Community structure and vertical distribution of chaetognaths in the Celebes and Sulu Seas

Chaetognath community structure, vertical distribution and migration patterns were investigated in two marginal basins-the Celebes and Sulu Seas. The two seas are part of an island arc located in the southeastern Asia region of the western equatorial Pacific Ocean. The Sulu Sea had higher species diversity (Shannon Index, H') and higher overall chaetognath abundance than the Celebes Sea. Twenty-two species from 4 genera were collected from the Sulu Sea and 19 species from 4 genera were collected from the Celebes Sea. Three species were collected exclusively in the Sulu Sea: Sagitta bipunctata, Sagitta nagae and an unidentified Sagitta species. Sagitta enflata was the numerically dominant species at both locations. Species diversity in the Celebes Sea was fairly even between the surface and 1000m. However, in the Sulu Sea substantial drops in diversity occurred at 175-200m-below the oxycline, thermocline and halocline-as well as at 900-1000m. The drop at 175-200m and 900-1000m was largely due to the numerical dominance of the species Sagitta decipiens and Sagitta macrocephala, respectively. In the Celebes Sea only, a large portion of the overall chaetognath population migrated downwards during the day into the 125-200m layer. Cluster analysis using Ward's method, along with weighted mean depth values, revealed that this migration was largely attributable to the species Sagitta pacifica, Sagitta ferox-robusta, Sagitta neglecta and Pterosagitta draco. A similar migration of the overall population was blocked in the Sulu Sea-apparently by the oxycline. It seems the group of migrating chaetognaths in the Celebes Sea prevented the numerical dominance of S. decipiens at the lower edge of the epipelagic region, possibly through competition, and consequently prevented the same drop in diversity (H') at 175-200m as was recorded in the Sulu Sea