In morphology, the novel alga was most similar to Delamarea attenuata Hariot. They shared terete rough thalli with an attenuated basal portion and blunt tip, thallus architecture composed of large inner cells and a cortical layer developing large clavate or barrel-shaped cortical cells, and the occurrence on the erect thalli of both unilocular and plurilocular zoidangia among large cortical cells. However, the novel alga was distinctive in having longer thalli with rare branching, shorter cortical cells, and occurrence of branches at the tips of the plurilocular zoidangia. Cladothele, Punctaria and Trachynema, Striaria and Asperococcus, which showed close phylogenetic relationships in our molecular phylogeny based on the concatenated DNA sequences of six genes, shared similar thallus morphological features: terete parenchymatous thalli of diffuse growth bearing unilocular and plurilocular zoidangia among large cortical cells, and terminal and lateral phaeophycean hairs. Although Punctaria spp. have foliose thalli, their juvenile thalli show similar terete stages with apical and opposite lateral hairs, which are common to those genera. Our multigene molecular phylogeny, including most of the related genera, showed that the novel alga is distinct from any of those genera. Therefore, for this novel alga we propose the establishment of a new genus and species, Setoutiphycus delamareoides gen. & sp. nov.. In contrast, rbcL of other ectocarpalean genera with similar thallus architecture (e.g., Coelocladia, Litosiphon, Pogotrichum and Stictyosiphon) showed distant phylogenetic relationships with the clade, suggesting convergent evolution of the thallus architecture in Ectocarpales.
Small brown algae having parenchymatous thalli and multiple chloroplasts with projected pyrenoids have been generally classified in Dictyosiphonales13,14,15 or in Ectocarpales s.l. including Ectocarpales s.s., Dictyosiphonales, Chordariales and Scytosiphonales9,16. Within these orders, members were placed in the families Asperococcaceae, Adenocystaceae, Coelocladiaceae, Delamareaceae, Punctariaceae, Striariaceae, etc.10,16,17. For Delamarea and Cladothele, a new family Delamareaceae comprised of Cladothele, Coelocladia, Delamarea and Stschapovia was proposed based on the anatomical similarity of possessing large cortical cells (paraphyses)18. Later, a new order Delamareales was proposed for the family, assuming an isomorphic life history alternating between macrothalli forming plurilocular gametangia (gametophyte) or unilocular zoidangia (sporophyte) 19. However, Coelocladia arctica Rosenvinge and Delamarea attenuata were shown by unialgal culture studies to have heteromorphic life histories20,21, and since then the order has not been cited.
Because monophyly of the families with parenchymatous thalli in Dictyosiphonales or Ectocarpales s.l., was not supported in molecular phylogenetic studies6,22, expansion of Chordariaceae to include members that used to be classified in independent families such as Punctariaceae and Striariaceae was proposed6. In contrast, small brown algae having parenchymatous thalli and a single chloroplast with projected pyrenoids have been classified in Scytosiphonales23, and in spite of this unique cytological feature and highly supported monophyly, they are classified as a family nested in Ectocarpales.
However, currently roughly 140 genera are included in Ectocarpales s.l., more than 100 of which are of Chordariaceae, and these numbers are exceptionally great, considering the genetic divergence within each order of the Phaeophyceae2,5,7,11 (Supplementary Information 4). The DNA sequence divergence of rbcL genes ranges from about 5–15% within each brown algal order. However, divergence was less than 10% in Ectocarpales s.l. and not especially high compared with other orders (Supplementary Information 4). In our molecular phylogeny, monophyly of several genera of former members of Dictyosiphonales (i.e., Delamareaceae [Delamarea, Cladothele], Punctariaceae [Punctaria, Trachynema] and Striariaceae [Striaria, Asperococcus] sharing the following morphological features was supported: Parenchymatous, terete or foliose thalli of diffuse growth, with terminal and lateral (often opposite) phaeophycean hairs; normally forming both unilocular and lanceolate to ovoid plurilocular zoidangia; cells with many discoid chloroplasts with projected pyrenoids. Remarkably, in spite of its close morphological similarity with these genera, Coelocladia was shown to have phylogenetic relationship instead with the clade of Cladothele, Delamarea, Punctaria, Setoutiphycus, and Trachynema. Indeed, an independent family Coelocladiaceae has been proposed for Coelocladia, based on the unique morphology of the plurilocular zoidangia showing a clustered or crown-like appearance and the occasional sympodial branching of the primary filament21. In contrast, Dictyosiphon, the type of Dictyosiphonaceae and Dictyosiphonales, does have parenchymatous terete thalli, but the genus is unique in showing apical growth by a single apical cell, and forming only unilocular zoidangia embedded in the subcortical and peripheral layers9.
Although the number of taxa we examined were rather limited, our multigene molecular phylogeny based on six genes showed considerable improvement of the phylogenetic resolution of families within Ectocarpales s.l.. Therefore, we expect that the application of multigene molecular phylogeny to additional taxa will give clues for obtaining a better taxonomy of the family. As to the taxonomy of the three families traditionally used for the genera comprising the clade including Setoutiphycus (i.e., Delamareaceae A.D.Zinova 1953, Punctariaceae (Thuret) Kjellman 1880, Striariaceae Kjellman 1890), Punctariaceae has taxonomic priority. Therefore, it is possible to reappraise Punctariaceae for the lineage in reorganizing current Chordariaceae by subdividing it to several monophyletic lineages sharing distinctive morphological features. However, for the moment, we suspend any taxonomic treatment, and provisionally place Setoutiphycus in Chordariaceae, Ectocarpales s.l..
As to the biogeography of Setoutiphycus delamareoides, at present it has been only found from the western end of the Seto Inland Sea, Japan, and it is possibly endemic to the region (Supplementary Information 1). Similarly, an endemic red alga Neorhodomela enomotoi Masuda & Kogame was described from the Seto Inland Sea, and has not been reported from any other coasts24. Members of Neorhodomela are cool-temperate or cold-water species, and the localities of the species appear to represent the southern limits of their distributional ranges24. In spite of the low latitude (34º N) and short distance from the main flow of the Kuroshio Warm Current, water temperatures at the locality are relatively low (monthly average is from 9.5 to 26.5ºC; Wanishi 2004), because of the enclosed geography of the area. The Seto Inland Sea repeatedly dried due to sea level regression during the glacial periods. Therefore, the history of the flora is rather recent, since the present sea level recovered after the LGM of only ca. 10,000 years ago25. Therefore, it is unlikely that the evolution of the genus and species occurred within this area. It has been noted that the macroalgal flora of the area is more similar to the cool-temperate Pacific coast of northern Honshu (Tohoku region) than that of the adjacent areas in Pacific Shikoku and Kyushu where the water temperature is higher26. This is explained as the result of separation of the populations that survived in the refugia in southern Japan during LGM27: during the northward expansion of the populations after the LGM, some of them survived in the Seto Inland Sea26,27,28. Therefore, it is possible that S. delamareoides has a broader distributional range, at least in Japan, such as northern Honshu. However, if not, the species can be endangered by the rise of seawater temperature in the area due to global climate change29,30, because the Seto Inland Sea is enclosed at its northern end, so the population cannot spread to colder northern coasts.
Description And Diagnosis
Setoutiphycus gen. nov. H. Kawai & T. Hanyuda
Typus: Setoutiphycus delamareoides H. Kawai & T. Hanyuda
Erect thalli, filiform, rarely branched, attenuated towards the base, blunt at the tip, parenchymatous, solid when young and becoming hollow with age, composed of large colorless inner cells and barrel-shaped cortical cells and phaeophycean hairs. Plurilocular and unilocular zoidangia formed on the same thallus at the end of subcortical cells among the large cortical cells. Plurilocular zoidangia conical to lanceolate, often branched at the tip. Unilocular zoidangia ovate. Each cell containing many discoidal chloroplasts with projected pyrenoids.
The new genus resembles Delamarea in gross morphology and anatomy, but differs in the longer, rarely branched thallus and shorter cortical cells. The species differs from Cladothele in the epilithic habit and rare branching, and from Trachynema in having large cortical cells. Nucleotide sequences of mitochondrial cox1 and cox3, chloroplast atpB, psbA and rbcL genes are also distinctive.
Setoutiphycus delamareoides sp. nov. H. Kawai & T. Hanyuda Figs. 1, 2.
Erect thalli, epilithic, solitary or caespitose, filiform, simple or rarely branched, attenuated towards the base, blunt at the tip, yellowish brown in color, up to about 15 cm in height, up to about 2 mm in diameter, parenchymatous, solid when young and becoming hollow with age, composed of 1–2 layers of large colorless inner cells, barrel-shaped cortical cells, and phaeophycean hairs. Cortical cells measure up to 100 µm in height and up to 60 µm in diameter. Plurilocular and unilocular zoidangia on the same thallus, at the end of cortical cells, among the cortical cells. Plurilocular zoidangia conical to lanceolate, often branched at the tip, projected from the cortical cells, up to 120 µm in height and up to 72 µm in diameter. Unilocular zoidangia ovate, up to ca. 60 µm in height and up to ca. 50 µm in diameter. Each cell containing many discoidal chloroplasts with projected pyrenoids. Nucleotide sequences of mitochondrial cox1 and cox3, chloroplast atpB, psbA and rbcL genes are also distinctive.
SAP115639, Suo-Oshima (33.9407 N 132.4016 E), Yamaguchi, Japan, 10, April 2017.
Etymology: The genus name refers to the original locality. The specific epithet refers to the morphological features of the thallus.