The hydrocoral Millepora squarrosa (Hydrozoa, Coelenterata) exhibits a limited, partly disjunct, distribution in the tropical western Atlantic. In the Caribbean the species is known from the Dominican Republic, Puerto Rico, Vieques, Culebra, the Lesser Antilles arc, and Barbados. It also occurs in Brazil. It is hypothesized that M. squarrosa is the descendant of an ancestor shared with the Indo-Pacific species M. platyphylla and that vicariance events prior to the Pliocene uplift of the Panamanian Isthmus have induced speciation of this ancestor into these species. The restricted Caribbean distribution of M. squarrosa is hypothesized to be the result of extinction of this species in certain parts of the Caribbean and its inability to re-intrude in these areas because of its ecological requirements and life strategy.