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Specimens of  Hildenbrandia  bearing conceptacles that contained small, bead-shaped reproductive cells, only 3–4 µm wide, features diagnostic of  Hildenbrandia sanjuanensis , were collected from the high intertidal zone at the type locality of the species. Examination of conceptacle contents by transmission electron microscopy was used to detail the differentiation of the catenate reproductive cells, and it revealed that these cells were joined by pit connections. Tetraspores are the only known reproductive cells in the Hildenbrandiales, but the process of cleaving tetrasporangia to form tetraspores has never been found to include the deposition of pit plugs in any red alga. An alternative interpretation of the nature of the reproductive cells of  H. sanjuanensis  is that they are spermatia or carpospores. The features of small size, sparse chloroplasts with few thylakoids, and near absence of starch contents in the reproductive cells favours the view that these cells are spermatia. If this interpretation is correct, it represents the first evidence of spermatia in any member of the Hildenbrandiophycidae, and it indicates that the possibility of a sexual life history in members of this order should be re-examined.

Evidence of spermatia in Hildenbrandia (Rhodophyta, Hildenbrandiales)