<i>Ceramium botryocarpum</i> and <i>C. secundatum</i> re-evaluated

In the British Isles the genus Ceramium is represented by 15 species, divided into to groups. The two groups without cortical spines are (1) fully corticated species and (2) those with ecorticated internodes. Group 1 species are very difficult to distinguish. In particular the key morphological features that discriminate between C. botryocarpum and C. secundatum include the number of periaxial cells and presence of adventitious branching (Maggs and Hommersand, 1993). However, these features may be influenced by the environment. By using various molecular markers, growing cultures in different conditions and crossing experiments we aim to clarify the relationship between the species of group 1.Analysis of the formalin preserved vouchers showed that C. botryocarpum and C. secundatum are morphologically almost identical. They only differ in the number of periaxial cells (6-7 for C. botryocarpum and 7-8 for C. secundatum) and by the more robust, larger thallus of C. secundatum. Culture studies showed that the morphology of Ceramium is highly influenced by the environment. There was crossing with formation of tetrasporophytes between C. botryocarpum and C. secundatum. The phylogenetic analysis with the chloroplast marker (tufA/rpl31) and the mitochondrial marker (cox2-3 spacer, Gabrielsen 2002) clearly demonstrate that C. botryocarpum and C. secundatum are not respectively monophyletic. Analysis of multiple samples and with different techniques confirmed that C. botryocapum Griffiths ex Harvey (1848) is a later synonym of C. secundatum Lyngbye (1819)