Hildenbrandia rubra is a red seaweed (Phylum Rhodophyta) that looks like a rusty stain on rocks; hence the common name - rusty rock. The name "rubra" is latin for red. A closely related species Hildenbrandia occidentalis also occurs on our coast, but it is much less common, is thicker and darker than H. rubra, and does not grow as high in the intertidal zone as H. rubra. It can only be positively distinguished from H. rubra by looking at microscopic features.
Description:
Hildernbrandia rubra forms very thin (0.2 - 0.5 mm) medium to dark red crusts on rocks. It adheres very strongly to the substratum at all points (unlike the many other erect algae which are only attached at the holdfast). The thinnest area of the algae are partially transparent.
Ecology:
Hildenbrandia rubra is found year-round from the splash zone to the subtidal zone and is the only encrusting red algae that grows in the upper region of the intertidal zone. It is very tolerant of extremes in temperature, light and salinity, and grows horizontally very slowly, often less than 1 mm/year. H. rubra can even survive for several years after being over-grown with barnacles, mussels and other macroalgae, which has been attributed to its very low metabolic needs. While growth is a form of asexual reproduction sexual reproduction in H. rubra has never been reported.
The thin encrusting growth form of H. rubra is potential food for limpets and littorine snails. However, grazing on H. rubra can cause considerable damage to these grazer's radula compared to algae with much larger cells (e.g. Ulva spp.). H. rubra avoids grazers by growing in cracks, crevices and other refuge sites that grazers cannot easily reach (e.g. in the high zone, under mussels or barnacles). If grazed, H. rubra can regenerate damaged tissues.
The thin encrusting growth form of H. rubra is potential food for limpets and littorine snails. However, grazing on H. rubra can cause considerable damage to these grazer's radula compared to algae with much larger cells (e.g. Ulva spp.). H. rubra avoids grazers by growing in cracks, crevices and other refuge sites that grazers cannot easily reach (e.g. in the high zone, under mussels or barnacles). If grazed, H. rubra can regenerate damaged tissues.
To learn more:
Berteness, M.D., Yund, P.O. & Brown, A.F. 1983. Snail grazing and the abundance of algal crusts on a sheltered New England rocky beach. Journal of Experimental Marine Biology & Ecology, 71:147-167.
Dethier, M.N. 1994. The ecology of intertidal algal crusts: variation within a function group. Journal of Experimental Marine Biology & Ecology, 177:37-71.