Chaetognaths of the Arabian Sea
315
e) South of the Equator between 50 - 68 0 E: S. minima;
f) Southwest Arabian Sea and western side of India and Ceylon: Pt. draco,
S. regularis;
g) Deep and subsurface waters: E. fowleri, S. decipiens, S. lyra and S. zetesios.
The distribution of certain species shows seasonal variation (Table 1 and Figs. 1 - 14).
Tables 1 and 2 show that maximum density is to be found in areas and seasons where
secondary production is high, such as the western boundary area, the equatorial upwelling
off India. The exceptions are few and apparently not too significant. They are S. ferox
and S. minima (Table 1) during the SW monsoon, and E. fowleri, K. subtilis, S. ferox,
S. minima and S. regularis during the NE monsoon. The numerical distribution of
chaetognaths as shown in these tables illustrates that numerical abundance is correlated
to food availability, while species presence or absence may be attributed to suitability of
the environment and tolerance of the species.
Table 2 shows that the average number per haul is higher during the SW monsoon for
nine species. In 2 species (K. subtilis and S. pacifica) the average number per haul during
the SW monsoon is 1.7 times that of the NE monsoon, while Pt. draco, S. bipunctata,
S. enflata, S. hexaptera and S. regularis showed an increase in number per haul by a
factor of 1 to 1.4. S. ferox is 6 times and S. minima 3 times more abundant during the
SW monsoon period. The average numbers per haul for the 2 species (S. robusta and
K. pacifica) are equal in both seasons. S. bedoti, S. neglecta and S. pulchra have more
numbers per unit haul in the NE monsoon period by a factor of 1.14 to 1.5 although 2 of
them showed highest density during the SW monsoon. This indicates that patterns of
distribution and swarming can be due to factors other than food availability. Species
like E. fowleri, S. decipiens and S. zetesios are obtained in small numbers only during
NE monsoon. S. lyra was found in small numbers at a single station in the SW monsoon
period. K. pacifica, S. enflata and S. neglecta showed maximum density (measured as
number of specimens per haul) during the NE monsoon. Further detailed studies on the
individual species should clarify the ecological factors involved.
It is significant that 2 of the 3 species showing affinity for the coastal area (S. bedoti
and S. neglecta) have an average number per haul greater during the NE monsoon, while
the other (S. robusta) has the same average number, because coastal species are more
affected by local factors other than large-scale upwelling.
There is remarkable similarity in the Indo-Pacific chaetognath fauna (ToKIOKA, 1952;
1965; HYMAN, 1959). In the present collections all the 15 epiplanktonic species are
common to the Indo-Pacific and 7 of them (S. bedoti, S. ferox, S. neglecta, S. pacifica,
S. pulchra, S. regularis and S. robusta) are endemic to this region. A critical study of
earlier work dealing with the Indo-Pacific reveals the fact that it sustains a variety of
species and many species are exclusive to this region. The maintenance of the Indo-Pacific
fauna may be due to the pattern of circulation in the Indian Ocean. The main part of the
Agulhas Current flowing along the SE coast of Africa is directed towards Australia across
the southern part of the Indian Ocean. During the NE monsoon there is a strong flow of
water from the Pacific to the Indian Ocean through the straits of Malacca into the Andaman Sea and then to the Bay of Bengal (SEWELL, 1948). During the SW monsoon the
direction of the currents is reversed and there is a westward flow between the Malay
Archipelago and Australia (SEWELL, 1948). Recent studies based on nOE data also indicate the influx of Pacific Ocean water through the Indonesian Archipelago (WYRTKI,
1973). There is much interchange of water between the 2 oceans, resulting in a wide distri-
315
e) South of the Equator between 50 - 68 0 E: S. minima;
f) Southwest Arabian Sea and western side of India and Ceylon: Pt. draco,
S. regularis;
g) Deep and subsurface waters: E. fowleri, S. decipiens, S. lyra and S. zetesios.
The distribution of certain species shows seasonal variation (Table 1 and Figs. 1 - 14).
Tables 1 and 2 show that maximum density is to be found in areas and seasons where
secondary production is high, such as the western boundary area, the equatorial upwelling
off India. The exceptions are few and apparently not too significant. They are S. ferox
and S. minima (Table 1) during the SW monsoon, and E. fowleri, K. subtilis, S. ferox,
S. minima and S. regularis during the NE monsoon. The numerical distribution of
chaetognaths as shown in these tables illustrates that numerical abundance is correlated
to food availability, while species presence or absence may be attributed to suitability of
the environment and tolerance of the species.
Table 2 shows that the average number per haul is higher during the SW monsoon for
nine species. In 2 species (K. subtilis and S. pacifica) the average number per haul during
the SW monsoon is 1.7 times that of the NE monsoon, while Pt. draco, S. bipunctata,
S. enflata, S. hexaptera and S. regularis showed an increase in number per haul by a
factor of 1 to 1.4. S. ferox is 6 times and S. minima 3 times more abundant during the
SW monsoon period. The average numbers per haul for the 2 species (S. robusta and
K. pacifica) are equal in both seasons. S. bedoti, S. neglecta and S. pulchra have more
numbers per unit haul in the NE monsoon period by a factor of 1.14 to 1.5 although 2 of
them showed highest density during the SW monsoon. This indicates that patterns of
distribution and swarming can be due to factors other than food availability. Species
like E. fowleri, S. decipiens and S. zetesios are obtained in small numbers only during
NE monsoon. S. lyra was found in small numbers at a single station in the SW monsoon
period. K. pacifica, S. enflata and S. neglecta showed maximum density (measured as
number of specimens per haul) during the NE monsoon. Further detailed studies on the
individual species should clarify the ecological factors involved.
It is significant that 2 of the 3 species showing affinity for the coastal area (S. bedoti
and S. neglecta) have an average number per haul greater during the NE monsoon, while
the other (S. robusta) has the same average number, because coastal species are more
affected by local factors other than large-scale upwelling.
There is remarkable similarity in the Indo-Pacific chaetognath fauna (ToKIOKA, 1952;
1965; HYMAN, 1959). In the present collections all the 15 epiplanktonic species are
common to the Indo-Pacific and 7 of them (S. bedoti, S. ferox, S. neglecta, S. pacifica,
S. pulchra, S. regularis and S. robusta) are endemic to this region. A critical study of
earlier work dealing with the Indo-Pacific reveals the fact that it sustains a variety of
species and many species are exclusive to this region. The maintenance of the Indo-Pacific
fauna may be due to the pattern of circulation in the Indian Ocean. The main part of the
Agulhas Current flowing along the SE coast of Africa is directed towards Australia across
the southern part of the Indian Ocean. During the NE monsoon there is a strong flow of
water from the Pacific to the Indian Ocean through the straits of Malacca into the Andaman Sea and then to the Bay of Bengal (SEWELL, 1948). During the SW monsoon the
direction of the currents is reversed and there is a westward flow between the Malay
Archipelago and Australia (SEWELL, 1948). Recent studies based on nOE data also indicate the influx of Pacific Ocean water through the Indonesian Archipelago (WYRTKI,
1973). There is much interchange of water between the 2 oceans, resulting in a wide distri-