Abstract
We describe a new species of capitellid polychaete, Mediomastus opertaculeus sp. nov., from an intertidal rocky shore in Hokkaido, northern Japan. This new species is distinguishable from all other 13 congeners by such characteristics as (1) staining pattern with methyl green (presumably indicating glandular cells in the epidermis), (2) absence of capillary chaetae in the abdomen, (3) absence of paddle-like chaetae in the thorax, and (4) shapes of the thoracic capillary chaetae and hooded hooks. Aligned partial sequences (624 bases) of cytochrome c oxidase subunit I gene from four paratypes of M. opertaculeus differed at 1-4 sites, corresponding to Kimura two-parameter (K2P) distances of 0.002-0.006. K2P distances were 0.216-0.218 between our sequences and that from an unidentified Mediomastus sp. from Darwin Harbor, Australia, indicating these are not conspecific.
