Ulva gametes are formed by mitosis, not meiosis. They come from haploid cells which resulted from meiosis, but those cells grow and divide to make the structure which mitotically produce the gametes.
Work Step by Step
Meiosis must be responsible for making haploid cells, which in animals are the gametes. However, the algae and plants have an alternation of generations lifecycle which separates gamete production from meiosis by mitotic divisions.