It seems intuitive that glacial advance/retreat played a
significant role in shaping red snapper distribution in the
northern Gulf. Several glacial/interglacial periods occurred
in North America (Shackleton and Opdyke 1973;
Williams et al. 1998), and undoubtedly impacted marine
species via altered sea-levels (Williams et al. 1998).
During the last glacial maximum (occurring
18,000 years ago), for example, sea levels in the
northern Gulf were as much as 130 m lower than they
are today (Rezak et al. 1985), meaning that little habitat
would have been available for continental-shelf spawners
such as red snappers (Allen 1985). The spatial/
temporal patterns indicated by nested clade analysis are
thus consistent with effects that might be expected with
glacial advance/retreat. Population expansion could
occur during glacial retreat and opening of continentalshelf
habitat; while restriction of gene flow could occur
either during glacial advance (if red snapper retreated to
spatially different glacial refuges) or retreat, if (re)colonization
of the continental shelf occurred at different
times at spatially different localities.