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Phylogenetic relationships in the Sargassaceae were explored using three DNA markers, and the monophyly of its genera was challenged. Nineteen out of 24 currently recognized genera were sampled, representing 63 species. The variable mt23S-tRNA Val intergenic spacer could only be aligned within genera and could not be used to infer intergeneric relationships. The partial mt23S was also useful to delineate genera and was alignable at the family level but provided few informative characters. Analysis of mt23S DNA sequences together with chloroplast-encoded psbA sequences resulted in a better resolved phylogeny. Hormophysa was the first genus to branch off within the Sargassaceae, followed by Myriodesma; then the three genera Caulocystis, Carpoglossum, and Scaberia in unresolved order; and then Acrocarpia. The other taxa studied here were divided over three major clades, but there was no branch support for the monophyly of two of these. The genera Bifurcaria, Cystoseira, Halidrys, and Sargassum appeared polyphyletic. The following taxonomic changes are proposed: a new genus Brassicophycus for Bifurcaria brassicaeformis (Kutzing) E. S. Barton; reinstatement of the genus Sargassopsis for Sargassum decurrens (R. Brown ex Turner) C. Agardh; reinstatement of the genus Sirophysalis for Indo-Pacific Cystoseira trinodis (Forsskal) C. Agardh; reinstatement of the genus Polycladia for the western Indian Ocean species Cystoseira indica (Thivy et Doshi) Mairh, Cystoseira myrica (S. G. Gmelin) C. Agardh, and Acystis heinii Schiffner; and reinstatement of the genus Stephanocystis for the North Pacific Cystoseira species and Halidrys dioica N. L. Gardner. The European Cystoseira species should be split into three genera, but no name changes are proposed yet, because diagnostic characters were found only for the clade including the type species. Some evolutionary trends could be discerned from the mt23S + psbA phylogeny.

The algal family Cystoseiraceae (order Fucales, class Phaeophyceae) includes the genera Acrocarpia, Acystis, Bifurcaria, Bifurcariopsis, Carpoglossum, Caulocystis, Coccophora, Cystophora, Cystoseria, Halidrys, Hormophysa, Landsburgia, Myriodesma, Scaberia and Stolonophora. Among these Cystoseira and Cystophora, the richest in species, are the most representative of the family. Cystoseira is a genus of worldwide distribution with about 80% of the species occurring along the Mediterranean and adjoining Atlantic coasts, while Cystophora is limited to the coasts of Australia and New Zealand. Members of the genus Cystoseira generally synthesise tetraprenyltoluquinols. Compounds of the same structural type have also been isolated from species belonging to the genera Halidrys (northern coasts of England), Bifurcaria (coasts of the Galapagos Islands), and Cystophora. Some species of these last two also elaborate linear diterpenoids as do some Cystoseira species. Tetraprenyltoluquinols have never been found in Caulocystis and Acrocarpia, which instead accumulate acetogenins. Finally, Landsburgia is the sole genus which produces naphthoquinone derivatives. Phytochemical studies of the secondary metabolites of the remaining genera are not reported. The chemistry of secondary metabolites and their distribution in the family Cystoseiraceae will be illustrated, and some taxonomic implications will be discussed.

Summary The spermatozoids of three genera of Cystoseiraceae (Halidrys, Cystoseira and Bifurcaria) have been investigated electron microscopically with respect to their external morphology, and two of them (Cystoseira and Halidrys) also with respect to their microanatomy for the purpose of comparison with Fucus on the one hand and a brown algal zoospore on the other. Special attention has been devoted to the flagellar ‘root’ which is shown to be the homologue of the proboscis of Fucus and to be present also in the zoospore of Scytosiphon. Comparison between Scytosiphon, Dictyota and the Fucoids suggests that the true function of the flagellar root is not mechanical but metabolic or conductional, perhaps in some relation to respiration or gaseous exchange. The evidence is interpreted as giving strong support to the view that Fucus is not primitive but highly specialized and that Cystoseira has remained closer to an ancestral condition in a number of different characters.