Two different genetic strains of stalked-Ulva (Ulvales, Chlorophyta) grow on intertidal rocky shores in Ebisujima, central Japan

Stalked‐Ulva has been recognized as an ecologic form of Ulva pertusa Kjellman with narrow and extensive stipes that grows on rocky shores with strong wave action. However, it is possible that stalked‐Ulva includes more than two taxa, because it has been shown previously that some isolates of stalked‐Ulva did not cross with U. pertusa. Therefore, further crossing tests, observations of swarmer behavior, culture studies and comparison of DNA sequences were made to clarify whether or not all stalked‐Ulva are included within U. pertusa. Crossing tests showed that stalked‐Ulva contained two different types, one that crossed with U. pertusa (Up strain) and one (N strain) that did not. The biflagellate swarmers of N strains showed negative phototaxis and were slightly larger than male and female gametes of Up strains. Thalli cultured from the swarmers of N strains released the same type of swarmers again. The N strains have an asexual life history, reproducing solely by biflagellate swarmers. Internal transcriber spacer sequence analysis indicated that these two entities are U. pertusa (Up strain) and U. fasciata Delile (N strain). In culture both strains differ morphologically from wild thalli with stipes; it seems that the two different taxa both show a petiolate morphology when growing under the same environmental conditions.