The conspecificity of Ceramium pacificum and Ceramium washingtoniense (Ceramiaceae, Rhodophyta)

Ceramium pacificum and Ceramium washingtoniense are two corticated Ceramium species recognized by their prolific side branches. Molecular analysis of specimens, including those from the type localities and the type specimens, indicates that these two species are conspecific. Two of the characters used to distinguish these species, colour and size, are plastic features that do not define these as separate species. The other character, degree of cortication in younger branches, is shared by both species. This study also shows the value of historic herbarium specimens and why they continue to be a vital resource for taxonomists.