Abundance and community structure of chaetognaths in the northern Indian Ocean

Abundance and community structure of chaetognaths in the northern Indian Ocean was investigated based on zooplankton samples collected during January to February 1990. Three types of sampling were done: from the upper 1100-1600 m water column, from discrete depths in the upper 1000m covering day and night and from the upper 150m water column using 0.33mm (GG 54) and 0.1 mm (XX 13) mesh nets. In the first set of collections, population density of chaetognaths was higher in the Arabian Sea than in the Bay of Bengal. Nineteen species were found in the area of which the predominant species was Sagitta enflata. S. decipiens was the dominant mesopelagic species while Eukrohnia fowleri was the only bathypelagic species in the collections. In the vertical realm, the chaetognath population reached maximum abundance in the upper 100m stratum and abruptly reduced on reaching the 1000m stratum. In general, population density was higher in the night than in the day samples. Population density of chaetognaths caught by the 0.1 mm mesh net was higher than with the other nets and the community was predominated by Krohnitta pacifica, Pterosagitta draco, Sagitta bipunctata, S. pacifica and S. regularis. Stratification of chaetognaths in the 1000m water column indicated variation within the community associated with the prevailing physico-chemical parameters. High species richness, high dominance/low evenness were obtained at low ambient temperature values. The relationship between dissolved oxygen concentration and community structure indices was negatively correlated , with lower DO values at higher community diversity indices. Data collected during the day revealed a high level of community similarity ( > 60% community coefficient values/similarity index values) between similar depths such as (50m, 100m), (300m, 400m), (500m, 600m), (600m, 700m) and (700m, 800m). At depths below 500m, chaethognath species had narrower distributions, being confined to 100m depth layers, centered around the depth of their maximum abundance. In the night collections similar trends were evident, in particular, the depth strata at 500 m and 600 m showed > 80% similarity in the composition of species.