Abstract
Adults of four species of the genus Caligus (Copepoda: Caligidae) were found in plankton samples collected from the coasts of Japan and Thailand: Caligus chiastos Lin and Ho, 2003 from the Gulf of Thailand; C. latigenitalis Shiino, 1954 from the Suo-nada basin of the Seto Inland Sea, Japan; C. undulatus Shen and Li, 1959 off the island of Mukaishima, Hiroshima, Japan; and C. longiramus sp. nov. from Amami Island, Kagoshima, Japan. The first two species respectively represent the first reported occurrences in plankton samples, although the third species has been encountered frequently in the pelagic fauna in Japanese waters. Caligus chiastos also represents the first report of pelagic caligids from Thailand. The present study provides support to the hypothesis that some species of Caligus occurring in the plankton alternate between two life modes: on their host fish and free in the water column. Caligus longiramus sp. nov. can be distinguished from its congeners by the following combination of features: (1) antennule long with its distal segment twice as long as its proximal one; (2) dentiform process of proximal segment of antenna small and blunt; (3) sternal furca long with sharp tines; (4) exopod of leg 4 armed with I-0; I, III elements; and (5) proximal outer spine on exopod of leg 2 longer than next more distal spine and crossing over it posteriorly.
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