Diversity and distribution of the Siphonophora (Cnidaria) in Sagami Bay, Japan, and their association with tropical and subarctic water masses

Siphonophores were sampled using stratified 1.67-m²-mouth 330-μm mesh nets during both day- and nighttime in Sagami Bay (35°0.50′N, 139°20′E), off south-eastern Japan during a 9-day period in March 2006, when the Kuroshio Current was in an offshore non-large meander phase. The samples were collected at 50-m intervals spanning from 1,000 m depth to the surface. Fifty-eight species and 11 eudoxid forms of unknown parentage were collected, of which 5 represent first-time records from Japanese waters, and Clausophyes laetmata Pugh and Pagès 1993, Eudoxia cf. galathea Moser 1925 and Lensia panikkari Daniel 1970 were recorded for the first time not only from Japan but from the entire Pacific Ocean. The highest abundance and diversity was found below the thermocline during the day, and above it at night. A second peak in abundance, around 400 m depth, associated with a decrease in diversity, could be linked to the increase in abundance of a single species—Dimophyes arctica (Chun 1897). The siphonophore communities could be related to the different water masses in the Bay, with an important influence of lateral transport of both tropical and subarctic species into the Bay by the different water masses.