Species diversity and vertical distribution of the deep-sea copepods of the genus Euaugaptilus in the Sulu and Celebes Seas

Abstract

The relationships between water-column structure, species diversity and patterns of vertical distribution were examined in the copepod genus Euaugaptilus in the Sulu and Celebes Seas. Euaugaptilus is among the most species-rich single genus of all calanoid copepods and is characterized by the specialized ‘button setae’ in their mouth appendages. The Sulu Sea is a semi-enclosed equatorial basin located in the center of the Indo-Malayan Archipelago, rimmed by sills shallower than 420 m, and characterized by homogeneous, warm water (ca. 10 °C) from the mesopelagic zone to the sea bottom of ca. 5000 m, while the adjacent Celebes Sea is of more typical oceanic conditions. Plankton samples were collected at two stations both day and night from 16 discrete layers in the upper 1000 m. A total of 29 species of Euaugaptilus were collected in the Celebes Sea, which is among the largest numbers for the genus so far reported from a single restricted sea area, but only 8 species were collected in the Sulu Sea. These 8 species occurred in the upper mesopelagic zone in the Celebes Sea, while in the Sulu Sea many of them extended their ranges and/or shifted into deeper zones. An additional 15 net tows to depths deeper than 1000 m added 6 species from the Celebes Sea and 8 species from the Sulu Sea, with all the deep Sulu species, except E. hyperboreus, being found above 1000 m in the Celebes Sea. This drastic reduction of species number in the Sulu Sea is attributed to the homogenous high-temperature deep water, which may have prevented settlement of many deep-water species from outside areas and co-existence of species sharing similar ecological niches. The species in the Sulu Sea showed discrete vertical distribution patterns according to the species or species groups, despite the essential absence of vertical gradients of temperature and salinity in the mesopelagic zone. The species pairs that showed similar vertical distributions in the Sulu Sea showed marked differences in their prosome length and/or in the morphology of the ‘button setae’ and the mandible blade. The species successfully inhabiting the Sulu Sea may have expanded their range into the deep waters, and vertical segregation and food-resource partitioning may be among the major factors allowing the observed coexistence of these congeneric species.