Abstract
Mona Island, a small uninhabited Caribbean island between Puerto Rico and Hispaniola, was surveyed for freshwater platyhelminths. Rainwater pools on the island's plateau produced two species of Typhloplanidae, a species close to Mesostoma georgianum and a new species of Mesostoma, M. tubiseminalis, herein described. The new species is diagnosed by the presence of a tubular receptaculum seminis that forms a distinct bifurcation of the female tract. Pools inhabited by both species are subject to high pH, high temperatures, and rapid evaporation. While M. cf. georgianum is quite conspicuous and creeps on the surface film, the new species lives concealed within the substrate and rock crevices of the pool. Studies on the free-living (turbellarian) platyhelminths inhabiting West Indian fresh-waters are few, probably reflecting the rather meager fauna of the region. Among triclad flatworms, five species of the dugesiid genus Girardia have been recorded from the Caribbean region (Sluys 1992); however, only two of these species appear to be endemic to the non-continental Antillean islands. The only other free-living flatworm group known to have representatives on the islands is the Rhabdocoela. The single species of this taxon known from the West Indies is Bothromesostoma pieti MARCUS 1960 (Typhloplanidae), so far known only from Curacao. Mona Island is a remote island occurring about halfway between the Dominican Republic and Puerto Rico, to which it politically belongs. Recent surveys have revealed the presence of two neorhabdocoel species in the family Typhloplanidae, living in rain-water filled pools on the island's interior plateau. Examination of a large series of specimens from each species has shown that they belong to the genus Mesostoma. One closely resembles a described species from the North American mainland. A comparison is made with type material of the mainland species and an expanded description is provided. The other species is new and a description is presented.
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