404
M. SAKTHlVEL: Biogeographical Change in the Latitudinal Boundary
equatorial zone. As the Pacific and Atlantic provide 2 subtropical zones one on either
side of the Equator, S. subula could easily have migrated from the warmer zone of the
Equator to the cooler zone of the subtropical latitudes. This is almost evident from the
paucity in the distribution of S. subula in the equatorial Pacific and Atlantic. In the Indian
Ocean, for lack of a subtropical zone in the northern hemisphere, this species can migrate
only towards the subtropical zone of the South Indian Ocean. This may account for the
persistence of S. subula in the equatorial zone.
A similar phenomenon has been observed in the distribution of euphausiids in the
Indian Ocean. Three species of euphausiids, Euphausia brevis, E. mutica and E. recurva,
have centres of distribution in the subtropical latitudes of the North and South Pacific but
are absent in the equatorial water mass. BRINTON (1962) has called this type of distribution biantitropical. BAKER (1965) says that E. brevis and probably E. mutica have a
continuous distribution from the subtropics to the southern Bay of Bengal, while E. recurva has a northerly restricted distribution in the partially enclosed Indian Ocean.
From the present study it is difficult to explain the tropical submergence of S. subula
in the northern part of the Indian Ocean as only 4 deep hauls (400 m to 880 m) were
available. Tropical submergence is normally reported for a cold-water surface-living
form inhabiting the deeper zone in the tropical latitudes (e.g. Eukrohnia hamata). Here
the temperature forms the major factor of distribution. As S. subula is mainly a sur£aceliving form, tolerating even the high temperature of the equatorial zone in the Indian
Ocean, tropical submergence based on temperature would hardly apply to. this species.
M. SAKTHlVEL: Biogeographical Change in the Latitudinal Boundary
equatorial zone. As the Pacific and Atlantic provide 2 subtropical zones one on either
side of the Equator, S. subula could easily have migrated from the warmer zone of the
Equator to the cooler zone of the subtropical latitudes. This is almost evident from the
paucity in the distribution of S. subula in the equatorial Pacific and Atlantic. In the Indian
Ocean, for lack of a subtropical zone in the northern hemisphere, this species can migrate
only towards the subtropical zone of the South Indian Ocean. This may account for the
persistence of S. subula in the equatorial zone.
A similar phenomenon has been observed in the distribution of euphausiids in the
Indian Ocean. Three species of euphausiids, Euphausia brevis, E. mutica and E. recurva,
have centres of distribution in the subtropical latitudes of the North and South Pacific but
are absent in the equatorial water mass. BRINTON (1962) has called this type of distribution biantitropical. BAKER (1965) says that E. brevis and probably E. mutica have a
continuous distribution from the subtropics to the southern Bay of Bengal, while E. recurva has a northerly restricted distribution in the partially enclosed Indian Ocean.
From the present study it is difficult to explain the tropical submergence of S. subula
in the northern part of the Indian Ocean as only 4 deep hauls (400 m to 880 m) were
available. Tropical submergence is normally reported for a cold-water surface-living
form inhabiting the deeper zone in the tropical latitudes (e.g. Eukrohnia hamata). Here
the temperature forms the major factor of distribution. As S. subula is mainly a sur£aceliving form, tolerating even the high temperature of the equatorial zone in the Indian
Ocean, tropical submergence based on temperature would hardly apply to. this species.