The life history of Hildenbrandia is mysterious. Unlike many other encrusting algae, Hildenbrandia is not an alternate life cycle stage of an erect alga. It is isomorphic, cycling from crust to crust (1, 2, 3). However, no sexual reproduction has been documented in this genus, not even in culture studies; the only observed form of propagation is via tetraspores (2, 3). As such, we can guess that most Hildenbrandia individuals we come across are haploid, but even these reproduce via tetraspores (i.e., mitotic tetrasporogenesis) (4).
As in the Corallinaceae, the tetrasporangia of Hildenbrandia develop in conceptacles. But conceptacle formation in Hildenbrandia is unusual, and it has been used to place Hildenbrandiaceae into its own order (3, 5). The tetrasporangia of coralline algae are derived from intercalary cells, which then form into conceptacles as surface cells are sloughed off. In contrast, the tetrasporangia of Hildenbrandia spp. are derived from surface cells, which "erode" the perithallus to form a conceptacle (5). This process of tetrasporogensis is reported to occur year round (6).
The anatomy of Hildenbrandia's conceptacles is debated. In addition to tetrasporangia, there are more ambiguous filamentous structures that have been identified by different scientists as paraphyses, trichogynes (suggesting sexual reproduction?), the walls of emptied tetrasporangia, and also fungal hyphae (3).