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P. N. ARAVINDAKSHAN: Distribution and Ecology of Pterotrachea coronata (FORSKAL)
The largest specimen measured only 104 mm and the smallest 7 mm in length. The
Indian Ocean specimens were smaller than those recorded elsewhere.
I. Food Habits
Gut contents of 21 specimens from 21 different stations were examined. The principal
food includes copepods, chaetognaths and ostracods. Inside the oesophagus of a 73 mm
female Pt. coronata numerous copepods of fairly large size (4 mm) were observed. In
another 13 mm specimen, a 5 mm copepod Rhincalanus cornutus was noted. In all
cases examined the species was exclusively carnivorous and had a varied diet in accordance with the relative abundance of the food species available.
II. Distribution
The distribution of Pt. coronata in the Arabian Sea, the Andaman Sea and the
western Indian Ocean was discussed in a preliminary study (ARAVINDAKSHAN, 1969).
The present observation is that the species was present in only 49 samples of the 1927
examined from the epipelagic zone, with a total number of 63 specimens. Fig. 1 shows
the total number of stations where Pt. coronata occured. It was absent in the eastern
Arabian Sea, in the northern and western parts of the Bay of Bengal and in the central zone
of the southern Indian Ocean. It was common around the mouth of the Gulf of Aden, in
the eastern African waters, in the eastern part of the Andaman Sea, and off the NW of
Australia.
The species were caught at 38 out of 920 night stations and 11 out of 1001 day
stations. Therefore, vertical diurnal migration is usual in this species.
Pt. coronata was found in almost all the water masses of the upper layer of the
Indian Ocean at different seasons of the year. Comparing the stations where the net
sampled a single water mass, it is seen that the species must be euryhaline and eurythermal;
the range of tolerance appears to be 33 -40%0 and 12-30° C respectively. It seems likely
that the species requires a relatively high O2 content since it was not taken in waters
with less than 2.0 mll- 1 dissolved oxygen.
Most specimens come from stations where the net fished through different layers,
whether the boundary between the two watermasses was shallow or deep, whether a
thermocline was present or not. This suggests that the species lives mostly at the boundary
between surface and subsurface water zooplankton aggregates as a deep scattering
layer. A similar distribution was found by NAVAS (1969) for a hydromedusa, Halistaura
bruuni, that is also a zooplankton fceder. Therefore, although Pt. coronata has been
described as a warm-water epipelagic species, the present data suggest that it is a warmwater, widely tolerant species that performs vertical migrations following the copepods
on which it feeds. For this rca son it is common in some upwelling areas, although very few
specimens were caught in the center of upwelling. The scattered and wide distribution
of the species and its comparatively greater abundance in enriched areas may be explained
by its eurytopic capacity and its euryphagous habits.
P. N. ARAVINDAKSHAN: Distribution and Ecology of Pterotrachea coronata (FORSKAL)
The largest specimen measured only 104 mm and the smallest 7 mm in length. The
Indian Ocean specimens were smaller than those recorded elsewhere.
I. Food Habits
Gut contents of 21 specimens from 21 different stations were examined. The principal
food includes copepods, chaetognaths and ostracods. Inside the oesophagus of a 73 mm
female Pt. coronata numerous copepods of fairly large size (4 mm) were observed. In
another 13 mm specimen, a 5 mm copepod Rhincalanus cornutus was noted. In all
cases examined the species was exclusively carnivorous and had a varied diet in accordance with the relative abundance of the food species available.
II. Distribution
The distribution of Pt. coronata in the Arabian Sea, the Andaman Sea and the
western Indian Ocean was discussed in a preliminary study (ARAVINDAKSHAN, 1969).
The present observation is that the species was present in only 49 samples of the 1927
examined from the epipelagic zone, with a total number of 63 specimens. Fig. 1 shows
the total number of stations where Pt. coronata occured. It was absent in the eastern
Arabian Sea, in the northern and western parts of the Bay of Bengal and in the central zone
of the southern Indian Ocean. It was common around the mouth of the Gulf of Aden, in
the eastern African waters, in the eastern part of the Andaman Sea, and off the NW of
Australia.
The species were caught at 38 out of 920 night stations and 11 out of 1001 day
stations. Therefore, vertical diurnal migration is usual in this species.
Pt. coronata was found in almost all the water masses of the upper layer of the
Indian Ocean at different seasons of the year. Comparing the stations where the net
sampled a single water mass, it is seen that the species must be euryhaline and eurythermal;
the range of tolerance appears to be 33 -40%0 and 12-30° C respectively. It seems likely
that the species requires a relatively high O2 content since it was not taken in waters
with less than 2.0 mll- 1 dissolved oxygen.
Most specimens come from stations where the net fished through different layers,
whether the boundary between the two watermasses was shallow or deep, whether a
thermocline was present or not. This suggests that the species lives mostly at the boundary
between surface and subsurface water zooplankton aggregates as a deep scattering
layer. A similar distribution was found by NAVAS (1969) for a hydromedusa, Halistaura
bruuni, that is also a zooplankton fceder. Therefore, although Pt. coronata has been
described as a warm-water epipelagic species, the present data suggest that it is a warmwater, widely tolerant species that performs vertical migrations following the copepods
on which it feeds. For this rca son it is common in some upwelling areas, although very few
specimens were caught in the center of upwelling. The scattered and wide distribution
of the species and its comparatively greater abundance in enriched areas may be explained
by its eurytopic capacity and its euryphagous habits.