Morphological variation in the seagrass Halodule wrightii near its southern distributional limit

In Rio de Janeiro state the seagrass Halodule wrightii Aschers. is reaching its southernmost limit, and populations are small, isolated and infrequently sexually reproductive. Leaf width and length, number of leaves short shoot −1, leaf sheath length, rhizome width, internode length, and root length and density were measured in 9 populations, and on three occasions within a single population. Each population featured at least one morphological attribute which differed from others. Leaf lengths varied 11-fold between populations, and each morphological feature was site dependent, but widths of leaves, diameters of rhizomes and the number of leaves per short shoot were the least, and root density the most variable features. Variation in a population over time was similar in scale to variation between plants, though leaf number varied very little over time. Leaf widths, lengths, sheath lengths, rhizome diameters and root densities showed more variation between than within populations. The variation in some characteristics within a single population was as high or higher than that reported within or between species of seagrasses. One population was polymorphic. The high degree of variation was consistent with the hypothesis of genetically mediated morphological differences between populations.