| Abstract: | Halimedales (Chlorophyta) are widely distributed in reefs and lagoons throughout the tropics and subtropics. One genus, Chlorodesmis, consists of simple, branching siphons that persist into the mature sexual phase. In most other genera, only the juvenile stages exhibit such simple anatomy. Their mature, often‐calcified thalli possess complex siphonal anatomy and differentiation depending on the location in the thallus. We obtained a nuclear ribosomal DNA phylogeny encompassing many genera in order to unravel the evolution of this complexity. Results pinpoint Caulerpa as outgroup to a clade with all other genera included in this study. The first dichotomy separates Halimeda from the remaining genera. Distinct gametangial anatomy defines these two principal clades. Within the remainder, Tydemania and its sister Flabellia are nearest neighbors to a clade containing Rhipocephalus, Udotea, Penicillus and Chlorodesmis. The latter three genera are polyphyletic. Therefore, character states of thallus superstructure defining these genera appear to be homoplasies. Chlorodesmis represents a case of neoteny where the juvenile stage persists into reproductive maturity. We will evaluate which morphological characters do change state in agreement with the molecular phylogeny and can, therefore, serve as diagnostic characters for a more natural classification. |
