Marine Botany

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Crust Aging and Mechanisms of Crust Death

In observing Mastocarpus crusts, about the biggest you will see is 1.0 meter in diameter. I found a few that were almost 70 cm on the shore along the marine station. I heard about people making estimates for crustose lichen ages by knowing their growth rate and measuring crust diameters.
Mastocarpus tetrasporophytes can grow from 1-2 cm in diameter in a year under laboratory regulated conditions. Taking this as a rough estimate of natural growth rate, some of those large crusts could be almost 70 years old. Considering that most of the algae are annuals or live only a few years, 70 years is ancient. Considering that bull kelp will grow a thick stipe about 20m in one year, 1-2cm per year is really slow. An experiment showed that flies with a higher metabolism died earlier than flies with a lower metabolism. Could the same thing be true for algae?

Finding out that an alga might grow to be 70 years old is really neat, but why not older? What are mechanisms of crust death?

Epiphyte smothering can occur, and other intertidal organisms could crowd out the slow-growing crust, but how then do we get a 70 cm wide crust?
Some other crustose algae have been observed to shed their cuticle to slough off epiphytes. Mastocarpus has not been observed to do the same, but it is a possibility. The crust could also produce secondary metabolites that ward off epiphyte colonization. There has to be some explanation.

Mastocarpus pages copyright W. Ludington 1999