Rissoina Indiscreta, a New Rissoid Species from the Tropical Southwestern Atlantic With Indo-West Pacific Affinities (Mollusca, Gastropoda, Rissooidea)
A new rissoid, Rissoina indiscreta, is described from the northeastern and eastern Brazilian coasts. The new species has been identified for more than 20 years in the literature as R. elegantissima d'Orbigny, 1842. Although apparently belonging in the same subgenus Rissalina, true elegantissima is a distinct species, restricted to the Caribbean Sea. Apart from the number of axial ribs per whorl, no other important variation in conchological characters is found in the different populations of the species. Comparisons indicate that indiscreta might be very close phylogenetically to R. turricula Pease, 1861, a Recent species with a large distribution range in both the Indian and Pacific Oceans, and also known from the Miocene of west Pacific islands. Three hypotheses accounting for the present-day disjunct distribution of the assumed species-group are discussed, and a vicariant proposition suggesting the existence of a widely-distributed, shallow-water Tethyan ancestor is favored. The other two interpretations, passive larval dispersal by currents across the East Pacific Barrier via the Pacific countercurrent system, or from the Indian Ocean, to South Africa, then to Brazil through the counterclockwise surface circulation of the South Atlantic, would imply the existence of long-lasting planktotrophic (teleplanic) larval stages in the ancestor, a condition unknown in the genus Rissoina.
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Document Type: Research Article
Publication date: July 1, 1989
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