General Features of Dinoflagellate Material Collected by the "Anton Bruun"
163
SUKHANOVA (1962a, b; 1964) has recognized the following group of species:
Pyrocystis pseudonoctiluca, Ceratium carriense, C. massiliense and C. trichoceros, as the
"basic tropical complex", being the chief tropical component within the tropical IndoWest Pacific region. The present study supports the fact that these are certainly among the
commonest species over most of the area studied. However, it should be noted that the
"complex" contains dissimilar elements, although P. pseudonoctiluca (Fig. 4d) is
exceptionally widespread and often abundant (Gulf of Aden, southern India), the other
species represent differing preferences. The "Anton Bruun" material tended to support
the views of STEEMANN NIELSEN (1934, 1939) and GRAHAM (1941) that of the 3 species
of Ceratium mentioned C. trichoceros (Fig. 4a) is the most highly stenothermal, being
incidentally, one of the very few species to show even moderate concentrations in the
oligotrophic South Equatorial Current water. C. carriense is slightly more thermotolerant,
and C. massiliense is not only thermotolerant but the var. macroceroides (Fig. 4 b)
dominates in warm temperate waters such as the SW Indian Ocean off South Africa
(TAYLOR, MS 1964) and in Australian waters (WOOD, 1964). The variety armatum is a
cold temperate to subantarctic variety. Consequently it can be seen that although these
species are common in the material as a whole, they represent different preferences and
degrees of tolerance. In fact, one could add other species which are also common and
match the type within the "complex". For example, Heteraulucus (= Goniodoma)
polyedricum is almost as widespread as Pyrocystis pseudonoctiluca, and Ceratocorys
horrida (Fig. 4c) is as stenothermal and common as Ceratium trichoceros.
A cautionary note may be added regarding the genus Pyrocystis. It has become
evident that this has been a "form genus", being a life-cycle stage of taxonomically
diverse dinoflagellate species. Thus although P. lunula has a non-thecate motile stage
which could be and has been assigned to Gymnodinium (KOFOID and SWEZY, 1921),
cysts resembling P. pseudonoctiluca and P. fusiformis give rise to Gonyaulax motiles
(BALECH, 1962; KOFOID and MICHENER in TAYLOR, 1972b; this material.). Further, as
there is so little morphology in the cysts on which to base distinctions (size seems highly
variable and can exist in 2 size-stages in the life cycle) there is a strong possibility that cysts
termed P. pseudonoctiluca may involve several species. For example, DOGIEL (1906)
showed that the lunate-cyst species P. lunula also has a spherical stage. TAYLOR (1972b)
has suggested that species with Gonyaulax-like stages be assigned to Pyrocystis, and
others with gymnodinoid stages be assigned to Dissodinium.
The southern boundary of most of the species spread throughout the tropical Indian
Ocean is situated close to 30° S where there is an abrupt depletion of dinoflagellates,
accompanied by a general sparseness of all protoplankton except for the coccolithophorids. One exception to this is in the southwest where the Agulhas Current represents a
southward extension of trophical and subtropical water carrying within its core many
tropical species (TAYLOR, 1967; NEL, 1968). The 18° C surface isotherm can be used as a
rough approximation of the southern distributional boundary, lying near the northern
edge of the Subtropical Convergence region. By comparing the position of the 18 ° and
13 ° C surface isotherms on the two meridians of cruise V (55° and 75° E), occupied during
the southern summer with the other southern meridional stations, it can be seen that the
distance between them varied seasonally and that the 18 ° C isotherm was situated furthest
south during the southern summer (NE monsoon) at approximately 36° S, whereas it was
close to 30° S in the southern winter (SW monsoon). It can be noted in passing that
SUKHANOVA (unpub\. MS) found that the so-called "tropical convergence" also moved
163
SUKHANOVA (1962a, b; 1964) has recognized the following group of species:
Pyrocystis pseudonoctiluca, Ceratium carriense, C. massiliense and C. trichoceros, as the
"basic tropical complex", being the chief tropical component within the tropical IndoWest Pacific region. The present study supports the fact that these are certainly among the
commonest species over most of the area studied. However, it should be noted that the
"complex" contains dissimilar elements, although P. pseudonoctiluca (Fig. 4d) is
exceptionally widespread and often abundant (Gulf of Aden, southern India), the other
species represent differing preferences. The "Anton Bruun" material tended to support
the views of STEEMANN NIELSEN (1934, 1939) and GRAHAM (1941) that of the 3 species
of Ceratium mentioned C. trichoceros (Fig. 4a) is the most highly stenothermal, being
incidentally, one of the very few species to show even moderate concentrations in the
oligotrophic South Equatorial Current water. C. carriense is slightly more thermotolerant,
and C. massiliense is not only thermotolerant but the var. macroceroides (Fig. 4 b)
dominates in warm temperate waters such as the SW Indian Ocean off South Africa
(TAYLOR, MS 1964) and in Australian waters (WOOD, 1964). The variety armatum is a
cold temperate to subantarctic variety. Consequently it can be seen that although these
species are common in the material as a whole, they represent different preferences and
degrees of tolerance. In fact, one could add other species which are also common and
match the type within the "complex". For example, Heteraulucus (= Goniodoma)
polyedricum is almost as widespread as Pyrocystis pseudonoctiluca, and Ceratocorys
horrida (Fig. 4c) is as stenothermal and common as Ceratium trichoceros.
A cautionary note may be added regarding the genus Pyrocystis. It has become
evident that this has been a "form genus", being a life-cycle stage of taxonomically
diverse dinoflagellate species. Thus although P. lunula has a non-thecate motile stage
which could be and has been assigned to Gymnodinium (KOFOID and SWEZY, 1921),
cysts resembling P. pseudonoctiluca and P. fusiformis give rise to Gonyaulax motiles
(BALECH, 1962; KOFOID and MICHENER in TAYLOR, 1972b; this material.). Further, as
there is so little morphology in the cysts on which to base distinctions (size seems highly
variable and can exist in 2 size-stages in the life cycle) there is a strong possibility that cysts
termed P. pseudonoctiluca may involve several species. For example, DOGIEL (1906)
showed that the lunate-cyst species P. lunula also has a spherical stage. TAYLOR (1972b)
has suggested that species with Gonyaulax-like stages be assigned to Pyrocystis, and
others with gymnodinoid stages be assigned to Dissodinium.
The southern boundary of most of the species spread throughout the tropical Indian
Ocean is situated close to 30° S where there is an abrupt depletion of dinoflagellates,
accompanied by a general sparseness of all protoplankton except for the coccolithophorids. One exception to this is in the southwest where the Agulhas Current represents a
southward extension of trophical and subtropical water carrying within its core many
tropical species (TAYLOR, 1967; NEL, 1968). The 18° C surface isotherm can be used as a
rough approximation of the southern distributional boundary, lying near the northern
edge of the Subtropical Convergence region. By comparing the position of the 18 ° and
13 ° C surface isotherms on the two meridians of cruise V (55° and 75° E), occupied during
the southern summer with the other southern meridional stations, it can be seen that the
distance between them varied seasonally and that the 18 ° C isotherm was situated furthest
south during the southern summer (NE monsoon) at approximately 36° S, whereas it was
close to 30° S in the southern winter (SW monsoon). It can be noted in passing that
SUKHANOVA (unpub\. MS) found that the so-called "tropical convergence" also moved