4.5
Meroplanktonic Stages of Anthozoa in the Indian Ocean
T. BALACHANDRAN
The international collection of 1927 zooplankton samples lodged at the Indian
Ocean Biological Centre (IOBC) forms the basis for the present study. The geographical
names used in the present report follow those adopted by the International Hydrographic
Bureau (IHB) (Sp. Pub!. 23, 1953). An aliquot of the catch, equalling about 3 to 4 ml
of the plankton irrespective of sample size, was fractionated either with a Lea's fractiona tor (WIBORG, 1955) or with a Folsom plankton splitter (Mc EVEN, JOHNSON and
FOLSON, 1954) and sorted. From this aliquot the catch per unit haul is estimated and
this is used for comparison of collections. Abundance indicates population density, the
term population being used not in the ecological sense but in a statistical sense. Biomass
is used here to mean wet zooplankton displacement volume.
I. Salient Features in the Distribution of Anthozoa Larvae
Numerical abundance - When compared to the copepods that constitute roughly 70%
of the zooplankton, the percentage of Anthozoa larvae is negligible. From Table 1 it can
be seen that 61 % of standard samples and 33% of nonstandard samples contained
anthozoan larvae.
Out of a possible total of 220 5 ° squares, collections are available from 170; of these,
39 squares (22% of total squares) were devoid of larvae; 79 squares (47%) contained
only 1 to 3 larvae; 29 squares (17%) contained 4 to 6 larvae; 16 squares (10%) contained
7 to 9 larvae; 4 squares contained 10 to 12 larvae; one square contained 12 to 25 larvae
and only 2 squares had more than 18 larvae. This extensive dispersal of anthozoan
larvae by currents indicates a long life span or extended larval life for most species.
Numbers ranged from 0 to 190 per hau!. The maximum average representation in a
5° square is 20, and that of a 10° zone of latitude 16. Of the 1065 samples which
contained anthozoan larvae, only 71 samples had more than 20. The 71 samples had
2858 larvae, i.e. 40 per unit sample, while the other 994 samples had 4891 larvae, hence
an average of 5 per sample. Most of the samples had about the same number of larvae.
Areas of greater larval abundance are (Fig. 1) a) along the West Australian coast.
The sampling in this area was done along the longitude 11 0° E from 12° S to 32° S.
Night hauls show a slightly bigger catch; b) between the Red Sea and the Gulf
of Aden and off the Arabian coast; c) in the central and western Bay of Bengal, to the
northwest and south of the Andaman and Nicobar Islands; d) along the Mozambique
coast, east of 30° E and 30° S; e) the Malabar coast, the Laccadive Sea and SW of the
Maldive Islands; f) the coast of Kenya and Somalia, 5° S to r N.
In addition, isolated places in the north and central Indian Ocean, such as the SW
coast of Sumatra, the southern tip of India, the northern and SW coasts of Ceylon, off
Visakhapatanam 13 ° Nand 80° E, and around the St. Brandon group of islands.
Meroplanktonic Stages of Anthozoa in the Indian Ocean
T. BALACHANDRAN
The international collection of 1927 zooplankton samples lodged at the Indian
Ocean Biological Centre (IOBC) forms the basis for the present study. The geographical
names used in the present report follow those adopted by the International Hydrographic
Bureau (IHB) (Sp. Pub!. 23, 1953). An aliquot of the catch, equalling about 3 to 4 ml
of the plankton irrespective of sample size, was fractionated either with a Lea's fractiona tor (WIBORG, 1955) or with a Folsom plankton splitter (Mc EVEN, JOHNSON and
FOLSON, 1954) and sorted. From this aliquot the catch per unit haul is estimated and
this is used for comparison of collections. Abundance indicates population density, the
term population being used not in the ecological sense but in a statistical sense. Biomass
is used here to mean wet zooplankton displacement volume.
I. Salient Features in the Distribution of Anthozoa Larvae
Numerical abundance - When compared to the copepods that constitute roughly 70%
of the zooplankton, the percentage of Anthozoa larvae is negligible. From Table 1 it can
be seen that 61 % of standard samples and 33% of nonstandard samples contained
anthozoan larvae.
Out of a possible total of 220 5 ° squares, collections are available from 170; of these,
39 squares (22% of total squares) were devoid of larvae; 79 squares (47%) contained
only 1 to 3 larvae; 29 squares (17%) contained 4 to 6 larvae; 16 squares (10%) contained
7 to 9 larvae; 4 squares contained 10 to 12 larvae; one square contained 12 to 25 larvae
and only 2 squares had more than 18 larvae. This extensive dispersal of anthozoan
larvae by currents indicates a long life span or extended larval life for most species.
Numbers ranged from 0 to 190 per hau!. The maximum average representation in a
5° square is 20, and that of a 10° zone of latitude 16. Of the 1065 samples which
contained anthozoan larvae, only 71 samples had more than 20. The 71 samples had
2858 larvae, i.e. 40 per unit sample, while the other 994 samples had 4891 larvae, hence
an average of 5 per sample. Most of the samples had about the same number of larvae.
Areas of greater larval abundance are (Fig. 1) a) along the West Australian coast.
The sampling in this area was done along the longitude 11 0° E from 12° S to 32° S.
Night hauls show a slightly bigger catch; b) between the Red Sea and the Gulf
of Aden and off the Arabian coast; c) in the central and western Bay of Bengal, to the
northwest and south of the Andaman and Nicobar Islands; d) along the Mozambique
coast, east of 30° E and 30° S; e) the Malabar coast, the Laccadive Sea and SW of the
Maldive Islands; f) the coast of Kenya and Somalia, 5° S to r N.
In addition, isolated places in the north and central Indian Ocean, such as the SW
coast of Sumatra, the southern tip of India, the northern and SW coasts of Ceylon, off
Visakhapatanam 13 ° Nand 80° E, and around the St. Brandon group of islands.