• This study was supported in part by North Atlantic Treaty Organization Grant No. 1130 (1977–79) to JAW and MDG, U.S. Department of Commerce National Oceanic Atmospheric Administration Sea Grant NOAA Or-6-158-44021 (1975–76) and 04-8-MOI-189 (1977–79) to JAW, U.S. National Science Foundation Grants GB-40550 (1974–75), INT-7946892 (1980–82) and PCM 7917342 (1980-82) to JAW, Irish National Board for Science and Technology Science Research Grant 14-1979 (1979-82) to MDG and the University of California, Berkeley, Miller Institute for Basic Research in Science which provided a one year research professorship (1976-77) to JAW. Portsmouth Polytechnic, U.K. and C. Thorp provided facilities at the Hayling Island Marine Laboratories during the early part of this study (1976-79). H. Blackler, J. Cabioch, T. Christensen, W. Farnham, P. Gray, N. Jephson, I. Munda, C. Thorp, I. Tittley, R. Vadas and P. Whelan provided material for culture studies. Eleanor Crump, Wendy Guiry and Deanna West provided invaluable technical assistance. Pat Whelan was particularly helpful in the early stages of the project. L. Irvine, C. Maggs, M. Masuda and others provided helpful discussion. To all these organizations and people, we extend our grateful thanks.

  • 1

    Accepted: 19 July 1983.


Fourteen isolates of the crustose marine red alga Petrocelis cruenta J. Agardh from various localities in the British Isles, France (including the type locality), Spain and Portugal gave rise in culture to dioecious foliose plants identifiable as Gigartina stellata (Stackhouse) Batters although two isolates formed only sterile foliose blades. A total of 145 isolates of Gigartina stellata were also grown in culture from various localities in the U.S.A. (Maine), the British Isles, Iceland, Denmark, France, Spain and Portugal using both carpospores and vegetative blade apices. Two basic types of life history were found among these isolates: a direct-type life history involving the formation of further foliose plants from carpospores, some isolates of which also form spermatangia on the same papillae as the cystocarps; and a heteromorphic-type in which only crustose plants resembling Petrocelis cruenta are formed from carpospores. Only heteromorphic-type life histories were found from Spain and Portugal. Both life history types were found in plants from the U.S.A., the British Isles and northern France. Only direct-type life histories were found in plants from Iceland and Denmark. Some Petrocelis-like crusts derived from field collected G. stellata carpospores and Petrocelis crusts of hybrid progeny formed tetrasporangia in 8:16 h LD, 10° C but not in 8:16 h LD, 15° C; 16:8 h LD 10° C or 15° C; and 10:6.5:1: 6.5 h LDLD, 10° C. The spores thus formed were viable and produced normal dioecious male and female gametophytes. Short day and low temperature conditions appear necessary for tetrasporogenesis. The results from crossing experiments with 32 male and 27 female isolates of the heteromorphic-type derived from both G. stellata and P. cruenta showed that two virtually non-interbreeding populations with a high degree of geographical separation exist in the north-eastern Atlantic. Morphological differences between plants from each population are described. On the basis of culture and crossing results, Petrocelis cruenta J. Agardh is placed in synonymy with Gigartina stellata (Stackhouse in Withering) Batters.