Species diversity of Gelidium (Gelidiales, Rhodophyta) from Indonesia evaluated by molecular and morphological data, with descriptions of G. balteum sp. nov. and G. brevifolium sp. nov.

Gelidium from tropical waters has frequently been overlooked due in part to its small size and superficial morphological similarity even between phylogenetically different species, presenting a major challenge to our understanding of global biodiversity. To assess the diversity of Gelidium in Indonesia, specimens were collected on intertidal rocky shores of Bali and Lombok Islands over the period of 2009–2026. Phylogenetic analyses based on 5′ region of cytochrome c oxidase subunit I and rbcL sequences revealed that the Indonesian specimens represent eight species: G. crinale, G. indonesianum, G. sentosaense, G. yangmeikengense, and four undescribed species. Herein, we describe two new species based on morpho-anatomical characteristics: Gelidium balteum sp. nov. can be distinguished by ribbon-like thallus, ovoid cystocarps, and elongate tetrasporangial sori, while G. brevifolium sp. nov. can be distinguished by blade-like thallus, spherical cystocarps, and ovoid tetrasporangial sori. In rbcL phylogeny, G. balteum is sister to G. brevifolium. Although both species form dense turfs, G. balteum occurs in several localities in Nusa Lembongan and Lombok, while G. brevifolium is currently restricted to the type locality. Our findings indicate that the species molecularly confirmed in the present study differ markedly from those reported in previous morphologybased studies and highlight the need for further sampling along the complex coastlines of Indonesia to reveal the hidden diversity of the tropical turf-forming Gelidium species in Southeast Asian waters.