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Prior to Portnoy and Gold, hypothesized reasons for the suture-zone and allopatric speciation in the northern Gulf included “(1) a physical barrier, similar to the Florida peninsula, that arose c.2.5 million years ago (Ma) during the Pliocene (Ginsburg, 1952), (2) an ecological barrier, perhaps a river that drained the Tennessee River basin directly into the Gulf, that existed approximately 2.4 Ma (Simpson, 1900; Ginsburg, 1952), (3) a strong current that flowed from the Gulf to the Atlantic through the Suwanee Straits approximately 1.75 Ma (Bert, 1986), and (4) extended cooling during early Pleistocene glaciations occurring c.
The seas have remained incredibly calm, again with waves normally no higher than 1 ft.
July 7, 2015 was a beautiful day, with few (FEW, 1-2 oktas) clouds in the sky (see above weather log from the bridge).
Visibility from the bridge was 10 nautical miles (nm) throughout the day.
Science and Technology Log After a run of around 16 hours, we finally arrived on the west coast of Florida to continue the survey. The organisms caught on the west coast of Florida were entirely different from those caught west of the Mississippi.
In our first trawl catch, I almost didn’t recognize a single species.
Fisheries biologist Kevin Rademacher shared with me an article, “Evidence of multiple vicariance in a marine suture-zone in the Gulf of Mexico” (Portnoy and Gold, 2012), that offers a potential explanation for the many differences observed.
The paper is based on what are called “suture-zones.” A suture-zone, as defined previously in the literature, is “a band of geographic overlap between major biotic assemblages, including pairs of species or semispecies which hybridize in the zone” (Remington, 1968).
In other words, it is a barrier zone of some kind, allowing for allopatric speciation, yet also containing overlap for species hybridization. (2009), such suture-zones are more difficult to detect in marine environments, and accordingly, have received less attention in the literature.
Such zones, however, have been discovered and described in the northern Gulf of Mexico, between Texas and Florida (Dahlberg, 1970; Mc Clure and Mc Eachran, 1992).
Portnoy and Gold note that “at least 15 pairs of fishes and invertebrates described as ‘sister taxa’ (species, subspecies, or genetically distinct populations) meet in this region, with evidence of hybridization occurring between several of the taxa” (Portnoy and Gold, 2012). On this table, I have highlighted species that we have found on this survey.
The figure below geographically illustrates distribution patterns of two pairs of sister species within the northern Gulf of Mexico.
We have seen all four of these species on this survey, and our observations have been consistent with these distribution patterns.