338
M. SARASWATHY: Distribution of Gaussia in the Upper 200 m in the Indian Ocean
area 20 0 S to 4 0 N along 170 0 E, GUEREDRAT found an equatorial increase of the species
in upwelling areas where there is enrichment of the upper layer. The pattern of distribution
in the Indian Ocean is seen to be different from that noted in the Pacific, the positive
stations being located mostly north of 50 N and south of 4 0 S (Fig. 1).
Our findings on the distribution pattern of the species in the Indian Ocean are not
inconsistent with the observations of SEWELL (1948). There may well be a population of
Gaussia in the southern areas of the Indian Ocean at deeper levels than towards the
north. This speculation is supported by records of the species at 3 stations south of the
Equator, the southernmost at 37
0
59' S (GRICE and HULSEMANN, 1967). Samples at all
these stations were taken with the Isaacs-Kidd mid-water trawl from 350 - 2394 m
depth at the southernmost station. The actual number of specimens collected from these
stations is not given. Further, as quoted by GRICE and HULSEMANN (1967), IVANENKOS
and GUBIN (1960) observed that "in the area between 10° and 16° S, the upper part of
the southward flowing North Indian Deep Water mixes with northward flowing SubAntarctic Intermediate Water and the lower part of the North Indian Deep Water mixes
with the Antarctic Bottom Water. This mixing results in the formation of two water
masses: the South Indian Deep Water (1500-3500 m) which flows southward and
North Indian Bottom Water (a layer of 200-700 m) which flows northward. The North
Indian Deep Water, formed in the northern Arabian Sea, is centered between 400 and
1 500 m north of the Equator and between 800 and 2 000 m south of the Equator". This
supports the view that cold water is encountered nearer the surface in the northern Indian
Ocean than in the southern.
It can be seen from Table 1 that at 2 stations the net had fished only above 200 m
depth, as evidenced by the wire angle; Fig. 2 shows water of 19° C temperature or lower
in the Bay of Bengal at approximately 130 m.
The hydrographic data available for water depths at everyone of the stations from
which G. princeps was collected, indicate the presence of cold water at subsurface levels.
This, considered along with time of collection and the inherent tendency of the species
to diurnal vertical migration, would explain the presence of this bathypelagic species
at depths of only 200 m. Moreover, latitudinal differences in temperature are insignificant
in deep-water layers as compared to the surface; hence bathypelagic fauna commonly
have a more extensive distribution than epigelagic fauna (EKMAN, 1953). Speciation
declines with increase in depth; it is, therefore, of interest that only 1 species of Gaussia
is known from all the oceans.
M. SARASWATHY: Distribution of Gaussia in the Upper 200 m in the Indian Ocean
area 20 0 S to 4 0 N along 170 0 E, GUEREDRAT found an equatorial increase of the species
in upwelling areas where there is enrichment of the upper layer. The pattern of distribution
in the Indian Ocean is seen to be different from that noted in the Pacific, the positive
stations being located mostly north of 50 N and south of 4 0 S (Fig. 1).
Our findings on the distribution pattern of the species in the Indian Ocean are not
inconsistent with the observations of SEWELL (1948). There may well be a population of
Gaussia in the southern areas of the Indian Ocean at deeper levels than towards the
north. This speculation is supported by records of the species at 3 stations south of the
Equator, the southernmost at 37
0
59' S (GRICE and HULSEMANN, 1967). Samples at all
these stations were taken with the Isaacs-Kidd mid-water trawl from 350 - 2394 m
depth at the southernmost station. The actual number of specimens collected from these
stations is not given. Further, as quoted by GRICE and HULSEMANN (1967), IVANENKOS
and GUBIN (1960) observed that "in the area between 10° and 16° S, the upper part of
the southward flowing North Indian Deep Water mixes with northward flowing SubAntarctic Intermediate Water and the lower part of the North Indian Deep Water mixes
with the Antarctic Bottom Water. This mixing results in the formation of two water
masses: the South Indian Deep Water (1500-3500 m) which flows southward and
North Indian Bottom Water (a layer of 200-700 m) which flows northward. The North
Indian Deep Water, formed in the northern Arabian Sea, is centered between 400 and
1 500 m north of the Equator and between 800 and 2 000 m south of the Equator". This
supports the view that cold water is encountered nearer the surface in the northern Indian
Ocean than in the southern.
It can be seen from Table 1 that at 2 stations the net had fished only above 200 m
depth, as evidenced by the wire angle; Fig. 2 shows water of 19° C temperature or lower
in the Bay of Bengal at approximately 130 m.
The hydrographic data available for water depths at everyone of the stations from
which G. princeps was collected, indicate the presence of cold water at subsurface levels.
This, considered along with time of collection and the inherent tendency of the species
to diurnal vertical migration, would explain the presence of this bathypelagic species
at depths of only 200 m. Moreover, latitudinal differences in temperature are insignificant
in deep-water layers as compared to the surface; hence bathypelagic fauna commonly
have a more extensive distribution than epigelagic fauna (EKMAN, 1953). Speciation
declines with increase in depth; it is, therefore, of interest that only 1 species of Gaussia
is known from all the oceans.