Marine prosobranch gastropods from oceanic islands off Brazil: Species composition and biogeography

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Donald R. Moore - Committee Chair

Second Committee Member

Nancy A. Voss - Committee Member


The distribution and biogeography of marine prosobranch gastropods from four oceanic islands and six seamounts off Brazil are investigated. The study is founded on an inventory of all Recent species known to occur in Atol das Rocas, Fernando de Noronha, Trindade and Martin Vaz Islands and the seamounts at the Vitoria-Trindade Seamount Chain, and deposited in several collections in Brazil, the U.S. and abroad. The species composition was determined for each island, and the relationships among assemblages from each island, and between islands and those from other land masses investigated. Evaluation of the modes of development in insular species made it possible to determine whether the assemblages are dominated by species with particular developmental strategies allowing for selective larval dispersal and eventual colonization of insular habitats.A total of 297 species was examined, equivalent to about 40% of the number of species known from the Brazilian coast. Cluster analyses of similarity indicate that assemblages from Rocas and Noronha are more related to each other than to those of any other study locality, probably due to geographic proximity and some ecological affinities. Similarities also suggest that, at the seamount chain, a 'stepping stones' effect should occur among the seamounts close to coast. The total number of species decreases with distance from the coast along the chain. Rates of endemism are relatively high (9 to 16%) in the four islands, in spite of their young age and small insular areas. Similar values were determined for exclusively Brazilian species present in the islands. Widely distributed western Atlantic tropical species are prevalent in each of the four islands. Species present in biogeographic units other than the western Atlantic were poorly represented. The effectiveness of a Mid-Atlantic barrier is confirmed by small percentages of amphi-Atlantic species, and by reduced similarities between study localities and Ascension Island.Frequencies of planktotrophic species among islands and seamounts were not significantly different from those with lecithotrophic development, suggesting that distances are not large enough to constitute a filter in regard to developmental strategies. However, species with broad, extra-western Atlantic distribution present in the four islands were prepoderantly planktotrophic. Conversely, lecithotrophy was prevalent in the groups of restricted or endemic species, indicating that speciation rates should be higher for species without the high dispersal abilities associated with feeding, long-lived planktonic stages. Finally, the species per family ratios in the the four islands did not support the hypothesis that oceanic islands can act in general as refugia for invertebrate faunas during periods of lowered sealevel. Some isolated cases, however, do indicate that these localities have sheltered particular groups of species that underwent extinction elsewhere.


Biology, Ecology; Biology, Oceanography; Biology, Zoology; Agriculture, Fisheries and Aquaculture

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