A new Hemicyclops (Copepoda, Cyclopoida, Clausidiidae) associated with the scleractinian coral Galaxea from the South China Sea

The genus Hemicyclops Boeck, 1873 is known for its association with various marine invertebrates, including cnidarians, crustaceans, polychaetes, and sponges, with some species also occurring in planktonic communities. Here, we report the first association of Hemicyclops with the scleractinian coral Galaxea fascicularis (Linnaeus, 1767) (Scleractinia, Euphylliidae). Hemicyclops cyanus sp. nov. is described based on a female specimen collected from this coral host in the lagoon (depth 10 m) of Dongsha Atoll, Pratas Islands, South China Sea.

The new species is readily distinguished from its congeners by its characteristic genital double-somite, which bears prominent anterolateral expansions, and by the flexed, elongated exopodal segment of leg 5, which is more than three times longer than wide. In H. cyanus sp. nov., the paired spermatophores attached to the female are fused into a butterfly-shaped, highly modified complex with large lateral wings and a central tube into which the female urosome is inserted.

To aid in species identification, we present the first comparative plate with schematic illustrations of the genital double-somites for the group of 25 species, including the type species.