Phylogenetic analysis of rhodolith formation in the Corallinales (Rhodophyta)

<div><p>Although the ecological importance of rhodolith (maerl, free-living coralline algae) beds is well-known, rhodolith-forming species have been neglected in molecular phylogenetic studies. This is the first molecular systematic study aimed at understanding whether the rhodolith habit is a fixed feature in lineages and determining the relationship (phylogenetic vs. environmental) between rhodolith and crustose habits. Phylogenetic relationships of rhodolith-forming species and encrusting coralline algae at generic and species levels were analysed using SSU rDNA and <i>psb</i>A sequences. Extensive sampling in the European North Atlantic, Pacific and Caribbean Mexico of <i>Phymatolithon, Lithothamnion, Lithophyllum</i> and <i>Neogoniolithon</i> taxa forming rhodoliths and crusts was accompanied by examination of type or topotype material. Phylogenetic reconstruction showed that <i>Neogoniolithon</i> contained a monophyletic group of rhodolith-forming species whereas other rhodolith-formers were closely related to encrusting forms in the genera <i>Phymatolithon, Lithothamnion, Mesophyllum, Hydrolithon, Spongites</i> and <i>Sporolithon</i>. DNA analysis showed that the crust-forming <i>Lithophyllum</i> cf. <i>incrustans/dentatum</i> also forms rhodoliths with a stone nucleus that occur on rocky shores. In contrast, species that form beds of non-nucleate rhodoliths (e.g. <i>Neogoniolithon spectabile, N. strictum, Lithophyllum</i> cf. <i>incrustans</i>/<i>dentatum</i> or sp. 1 and <i>Phymatolithon calcareum</i>) rarely form crusts. The rhodolith habit cannot be used to delimit species for taxonomic or identification purposes. Extensive taxonomic revision will be required to deal with problems such as the position of specimens identified as <i>Lithophyllum margaritae</i> in two unrelated lineages.</p></div>