentage up to 30 0 S. Between 30 0 Sand 35 0 S there is a sudden
decrease to 18% with a further reduction (5%) beyond 35 0 S and total disappearance
beyond 40 0 S. The peak percentage occurrence between 150 and 30 a S indicates clearly
that S. subula flourishes in the subtropical zone rather than in the tropical zone of the
Indian Ocean. The unique feature in the distribution of S. subula is the change in occurrence with latitude. The absence of this species in the northern latitudes and its continuous
occurrence from 100 N to 40 0 S suggests a possible change in the bisubtropical distribution
of a subtropical species in a unipolar ocean.
The diurnal variation of S. subula is clearly seen in Fig. 2. A distinct rise in percentage
occurrence is observed during the night, especially between 50 Nand 30 0 S. The largest
difference in the percentage between day and night is found between 15 a Sand 30
0
S.
This diurnal variation is attributed to the position of the sun at different latitudes in
different months of the year, the intensity of light, the transparency of water and other
local environmental conditions which determine the vertical division of the hydrosphere
into photic, dysphotic and aphotic zones.
From the present state of knowledge of the hydrography of the Indian Ocean it is
really difficult to define the exact barrier to the distribution of S. subula in the northern
part of the north Indian Ocean. It is probably due to the combination of several factors
caused by the land barrier in the north.
With regard to surface salinity, the waters of the Bay of Bengal are less saline than the
waters of the Arabian Sea. The average values are 30-33%0 for the Bay of Bengal and
34-370/00 for the Arabian Sea (PANIKKAR and jAYARAMAN, 1966). Salinity increases
from north to south in the Bay of Bengal and decreases from north to south in the
Arabian Sea (GALLAGHER, 1966). As RAMPAL (1966) has observed, this species as a
stenohaline form is avoiding the area of high salinity (380/00) in the Mediterranean, the
water mass with low salinity and high temperature in the northern Bay of Bengal and
the water mass of high salinity and high temperature in the northern Arabian Sea, might
possibly act as hydrological barrier for the distribution of S. subula. The unique subsurface O2 minimum (0.5 mil-I) found north of 8 0 N might also be a major reason for
the absence of this species in the northern part of the Indian Ocean.
In support of the relict theory of bipolarity, MEISENHEIMER (1905) traced the various
stages in the migration of pteropods. He believed that the pteropods originally spread
evenly from the Equator to the north and south without any gap. The next step was
moving away from the warmest water on either side of the Equator. He quotes S. subula
and Cavolinia gibbosa as examples of this migration from the Equator because they are
rare in the equatorial zone and abundant in the subtropical zone. The gradual increase
in the percentage occurrence of S. subula from equatorial to subtropical latitudes in the
Indian Ocean supports this hypothesis, even though this species still occurs in the