Occurrence of Caligid Copepods (Crustacea) in Plankton Samples Collected from Japan and Thailand, with the Description of a New Species

Abstract

Adults of four species of the genus Caligus (Copepoda: Caligidae) were found in plankton samples collected from the coasts of Japan and ailand: Caligus chiastos Lin and Ho, 2003 from the Gulf of ailand; C. latigenitalis Shiino, 1954 from the Suo-nada basin of the Seto Inland Sea, Japan; C. undulatus Shen and Li, 1959 o the island of Mukaishima, Hiroshima, Japan; and C. longiramus sp. nov. from Amami Island, Kagoshima, Japan. e rst two species respectively represent the rst 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 rst report of pelagic caligids from ailand. e present study provides support to the hypothesis that some species of Caligus occurring in the plankton alternate between two life modes: on their host sh 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) denti-form 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.