VERTICAL-DISTRIBUTION AND DIEL MIGRATION IN THE IRIDESCENT COPEPODS OF THE FAMILY SAPPHIRINIDAE - A UNIQUE EXAMPLE OF REVERSE MIGRATION

The vertical distributions and diel vertical migrations of the sapphirinid copepods of the genera Sapphirina and Copilia were investigated in the tropical and subtropical waters in the eastern Indian Ocean, the South China Sea and the western Pacific. Vertically stratified plankton samples were collected at a total of 11 stations including 5 day-night stations and 1 station occupied for a 20 h serial sampling, in the years 1977, 1981, 1988 and 1991. The sapphirinids, the family as a whole, were collected essentially in the upper 200 m, and are considered as typical epipelagic copepods. Three groups of species were recognized according to their patterns of vertical distributions: the upper epipelagic species, the lower epipelagic species, both of the genus Sapphirina, and 2 species of Copilia which showed relatively deep and broad vertical distributions. Marked reverse diel vertical migrations were observed in the upper epipelagic species, S. gastrica, S. stellata, S. nigromaculata and S, auronitens, and, to a lesser extent, in the lower epipelagic species, S, metallina. In the species exhibiting reverse migrations, the vertical distributions in the daytime were mostly unimodal, while bimodal distributions were occasionally observed at night. A comparison of these vertical patterns with the specific iridescent color patterns in the male and the distribution of light in the ocean suggested that the daytime distributions of the sapphirinids are determined by the underwater light condition as a proximate cue. It was also hypothesized that the well-developed eye, the iridescence of the male and the daytime shoaling in the sapphirinids are closely related and constitute a presumed mate-finding mechanism which may be unique in the oceanic plankton.