Branchiomma bairdi, a fouling non-indigenous sabellid polychaete has been detected in the south-eastern Gulf of California, representing the first record in the eastern tropical Pacific, as its original distribution is the Caribbean Sea. The species was commonly found as isolated specimens but small aggregates with 4–12 individuals were also found on floating docks, on dock pilings, buoys and on hulls of vessels in the port of Mazatlán. Branchiomma bairdi is a simultaneous hermaphrodite with male and female gametes developing separately in the same segments. The sperm morphology suggests that the species is a free-spawner with external fertilization (ect-aquasperm type). A complete diagnosis is provided and a discussion about its systematics, reproduction and ecology is included.
