The massive aggregations of serpulids have been studied in several regions of the
world (Mediterranean, northern Gulf of Mexico, Argentina, California, Malaysia, Australia); however,
there have been few studies on the Mexican coasts that record aggregations of small serpulids in the Gulf
of California, as Salmacina tribranchiata. In this work, we recorded, for the first time, the massive
aggregations of the invasive serpulid Hydroides sanctaecrucis KRØYER [IN] MÖRCH, 1863 in a coastal
lagoon system from Oaxaca, Southern Mexican Pacific. The serpulid is native to the Caribbean Sea;
nevertheless, it was recorded as non-native in the Tropical Eastern Pacific 24 years ago, as solitary
specimens or, less frequently, in small aggregations of some specimens, mainly associated with marinas
and harbor facilities. With the spreading of this species in several localities of the Tropical Eastern Pacific
and Western Pacific, both in ports and less human-impacted habitats, H. sanctaecrucis is considered an
invasive species. This serpulid formed aggregations in a coastal lagoon system of Oaxaca; the aggregations
have between 10 and 70 cm diameter, including thousands of calcareous tubes. The conditions of the
lagoons during the sampling were hypersaline (42.37-44.19 ‰) in Superior Lagoon, and brackish (30.77
‰) in Inferior Lagoon; the turbidity was high so visibility was almost null. The aggregations include three
other native serpulids, H. crucigera MÖRCH, 1863, H. panamensis BASTIDA-ZAVALA & TEN HOVE, 2003,
and Spirobranchus minutus (RIOJA, 1941), but with less abundance. The flora and fauna associated with
these serpulid aggregations included two macroalgae, other polychaetes, gastropods, bivalves, porcelain
crabs, barnacles, amphipods, and isopods.
