TEMPORAL VARIATION IN ISOMORPHIC PHASE AND SEX RATIOS OF A NATURAL POPULATION OF ULVA PERTUSA (CHLOROPHYTA)1

The green macroalga Ulva L. is well known as having an alternation of isomorphic biphases: gametophyte and sporophyte. However, an examination of the temporal alternation in phase dominance has not been carried out. By inducing reproduction of thallus samples in the laboratory, this study reports the temporal changes in the two phases and sex ratios in a natural population of Ulva pertusa Kjellm. in the Seto Inland Sea, Japan, over a period of >3 years. The results showed that a temporal alternation in phase dominance occurred after 11–20 months; seasonal changes in phase dominance were not observed in the Ulva population. The sex ratio of male and female gametophytes remained ∼1:1 throughout the investigated period, although skewed sex ratios were observed in several collections.