The rockworm, Marphysa sanguinea is a large, often conspicuous benthic polychaete. Like other worms belonging to the phylum Annelida, the opalescent body of M. sanguinea is divided into numerous segments. Several of these divisions are called setigers, so named because of the presence of small hairs called setae located on either side of the segment. In M. sanguinea, long scarlet-red gill filaments arch up over the body, beginning at the 21st to 24th setiger (Ruppert & Fox 1988). Each gill may contain several filaments.