Pérez-Farfante (1969) postulated that Gulf of Mexico and
western Atlantic L. setiferus populations have been separated
at least since the last glacial maximum, when the sea level in the North Atlantic (as measured in Barbados) may
have been 120 metres below the present level (Bard et al.
1990; Yokoyama et al. 2000). Changes in sea levels over the
past 500 000 years may have provided the means for Gulf
of Mexico and western Atlantic populations to intermingle
(Cronin et al. 1981; McManus et al. 1999). Many species or
species pairs split along the warm temperate to tropical
break across Florida (Briggs 1974). L. setiferus, with a large
Ne, migratory adults and pelagic larvae, appeared to follow
the pattern of only weak differentiation between the
Gulf of Mexico and western Atlantic despite a disjunct
distribution. In some organisms, gene flow from the Gulf
of Mexico to the western Atlantic is indicated by the presence
of Gulf of Mexico alleles in the western Atlantic but
not the reverse (Bowen & Avise 1990). In this data set rare
alleles were found everywhere, and thus probably merely
reflected the high mutation rate of these loci rather than
directional gene flow. The Mississippi River outflow is
thought to present another barrier to gene flow for
many marine species, including white shrimp (Lindner
& Anderson 1956; Hoese & Moore 1977). Although we
looked for evidence of a barrier to gene flow across the
Mississippi River, none of the analyses revealed any genetic
break.