314
V. R. NAIR and T. s. s. RAo:
common species in the order of their numerical abundance (Table 1). S. ferox, S. robusta,
S. neglecta and K. pacifica are moderately common, while S. hexaptera, S. pulchra,
S. minima and K. subtilis are occasionally encountered, S. bombayensis is endemic to the
coasts of India but specimens were obtained only from a single collection taken from the
Gulf of Cambay.
The Arabian Sea is in the NW of the monsoon area of the Indian Ocean and its surface
currents and upwelling systems are regulated by the monsoonal regime. The western
Arabian Sea, especially the Somalian and Arabian areas, has the maximum density for all
abundant and frequent species excluding S. ferox and S. minima (Table 1). During the
SW monsoon upwelling occurs along the east coast of Africa and west coast of India.
According to PANIKKAR and ]AYARAMAN (1966), upwelling is prevalent along the coast
between 7 -10 0 N lat. during August to early October. Even during the NE monsoon
period, the western Arabian Sea and coast of India are rich in most of the chaetognath
species. Investigations made during the HOE indicate that the Arabian Sea has a very high
concentration of nutrients (BEZRUKOV, 1961; RYTHER and MENZEL, 1965). The western
side of the Arabian Sea is more productive (LA FOND, 1965; WOOSTER, SCHAEFER
and ROBINSON, 1967) and hence it is no wonder that the chaetognaths, which occupy a
high level in the food web, also have maximum abundance in this part of the Arabian
Sea. The region 40 - 60 0 E between the Equator and northern limit of the ocean is
richest in chaetognaths during both seasons (NAIR, 1969). The HOE data on zooplankton (IOBC, 1968 a, b) suggest that primary production is able to support a good
standing stock of higher organisms in the northern and western part of the Arabian Sea,
with a peak during the SW monsoon upwelling season. The biomass values (IOBC,
1968 a, b) for both monsoon periods show the highest values along Somalia, Arabia and
SW coast of India. However, during the NE monsoon period the areas of highest values
for biomass are located further south than in the SW monsoon period.
In attempting to determine the primary cause or causes of abundance it is important
to compare the distribution with that of zooplankton. Examination of the stomach content
of the chaetognaths in the HOE samples indicates that copepods form the main food
item (see also VARADARAJAN and CHACKO, 1943; THOMSON, 1947; REEVE, 1966).
Chaetognaths feed mainly on calanoid copepods, the commonest forms being Centropages, Eucalanus and Lucicutia, and cyclopoid copepods like Corycaeus, Oithona
and Oncaea. Rarely harpacticoid copepods and other groups like fish larvae, ostracods,
euphausiids and polychaete larvae were found inside the gut. The HOE data on copepod
distribution for the NE and SW monsoon periods show maximum population along
the Somalian coast of Africa extending northwards to the Arabian coast (lOBC, 1970 a).
Lowest population density for both monsoon periods was observed in the central part of
the Arabian Sea. Most of the chaetognath species except S. enflata, S. pacifica and
Pt. draco also have sparse representation in the central part of the Arabian Sea. In general,
the abundance and scarcity of different chaetognath species corresponds to the pattern of
distribution of copepods.
Although the distribution of each species is different in itself, based on the distribution
pattern, the various species can be grouped by their regional dominance as follows:
a) Widely distributed: S. enflata, S. pacifica;
b) Localized areas: S. bedoti, S. bombayensis, S. neglecta, S. robusta;
c) Patchy: K. pacifica, K. subtilis, S. bipunctata, S. hexaptera, S. pulchra;
d) Equatorial: S. ferox;
V. R. NAIR and T. s. s. RAo:
common species in the order of their numerical abundance (Table 1). S. ferox, S. robusta,
S. neglecta and K. pacifica are moderately common, while S. hexaptera, S. pulchra,
S. minima and K. subtilis are occasionally encountered, S. bombayensis is endemic to the
coasts of India but specimens were obtained only from a single collection taken from the
Gulf of Cambay.
The Arabian Sea is in the NW of the monsoon area of the Indian Ocean and its surface
currents and upwelling systems are regulated by the monsoonal regime. The western
Arabian Sea, especially the Somalian and Arabian areas, has the maximum density for all
abundant and frequent species excluding S. ferox and S. minima (Table 1). During the
SW monsoon upwelling occurs along the east coast of Africa and west coast of India.
According to PANIKKAR and ]AYARAMAN (1966), upwelling is prevalent along the coast
between 7 -10 0 N lat. during August to early October. Even during the NE monsoon
period, the western Arabian Sea and coast of India are rich in most of the chaetognath
species. Investigations made during the HOE indicate that the Arabian Sea has a very high
concentration of nutrients (BEZRUKOV, 1961; RYTHER and MENZEL, 1965). The western
side of the Arabian Sea is more productive (LA FOND, 1965; WOOSTER, SCHAEFER
and ROBINSON, 1967) and hence it is no wonder that the chaetognaths, which occupy a
high level in the food web, also have maximum abundance in this part of the Arabian
Sea. The region 40 - 60 0 E between the Equator and northern limit of the ocean is
richest in chaetognaths during both seasons (NAIR, 1969). The HOE data on zooplankton (IOBC, 1968 a, b) suggest that primary production is able to support a good
standing stock of higher organisms in the northern and western part of the Arabian Sea,
with a peak during the SW monsoon upwelling season. The biomass values (IOBC,
1968 a, b) for both monsoon periods show the highest values along Somalia, Arabia and
SW coast of India. However, during the NE monsoon period the areas of highest values
for biomass are located further south than in the SW monsoon period.
In attempting to determine the primary cause or causes of abundance it is important
to compare the distribution with that of zooplankton. Examination of the stomach content
of the chaetognaths in the HOE samples indicates that copepods form the main food
item (see also VARADARAJAN and CHACKO, 1943; THOMSON, 1947; REEVE, 1966).
Chaetognaths feed mainly on calanoid copepods, the commonest forms being Centropages, Eucalanus and Lucicutia, and cyclopoid copepods like Corycaeus, Oithona
and Oncaea. Rarely harpacticoid copepods and other groups like fish larvae, ostracods,
euphausiids and polychaete larvae were found inside the gut. The HOE data on copepod
distribution for the NE and SW monsoon periods show maximum population along
the Somalian coast of Africa extending northwards to the Arabian coast (lOBC, 1970 a).
Lowest population density for both monsoon periods was observed in the central part of
the Arabian Sea. Most of the chaetognath species except S. enflata, S. pacifica and
Pt. draco also have sparse representation in the central part of the Arabian Sea. In general,
the abundance and scarcity of different chaetognath species corresponds to the pattern of
distribution of copepods.
Although the distribution of each species is different in itself, based on the distribution
pattern, the various species can be grouped by their regional dominance as follows:
a) Widely distributed: S. enflata, S. pacifica;
b) Localized areas: S. bedoti, S. bombayensis, S. neglecta, S. robusta;
c) Patchy: K. pacifica, K. subtilis, S. bipunctata, S. hexaptera, S. pulchra;
d) Equatorial: S. ferox;