Assessing morphological variations in the seagrass genus <i>Halodule</i> (Cymodoceaceae) along the Brazilian coast through genetic analyses

We identified <i>H. beaudettei</i> morphology at two sites in Northeast Brazil, while <i>H. wrightii</i> was found in all the ecoregions in the remaining areas, distinguished by the leaf tip shape that occurred at each site. Leaf width and length varied across the five sites, and leaf length differed between <i>H. wrightii</i> and <i>H. beaudettei</i>, with higher values observed in <i>H. beaudettei</i>. Variations in morphological measurements may be related to habitat conditions at each site studied. No divergence was observed for the DNA sequences of two molecular markers, except for a single base in the ITS region, resulting in the Brazilian specimens merging at a single node in the phylogenetic trees. AMOVA and genetic distance analysis revealed low genetic variation but high structuring within populations. The ITS marker showed insufficient genetic variance to delineate the two morphologies as different species which indicating <i>H. wrightii</i> and <i>H. beaudettei</i> are closely related. A genomic approach is needed to fully resolve this issue. This study represents the first step toward the complete determination of the <i>Halodule</i> genus on the Brazilian coast.