4.14
Studies on Limacina inflata d'Orbigny (Thecosomata,
Gastropoda) in the Indian Ocean
M. SAKTHIVEL
This paper deals with the seasonal and diurnal variations in spatial abundance, the
annual variations along 78 0 E, latitudinal and seasonal variations along 110 0 E and
also the distribution in relation to the thermocline. This study is based on the identification
of the sorted fractions of Euthecosomata from 1927 zooplankton collections of the HOE
and 180 collections of the seasonal biological cruises of the Division of Fisheries and
Oceanography CSIRO Australia. Because of remarkable diurnal changes of Limacina
inflata only night collections were taken into consideration for studying seasonal
variations.
I. Present Observation on Geographical Abundance
Out of 1927 samples, Limacina inflata was present in 1040, of which 438 were
day hauls, and 602 night hauls. The day distribution (Fig. 1) shows that a large number
of stations in the central water mass of the southern Indian Ocean and the areas which
fall away from the zones of upwelling and divergence either caught none of this species
or very few « 10 per haul). This species has been caught in moderate numbers in the
areas where coastal upwelling or divergence is reported, for example, the coasts of Kenya
and Somalia, the Gulf of Aden, the south Arabian coast, the SW coast of India, a part
of the western Bay of Bengal, near Andaman, off Sumatra, off Java and in isolated areas
in the equatorial zone. The areas of greater abundance are restricted to small areas off
the Kenya and Somali coast, off Goa, off Cochin and off Madras. The night distribution
(Fig. 2) shows that the range B (10-100 per haul) covers about 3 / 4 of the area of the
Indian Ocean, whereas the same range of abundance was found mainly in the North
Indian Ocean during the day. Except in a few coastal areas (West Australian coast,
Malaya and Burmese coast, NE coast of India) and a large area off South Africa, Gulf of
Oman, the eastern Bay of Bengal, east of Ceylon, a few areas in the equatorial zone and
the area near West Java, Limacina inflata is more common at night in the entire area
of the Indian Ocean. The area of greater abundance (range C, 100 -1 000 per haul
Fig. 2) is comparatively many times larger than during the day; it covers almost the entire
western half of the Indian Ocean from Madagascar to the south Arabian coast, off Goa,
off Cochin, a meridional zone along 78
0
E, specified areas in the equatorial zone, off
Madras, a part of the northern Bay of Bengal, Andaman Sea and off Java. The areas
of highest abundance (range D, > 1 000 per haul) were noted in the form of patches off
the Somali coast, near Socotra, off Goa, off Madras and near the Equator at 82
0
E. The
average number of organisms in the day hauls is 39 and night hauls 105.
The distinct variation in spatial abundance of Limacina inflata clearly indicates that
Studies on Limacina inflata d'Orbigny (Thecosomata,
Gastropoda) in the Indian Ocean
M. SAKTHIVEL
This paper deals with the seasonal and diurnal variations in spatial abundance, the
annual variations along 78 0 E, latitudinal and seasonal variations along 110 0 E and
also the distribution in relation to the thermocline. This study is based on the identification
of the sorted fractions of Euthecosomata from 1927 zooplankton collections of the HOE
and 180 collections of the seasonal biological cruises of the Division of Fisheries and
Oceanography CSIRO Australia. Because of remarkable diurnal changes of Limacina
inflata only night collections were taken into consideration for studying seasonal
variations.
I. Present Observation on Geographical Abundance
Out of 1927 samples, Limacina inflata was present in 1040, of which 438 were
day hauls, and 602 night hauls. The day distribution (Fig. 1) shows that a large number
of stations in the central water mass of the southern Indian Ocean and the areas which
fall away from the zones of upwelling and divergence either caught none of this species
or very few « 10 per haul). This species has been caught in moderate numbers in the
areas where coastal upwelling or divergence is reported, for example, the coasts of Kenya
and Somalia, the Gulf of Aden, the south Arabian coast, the SW coast of India, a part
of the western Bay of Bengal, near Andaman, off Sumatra, off Java and in isolated areas
in the equatorial zone. The areas of greater abundance are restricted to small areas off
the Kenya and Somali coast, off Goa, off Cochin and off Madras. The night distribution
(Fig. 2) shows that the range B (10-100 per haul) covers about 3 / 4 of the area of the
Indian Ocean, whereas the same range of abundance was found mainly in the North
Indian Ocean during the day. Except in a few coastal areas (West Australian coast,
Malaya and Burmese coast, NE coast of India) and a large area off South Africa, Gulf of
Oman, the eastern Bay of Bengal, east of Ceylon, a few areas in the equatorial zone and
the area near West Java, Limacina inflata is more common at night in the entire area
of the Indian Ocean. The area of greater abundance (range C, 100 -1 000 per haul
Fig. 2) is comparatively many times larger than during the day; it covers almost the entire
western half of the Indian Ocean from Madagascar to the south Arabian coast, off Goa,
off Cochin, a meridional zone along 78
0
E, specified areas in the equatorial zone, off
Madras, a part of the northern Bay of Bengal, Andaman Sea and off Java. The areas
of highest abundance (range D, > 1 000 per haul) were noted in the form of patches off
the Somali coast, near Socotra, off Goa, off Madras and near the Equator at 82
0
E. The
average number of organisms in the day hauls is 39 and night hauls 105.
The distinct variation in spatial abundance of Limacina inflata clearly indicates that