The photograph below actually depicts a giant clam, Tridacna, from the south Pacific, but notice the pink encrusting material on the outsides of the shell. This is a "coralline" alga, so called because it secretes a calcium carbonate skeleton. Almost all coralline algae are members of the order Corallinales, in the Rhodophyta, but there are calcifying algae in other rhodophyte orders, and even a few cyanobacteria, green "algae", and one kelp which also form coralline structures. Corallines may be ubiquitous encrusters of hard surfaces, and in some reefs are the major structural component, even outnumbering the corals. (See the "Fossil Record" page for more information about coralline rhodophytes.)
It was discovered that when the edible nori stage begins to release gametes, that these fuse together upon the female to produce a stage which then releases spores. The dispersed spores germinate into a smaller alga, which had been described as a separate genus because of its radically different appearance and ecology. This conchocelis phase burrows into the shells of marine animals, where it grows and matures, eventually dispersing conchospores which germinate into the edible phase of the life cycle. When K. M. Drew made this discovery circa 1950, the nori industry was revolutionized, allowing for cultivation on a previously unknown scale.
Additional information about the ecology of rhodophytes may be found at the Geothermal Aquaculture Research Foundation, whose site includes information on coralline algae propogation.
Read also about the importance of coralline algae and about how they reproduce. Find out about competition and grazing effects. These pages are part of Derek W. Keats' Introduction to nongeniculate coralline algae.
You may also be interested in work being done on the relationship between water motion and rhodolith movement being done by Lisa Marrack.
Drew, K. M. 1949. Conchocelis phase in the life history of Porphyra umbilicalis. Nature 164:748.
Littler, M. M. & Littler, D. S. 1984. Models of tropical reef biogenesis: the contribution of algae. Pp. 323-364 in Progress in Phycological Research, Vol. 3, Round and Chapman, eds.
C. S. Lobban & Paul J. Harrison. 1994. Seaweed Ecology and Physiology. Cambridge University Press, Cambridge and New York.
Seaweed soup provided by Go-Bu-Li restaurant in Berkeley.