Morphological and molecular characterization of four newly recorded species of the genus <i>Dictyota</i> (<scp>Dictyotales, Phaeophyceae</scp>) from <scp>Japan</scp>

Summary The genus Dictyota (Dictyotales, Phaeophyceae) is a diverse group primarily found in tropical and subtropical waters, with significant ecological roles in coastal ecosystems. In this study, the taxonomy of Dictyota species from Japan was reassessed using a combination of molecular and morphological data. Molecular phylogenetic analyses were conducted using sequences from five genetic markers: chloroplast rbc L, psa A, psb A genes and mitochondrial cox 1 and cox 3 genes. Analyses of concatenated and single gene datasets showed that the Japanese specimens clustered into four well‐supported clades, corresponding to four known species: D. acutiloba J. Agardh, D. humifusa Hörnig, Schnetter &amp; Coppejans, D. pulvinata Sadeghi, Sohrabipour &amp; De Clerck, and D. sandvicensis Sonder. Morphological examinations further supported their identification, with distinctive characteristics, such as the narrow elongated linear thallus with pointed tips and reproductive organs longitudinally zoned in the middle of the thallus in D. acutiloba , the prostrate, overlapping or cushion‐form thallus in D. humifusa and D. pulvinata , and erect thalli with teardrop‐shaped marginal proliferations in D. sandvicensis . The presence of these four species in southern Japan is confirmed, providing their genetic data of multigene markers and detailed morphological descriptions for the first time. Although genetic data of D. acutiloba from Japan was reported in previous studies, this study provides the first detailed morphological description of Japanese specimens. Our findings, along with previous genetic‐based identifications, have provided new insights into the global distributional patterns of these four species, particularly highlighting disjunct distributions of D. humifusa and D. pulvinata , from the Caribbean (Colombia) to the West Pacific and West Indian Ocean, and from the West Indian Ocean to the West Pacific, respectively, showing extensions across very widely separated oceanic regions.