Assessing morphological variations in the seagrass genus Halodule (Cymodoceaceae) along the Brazilian coast through...

Seagrass meadows are distributed globally and provide critical ecological functions and ecosystem services, but seagrasses are still poorly studied compared with other coastal and marine foundation species. Species taxonomy is uncertain in various seagrass genera, such as the genus Halodule. Until recently, the morphological characteristics of leaves were the major criteria for species identification. In Brazil, three species of Halodule are recognized and separated solely using leaf morphology criteria by some authors; however, the leaves present high variability and plasticity, resulting in great uncertainty about species diversity. A review of seagrass species validation using both morphological and phylogenetic methods is needed. This includes examining the genus Halodule with the aim of better understanding its diversity and spatial distribution and consequently supporting management and conservation goals.

Plant samples with the morphological forms of H. beaudettei and H. wrightii were collected at five sites across three Brazilian marine ecoregions. Leaf tip format and leaf width and length were compared among all the sites and between the two populations with different leaf tip forms. Molecular diversity and divergence indices and analyses were used to estimate the genetic distance between H. wrightii and H. beaudettei populations. To determine the phylogenetic relationship between the two morphologies, we sequenced two molecular markers, the internal transcribed spacer (ITS) fragment and the rbcL gene, to construct phylogenetic trees using Bayesian inference.

We identified H. beaudettei morphology at two sites in Northeast Brazil, while H. wrightii 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 H. wrightii and H. beaudettei, with higher values observed in H. beaudettei. 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 H. wrightii and H. beaudettei 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 Halodule genus on the Brazilian coast.