386
M. SAKTHIVEL:
it undergoes vertical migration and avoids surface waters during the day. The mean
level of this species might be around 200 m in the areas of coastal upwelling and divergence where, in general, light penetration is less. In dear waters where light penetrates
deeply, this species lives mainly below 200 m. This is evident from its occurrence in the
night hours in the central water mass of the South Indian Ocean where it is absent in the
daytime. The ascending migration at night is for the purpose of feeding on phytoplankton, generally most abundant at 75 m (HUMPHREY and KERR, 1969). The areas
of greater abundance of Limacina inflata are approximately the areas of the vertical
extension of monsoonal effects, as reported by DOING (1970). As this result almost
coincides with the similar finding of MCGOWAN (1960) in the North Pacific, Limacina
inflata could be considered an indicator of environments where the nutrient-rich, deep
water is brought up, in the form of upwelling, divergence or other vertical mixing
processes.
II. Seasonal Variation in Spatial Abundance
From mid-April to mid-October, a large area in the western Indian Ocean between
Madagascar and the south Arabian coast and a large area in the eastern Indian Ocean,
especially the western Bay of Bengal, the Andaman Sea, the areas around Ceylon, off
Sumatra and the areas between Java and Australia, show abundance (range B) of Limacina
inflata (Fig. 3). The areas of greater abundance (range C) are found off Kenya, NE of
Madagascar, the Somali coast, the south Arabian coast.
The highest abundance (range D) was seen only in the area off the Somali coast.
A large area off South Africa, part of the central and eastern Bay of Bengal including
the Malacca Strait, a few areas in the equatorial zone and a large area in the southern
Indian Ocean are poorly populated. From mid-October to mid-April (Fig. 4), the areas
of abundance (range B) are seen off Kenya and Somali coast, the Gulf of Aden, the west
coast of India, a meridional zone along 78
0
E, the areas around Ceylon, a part of western
Bay of Bengal, a few areas in the equatorial zone and the NW Australian Sea. The areas
of greater abundance (range C) are found in the areas off the Kenya and Somali coast,
near Socotra, the Gulf of Aden, off Kathiawar, the SW coast of India, a meridional zone
along 78
0
E, off Madras and off Java. The highest abundance (range D) is noted in the
area near Socotra, off Goa, off Madras, and a patch near the Equator at 82 0 E. The area
off South Africa, the Gulf of Oman, the central and southern Arabian Sea, the eastern
Bay of Bengal, the west coast of Australia and a large area in the western half of the
Indian Ocean are poorly populated by Limacina inflata. The fact that the period from
mid-April to mid-October which is dominated by the SW monsoon shows greater
abundance than that from mid-October to mid-April; that might be due to the different
intensities and duration of the SW and NE monsoons which bring about a number of
changes in the properties of the environment according to the vertical extension.
The finding of a large number of well-preserved empty shells of Limacina inflata
off the eastern part of the Somali coast (Table 1 and Fig. 2) could indicate mass mortality
of this species, due to the toxic effects of phytoplankton blooms or other unknown
environmental factors. The subsurface O2 minimum is also recorded.
M. SAKTHIVEL:
it undergoes vertical migration and avoids surface waters during the day. The mean
level of this species might be around 200 m in the areas of coastal upwelling and divergence where, in general, light penetration is less. In dear waters where light penetrates
deeply, this species lives mainly below 200 m. This is evident from its occurrence in the
night hours in the central water mass of the South Indian Ocean where it is absent in the
daytime. The ascending migration at night is for the purpose of feeding on phytoplankton, generally most abundant at 75 m (HUMPHREY and KERR, 1969). The areas
of greater abundance of Limacina inflata are approximately the areas of the vertical
extension of monsoonal effects, as reported by DOING (1970). As this result almost
coincides with the similar finding of MCGOWAN (1960) in the North Pacific, Limacina
inflata could be considered an indicator of environments where the nutrient-rich, deep
water is brought up, in the form of upwelling, divergence or other vertical mixing
processes.
II. Seasonal Variation in Spatial Abundance
From mid-April to mid-October, a large area in the western Indian Ocean between
Madagascar and the south Arabian coast and a large area in the eastern Indian Ocean,
especially the western Bay of Bengal, the Andaman Sea, the areas around Ceylon, off
Sumatra and the areas between Java and Australia, show abundance (range B) of Limacina
inflata (Fig. 3). The areas of greater abundance (range C) are found off Kenya, NE of
Madagascar, the Somali coast, the south Arabian coast.
The highest abundance (range D) was seen only in the area off the Somali coast.
A large area off South Africa, part of the central and eastern Bay of Bengal including
the Malacca Strait, a few areas in the equatorial zone and a large area in the southern
Indian Ocean are poorly populated. From mid-October to mid-April (Fig. 4), the areas
of abundance (range B) are seen off Kenya and Somali coast, the Gulf of Aden, the west
coast of India, a meridional zone along 78
0
E, the areas around Ceylon, a part of western
Bay of Bengal, a few areas in the equatorial zone and the NW Australian Sea. The areas
of greater abundance (range C) are found in the areas off the Kenya and Somali coast,
near Socotra, the Gulf of Aden, off Kathiawar, the SW coast of India, a meridional zone
along 78
0
E, off Madras and off Java. The highest abundance (range D) is noted in the
area near Socotra, off Goa, off Madras, and a patch near the Equator at 82 0 E. The area
off South Africa, the Gulf of Oman, the central and southern Arabian Sea, the eastern
Bay of Bengal, the west coast of Australia and a large area in the western half of the
Indian Ocean are poorly populated by Limacina inflata. The fact that the period from
mid-April to mid-October which is dominated by the SW monsoon shows greater
abundance than that from mid-October to mid-April; that might be due to the different
intensities and duration of the SW and NE monsoons which bring about a number of
changes in the properties of the environment according to the vertical extension.
The finding of a large number of well-preserved empty shells of Limacina inflata
off the eastern part of the Somali coast (Table 1 and Fig. 2) could indicate mass mortality
of this species, due to the toxic effects of phytoplankton blooms or other unknown
environmental factors. The subsurface O2 minimum is also recorded.