316
V. R. NAIR and T. S. S. RAo:
bution of chaetognaths in the Indo-West-Pacific. Thus the similarity of the fauna can
naturally be expected.
There is a distinct difference in the distribution of chaetognaths in the Arabian Sea
and the Red Sea. There are 2 samples available from the Red Sea, 5 from the Bab-e1Mandeb and 12 from the Gulf of Aden. All the epiplanktonic forms (excluding S. bombayensis) recorded from the Arabian Sea are found in the Gulf of Aden. S. enflata,
S. bedoti, S. robusta and Pt. draco are the common forms in these samples. At Bab-e1Mandeb all the species found in the Gulf of Aden (except K. subtilis) are encountered.
In these samples S. enflata, S. regularis, S. bedoti and K. pacifica are the common species
and S. hexaptera, S. pulchra and S. ferox are sparse. In the Red Sea samples S. enflata,
S. pacifica, S. bipunctata, S. regularis, S. neglecta, S. ferox, S. heraptera, and S. bedoti
are found. These collections from the Gulf of Aden and Red Sea are comparable since
they cover the NE monsoon period, except the one located towards the northernmost
part. Along with the reduction in specific number, there is a general lowering of population density of the different species of chaetognaths from the Gulf of Aden into the Red
Sea. The hydrographic conditions prevailing in the Red Sea are high salinity (36.6640.500/00) and high temperature (21.96 - 29.53 0 C) down to 200 m depth, and thus may
not be congenial for all the species which occur in the Arabian Sea and Gulf of Aden.
FURNESllN and BALANc;A (1968) have commented on the sporadic transportation of
chaetognaths from the Gulf of Aden into the southern section of the Red Sea. STUBBINGS
(1938) has reported mass mortality of pteropods entering the Red Sea from the Arabian
Sea. The hot, highly saline waters of the Red Sea, and perhaps other unfavourable physical
and chemical factors, are a barrier to the survival of many chaetognath species. The
species which are able to withstand this peculiar environment might have maintained
the population due to lack of competition from other species and availability of abundant
food, as is evidenced by the copepod distribution in the area (lOBC, 1970 a).
All the samples from which mesoplanktonic and bathyplanktonic chaetognaths were
taken belong to the upper 200 m. The 0 - 200 m water column includes the Arabian Sea
surface water (ASSW), extending from the surface to 100 -150 m depth, and the Arabian
Sea subsurface water (ASSSW) extending from a depth below the ASSW. At "Varuna"
station 2009, where E. fowleri is found, the surface water is diluted by run-off from the
Cochin backwater system down to 30 - 40 m depth and O2 content is high down to
200 m depth. ASSSW has an unusually high dissolved O 2 content.
According to GALLAGHER (1966) the ASSSW has a temperature range of 10 -19 0 C,
salinity varies from 35.0-36.0%0 and O2 content from 0.21-2.09 mll-l. The lowest
level in the column sampled in the present collection had temperatures 13.5 -16.5 0 C.
These features may explain the presence of species described as meso- and bathyplanktonic
at depths as low as 200 m.
At the stations from which S. decipiens (station AB5 only), S. lyra and S. zetesios
were taken, upwelled waters were present; however, since these species were not taken
when and where upwelling was at its peak intensity, or in upwelled waters only, other
factors must have been involved. Temperatures at 40 m at stations 170 and 190 were
22.88
0
C and 23.41
0
C, respectively. At the station at which S. decipiens occurred,
temperature was low even at the surface (25.32
0
C). Previous records of these species from
the Indian Ocean are from depths below 200 m except for S. lyra (Table 4) which has
already been reported from the upper 200 m (TOKIOKA, 1956; TSURUTA, 1963). E. fowleri
is considered to be bathyplanktonic and BURFIELD and HARVEY (1926) found it mainly
V. R. NAIR and T. S. S. RAo:
bution of chaetognaths in the Indo-West-Pacific. Thus the similarity of the fauna can
naturally be expected.
There is a distinct difference in the distribution of chaetognaths in the Arabian Sea
and the Red Sea. There are 2 samples available from the Red Sea, 5 from the Bab-e1Mandeb and 12 from the Gulf of Aden. All the epiplanktonic forms (excluding S. bombayensis) recorded from the Arabian Sea are found in the Gulf of Aden. S. enflata,
S. bedoti, S. robusta and Pt. draco are the common forms in these samples. At Bab-e1Mandeb all the species found in the Gulf of Aden (except K. subtilis) are encountered.
In these samples S. enflata, S. regularis, S. bedoti and K. pacifica are the common species
and S. hexaptera, S. pulchra and S. ferox are sparse. In the Red Sea samples S. enflata,
S. pacifica, S. bipunctata, S. regularis, S. neglecta, S. ferox, S. heraptera, and S. bedoti
are found. These collections from the Gulf of Aden and Red Sea are comparable since
they cover the NE monsoon period, except the one located towards the northernmost
part. Along with the reduction in specific number, there is a general lowering of population density of the different species of chaetognaths from the Gulf of Aden into the Red
Sea. The hydrographic conditions prevailing in the Red Sea are high salinity (36.6640.500/00) and high temperature (21.96 - 29.53 0 C) down to 200 m depth, and thus may
not be congenial for all the species which occur in the Arabian Sea and Gulf of Aden.
FURNESllN and BALANc;A (1968) have commented on the sporadic transportation of
chaetognaths from the Gulf of Aden into the southern section of the Red Sea. STUBBINGS
(1938) has reported mass mortality of pteropods entering the Red Sea from the Arabian
Sea. The hot, highly saline waters of the Red Sea, and perhaps other unfavourable physical
and chemical factors, are a barrier to the survival of many chaetognath species. The
species which are able to withstand this peculiar environment might have maintained
the population due to lack of competition from other species and availability of abundant
food, as is evidenced by the copepod distribution in the area (lOBC, 1970 a).
All the samples from which mesoplanktonic and bathyplanktonic chaetognaths were
taken belong to the upper 200 m. The 0 - 200 m water column includes the Arabian Sea
surface water (ASSW), extending from the surface to 100 -150 m depth, and the Arabian
Sea subsurface water (ASSSW) extending from a depth below the ASSW. At "Varuna"
station 2009, where E. fowleri is found, the surface water is diluted by run-off from the
Cochin backwater system down to 30 - 40 m depth and O2 content is high down to
200 m depth. ASSSW has an unusually high dissolved O 2 content.
According to GALLAGHER (1966) the ASSSW has a temperature range of 10 -19 0 C,
salinity varies from 35.0-36.0%0 and O2 content from 0.21-2.09 mll-l. The lowest
level in the column sampled in the present collection had temperatures 13.5 -16.5 0 C.
These features may explain the presence of species described as meso- and bathyplanktonic
at depths as low as 200 m.
At the stations from which S. decipiens (station AB5 only), S. lyra and S. zetesios
were taken, upwelled waters were present; however, since these species were not taken
when and where upwelling was at its peak intensity, or in upwelled waters only, other
factors must have been involved. Temperatures at 40 m at stations 170 and 190 were
22.88
0
C and 23.41
0
C, respectively. At the station at which S. decipiens occurred,
temperature was low even at the surface (25.32
0
C). Previous records of these species from
the Indian Ocean are from depths below 200 m except for S. lyra (Table 4) which has
already been reported from the upper 200 m (TOKIOKA, 1956; TSURUTA, 1963). E. fowleri
is considered to be bathyplanktonic and BURFIELD and HARVEY (1926) found it mainly