Abstract
Currently, only two species are known in the sabellid genus Aracia: A. riwo (Rouse, 1996) and A. heterobranchiata (Nogueira, López & Rossi, 2004). The first was described from the surface of a Teredo-riddled log lying in 1 m of water among mangrove roots, in Papua (New Guinea), whilst the second was found on an organically enriched beach at São Sebastião (Brazil). In this contribution, a third species of Aracia is described from settling panels and red mangrove roots in an estuarine system located in southern Gulf of California (Mexico). The new species is a simultaneous hermaphrodite and brooder, unique in the presence of a rectangular ventral shield on the collar, shorter than those in posterior thoracic segments; and a high number of rows of teeth in thoracic and abdominal uncini, covering from one-half to three-quarters of the main fang length. Information about color patterns in live specimens, glandular patterns, and reproduction is provided.