4.3
Distribution of Hydromedusae in the Indian Ocean
M. VANNUCCI and D. NAVAS
We have endeavoured in this work to characterize the various oceanographic systems
of the Indian Ocean at different depths, using the medusae as indicators of the prevalence
of plankton at the trophic level of obligate carnivores.
I. Material and Methods
We chose hydromedusae as our example because:
a) most species are in good taxonomic order and we use the species as the basic
biological unit; b) all medusae are exclusive carnivores, and most fish passively for their
food by the hook and line technique and thus fall into a single ecological category with
regard to food acquisition habits; c) they are too sluggish to actively avoid the plankton
sampler, thus sampling bias brought about by avoidance behaviour is minimized and the
numbers in the samples should adequately represent the association at sea. Here the main
source of error lies in the very great fragility of many species; vulnerability varies between
species, so they are not numerically comparable except in a gross manner; however, this
error is minimized by considering each species separately and disregarding those that for
various reasons did not provide reliable information. Special difficulties arise when the
juveniles cannot be determined to species level and when they are specially abundant, as
for instance in the genus Aequorea, a group of slow-growing species.
Given the rarity of hydromedusae, all specimens in each sample were sorted,
determined and counted. About 150 species were found in about 900 samples from the
Indian Ocean, which were made available for study from the Smithsonian Institution.
Some of the samples are duplicates or insufficiently labelled, and some of the specimens
are doubtful or undeterminable because they are young stages, new species, or damaged
beyond recognition. About 45000 specimens were determined and counted; 34 species
were not previously recorded for the Indian Ocean and were dealt with separately
(NAVAS, 1971), 1 new species Halistaura bruuni (NAVAS, 1969) is, as far as known,
endemic in the Bay of Bengal.
II. Results
Two species predominate (45%): Aglaura hemistoma e13) and Lyriope tetraphylla
(1/3)' both warm-water, holoplanktonic, frequent, surface-water species; their abundance
in the collection is due to the geographic distribution of the stations, mostly oceanic and
far from land. The scarcity or absence of inshore and coastal species is noteworthy. Fig. 1
shows frequency distribution with depth, and the identical behaviour of the two species:
both are euryhaline and eurythermal and Aglaura is morc abundant in tropical oceanic
samples (VANNUCCI, 1957; 1963). Lyriope feeds on larger animals than Aglaura and this
Distribution of Hydromedusae in the Indian Ocean
M. VANNUCCI and D. NAVAS
We have endeavoured in this work to characterize the various oceanographic systems
of the Indian Ocean at different depths, using the medusae as indicators of the prevalence
of plankton at the trophic level of obligate carnivores.
I. Material and Methods
We chose hydromedusae as our example because:
a) most species are in good taxonomic order and we use the species as the basic
biological unit; b) all medusae are exclusive carnivores, and most fish passively for their
food by the hook and line technique and thus fall into a single ecological category with
regard to food acquisition habits; c) they are too sluggish to actively avoid the plankton
sampler, thus sampling bias brought about by avoidance behaviour is minimized and the
numbers in the samples should adequately represent the association at sea. Here the main
source of error lies in the very great fragility of many species; vulnerability varies between
species, so they are not numerically comparable except in a gross manner; however, this
error is minimized by considering each species separately and disregarding those that for
various reasons did not provide reliable information. Special difficulties arise when the
juveniles cannot be determined to species level and when they are specially abundant, as
for instance in the genus Aequorea, a group of slow-growing species.
Given the rarity of hydromedusae, all specimens in each sample were sorted,
determined and counted. About 150 species were found in about 900 samples from the
Indian Ocean, which were made available for study from the Smithsonian Institution.
Some of the samples are duplicates or insufficiently labelled, and some of the specimens
are doubtful or undeterminable because they are young stages, new species, or damaged
beyond recognition. About 45000 specimens were determined and counted; 34 species
were not previously recorded for the Indian Ocean and were dealt with separately
(NAVAS, 1971), 1 new species Halistaura bruuni (NAVAS, 1969) is, as far as known,
endemic in the Bay of Bengal.
II. Results
Two species predominate (45%): Aglaura hemistoma e13) and Lyriope tetraphylla
(1/3)' both warm-water, holoplanktonic, frequent, surface-water species; their abundance
in the collection is due to the geographic distribution of the stations, mostly oceanic and
far from land. The scarcity or absence of inshore and coastal species is noteworthy. Fig. 1
shows frequency distribution with depth, and the identical behaviour of the two species:
both are euryhaline and eurythermal and Aglaura is morc abundant in tropical oceanic
samples (VANNUCCI, 1957; 1963). Lyriope feeds on larger animals than Aglaura and this