First record on the occurrence of Ryukyua circularis (Pillai, 1954), a parasitic cymothoid (Crustacean: Isopoda) infesting the clupeid fish Amblygaster sirm (Walbaum) from Andaman Islands, India.

In the present study, occurrence of branchial parasitic cymothoid, Ryukyua circularis from the marine finfish, Amblygaster sirm is reported for the first time from Andaman Islands. Morphological characterization was carried out which confirmed the parasite as R. circularis. Molecular characterization using 28S rDNA revealed 443 bp sequence which has been submitted to NCBI GenBank under the accession no. KX090447. Ten female and one male parasite (R. circularis) were collected from the branchial cavity of the individual host fishes (A. sirm). The displacement of gill filament and cavity formation in the gill chamber of the host fish was observed. The report of this species from Andaman Islands gives further information on its geographic range extension, since they are currently known from West coast of India and Thailand.