Morphological and molecular differentiation between three allopatric populations of the littoral crabPachygrapsus transversus(Gibbes, 1850) (Brachyura: Grapsidae)

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Abstract
The littoral grapsid crab species Pachygrapsus transversus (Gibbes, 1850) has a wide distribution ranging from the eastern Pacific to the eastern Atlantic and Mediterranean. The American continent and the Atlantic Ocean are believed to act as barriers for dispersal of planktotrophic larvae. The crab populations studied, from the eastern Pacific (Panama), the western Atlantic (Caribbean), and the eastern Atlantic (Spain) were, therefore, assumed to be genetically isolated. Remarkable differences in DNA sequence of the 16S mt-DNA gene as well as in larval and adult morphometry and coloration were found between these populations. A closer genetic similarity between the amphi-Atlantic populations in comparison to the Pacific animals, suggests the occurrence of occasional gene flow across the Atlantic Ocean.