Complex patterns of genetic population structure in the mouthbrooding marine catfish, Bagre marinus, in the Gulf of Mexico and U.S. Atlantic

Patterns of genetic variation reflect interactions among microevolutionary
forces that vary in strength with changing demography. For marine species,
these patterns are often interpreted under the expectation that larval
movement drives connectivity because most marine species exhibit broadcast
spawning dispersal strategies. Here, patterns of variation within and
among samples of the mouth brooding gafftopsail catfish (Bagre marinus,
Family Ariidae) captured in the U.S Atlantic and throughout the Gulf of
Mexico were analyzed using genomics to generate neutral and non-neutral
SNP data sets. Because genomic resources are lacking for ariids, linkage
disequilibrium network analysis was used to examine patterns of putatively
adaptive variation. Finally, historical demographic parameters were
estimated from site frequency spectra. The results show four
differentiated groups, corresponding to the (1) U.S. Atlantic, and the (2)
northeastern, (3) northwestern, and (4) southern Gulf of Mexico. Patterns
of genetic variation for the neutral data resemble that of other fishes
that use the same estuarine habitats as nurseries, regardless of the
presence/absence of a dispersive larval phase, supporting the idea that
adult/juvenile behavior and habitat are important predictors of
contemporary patterns of genetic structure. The non-neutral data presented
two contrasting signals of structure, one due to increases in diversity
moving west to east and north to south, and another to increased
heterozygosity in the Atlantic. Demographic analysis suggested recently
reduced long-term effective population size in the Atlantic is likely an
important driver of patterns of genetic variation and is consistent with a
known reduction in population size potentially due to an epizootic.