Age and growth of Lithothamnion muelleri (Corallinales, Rhodophyta) in the southwestern Gulf of California, Mexico

 The nongeniculate coralline alga Lithothamnion muelleri Lenormand ex Rosanoff is commonly found as a free-living rhodolith in the wave-exposed shallow waters of the southwestern Gulf of California. This species has bands along the main branch axes suggesting that growth is periodic and that may be used to determine age and growth. As part of our ongoing research related to rhodoliths and their ecology, we have made observations on the nature of these bands based on their structure in entire plants and on field and laboratory growth experiments. Individual plants from different size classes were tagged, stained with alizarin and returned to the field, while another set was tested in the laboratory. Half of the plants were sampled after 6 months and the rest after 12 months. The data indicate that this species grew at a rate of 0.60 mm yr–1 in the field and 0.87 mm yr–1 in the laboratory. There were no significant differences among branches within a plant, or among plants of different size. Growth was seasonal, with high rates in winter and spring, and low rates in summer and fall. This may explain the observed alternating light-dark bands (up to 4) along branch axes. The growth rates suggest that larger plants (ca 15 cm diameter) may be over 100 years old.