In the sample analysed, Ulva rigida, a dark green, firm, foliaceous marine green alga, had a thallus of about 11.5 × 9.5 cm. Thallus flattened, lobed, attached to substrate by a small basal disc. It has a great morphological variability according to the environment and stage of development, from orbicular to umbilical in shape and wavy in margins. Cartilaginous at the base, leathery when dry, thickened and stiff in the middle part of the blade. In cross section it is dystromatic with two layers of elongate cells and not quadrate cells as in Ulva lactuca.