Morphology of Laurencia clavata and L-elata (Ceramiales, Rhodophyta) in relation to generic circumscription in the Laurencia complex

Laurencia Lamouroux (Rhodophyta) was recently separated into three genera - Laurencia, Chondrophycus (Tokida et Saito) Garbary et Harper and Osmundea Stackhouse - each of which was newly defined based on vegetative and reprod uctive structures. In this study, the previously unknown vegetative and rep roductive morphology of two Australian endemic species of Laurencia, L. cla vata Sonder and L. elata (C. Agardh) Harvey, was studied, particularly in t he context of the revised generic delineation. These species exhibit vegeta tive axes with four pericentral cells and trichoblast-type spermatangial de velopment. Tetrasporangia are abaxially produced from the existing third an d fourth pericentral cells. L. clavata has terete thalli with distinctive v erticillate branching and is similar to Chondria C. Agardh, rather than to Laurencia, in having an unusually marked constriction at the base of the br anches and starch accumulation in subcortical and medullary cells. Compared to Laurencia, apical cells of this species exhibit a less oblique division ; the resulting recognizable axial cell rows extend somewhat below the bran ches, and particularly at a young stage they are also clear throughout bran chlets. However, other vegetative and spermatangial structures show that L. clavata is more closely allied to Laurencia than to Chondria, and it is pl aced in Laurencia. By contrast, L. elata exhibits morphology typical of Lau rencia and is characterized by large, robust, compressed thalli with fastig iately distichous branching and an extensive secondary cortex. Furthermore, it appears to be distinct from similar species in sometimes having a paras itic species of Janczewskia Solms-Laubach (Rhodophyta). Taxonomy of Laurenc ia is discussed on the basis of these and previous studies.