| Summary: | In 1984, a previously unreported species of hydroid, Samuraia tabularasa Mangin, 1991, appeared in the rocky intertidal zone of the northern Gulf of California, Mexico. Samuraia tabularasa, whose common name is the samurai hydroid, is a new genus and species in the athecate family Hydrocorynidae, class Hydrozoa, phylum Cnidaria. A key character that refers this species to a new genus is its mode of sexual reproduction by eumedusoids. Additionally, Samuraia tabularasa is able to withstand extreme desiccation in the intertidal zone, an evolutionary novelty for hydrozoans. Since its appearance, the samurai hydroid has had a dramatic effect on the community by causing the death of barnacles (Chthamalus anisopoma), the dominant occupier of space in this system, and thereby increasing the amount of primary substrate available for use by other species. Each hydroid colony is surrounded by an elliptical clearing approximately 2.5 cm in diameter that is kept clear of adult barnacles by causing the death of all newly settled barnacles that settle within reach of its tentacles. The clearings are maintained free of barnacles for sufficient time to allow for the growth of crustose algae (Ralfsia sp., and others) and subsequent grazing by limpets (Collisella strongiana) that live in the clearings. The addition of the samurai hydroid thus has led to an increase in local abundance of these two native species. This study shows that communities can be very resilient to the changes caused by the addition of a new species. Not only was there "room" for the samurai hydroid, but its activity promoted local species diversity by increasing habitat complexity. Hydroid mortality has exceeded recruitment from 1985-1990. This population decline, along with a 60% decrease in the average area of a hydroid clearing, may be associated with the hydroid's status as a recent invader and suggests that it is no longer favored in this region.
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