The existence of widespread or cosmopolitan species has been widely accepted in marine organisms because of their high dispersal potential and the continuity of the marine environment (Avis, 1998). However, recent studies using molecular data have revealed that many supposedly widespread macroalgal species consist of cryptic species complexes (Díaz-Tapia et al., 2018; Nauer et al., 2019; Vieira et al., 2019), raising questions concerning the real distribution of such cosmopolitan species. Widespread species have frequently been found to be highly genetically differentiated across regional boundaries (Díaz-Tapia et al., 2018; Yang et al., 2020) and sometimes exhibit fine-scale endemism (Leliaert et al., 2018; Yang and Kim, 2018). Generally, regional endemism is considered to have evolved in regions that have experienced a stable environment over a long period (Myers and Giller, 1988).

With the discovery of cryptic speciation, molecular analyses have been developed to identify and delimit species (Díaz-Tapia et al., 2020; Lagourgue et al., 2022). Among the widespread macroalgal species, molecular data have clarified the real distribution of traditionally identified species solely based on morphological characteristics (Yang et al., 2015; Boo et al., 2019; Kang et al., 2021), with deep genetic divergence seen between regional populations owing to their limited natural dispersal ability (Nauer et al., 2019; Yang et al., 2021b). Other widely distributed species showed genetic affinities among populations despite separation by large oceanic distances, suggesting other dispersal mechanisms for the present distribution pattern (Fraser et al., 2013; Sherwood et al., 2014). Such long dispersal mechanisms can be explained by the artificial distribution of non-buoyant macroalgae (Fraser et al., 2013). Although the hidden algal diversity is increasingly being discovered, our knowledge on the global diversity and distribution of red macroalgae is still incomplete.

The turf-forming red algal genus, Caulacanthus Kützing, is a component of intertidal ecosystems and is commonly found in temperate and tropical marine waters (Guiry and Guiry, 2023). The genus Caulacanthus includes three currently accepted species, Caulacanthus okamurae Yamada, Caulacanthus salifugus A. B. Cribb, and Caulacanthus ustulatus (Turner) Kützing (Guiry and Guiry, 2023). The type species of the genus, C. ustulatus, was described from Cádiz, Spain, and subsequently reported in various regions, including the Western Pacific Ocean, western North America, Mediterranean Sea, Africa, Australia, and New Zealand (Guiry and Guiry, 2023). Whether the actual distribution of this species belong to the widespread species, Caulacanthus was investigated by Zuccarello et al. (2002), who conducted molecular analyses using Rubisco and cox2‐3 spacers to confirm the phylogenetic relationships between collections from the Northwest (NW) Pacific (including Korea and China), tropical Western (W) Pacific (including the Philippines), Southwest (SW) Pacific (including Australia), Northeast (NE) Pacific (including the USA), and the NE Atlantic (including France, Spain, and Portugal). They found that C. ustulatus included two distinct phylogenetic groups (Pacific and Atlantic) and concluded that the Pacific lineage indicated C. okamurae, which has long been regarded as a synonym of C. ustulatus (Searles, 1968; West and Calumpong, 1990; Rueness, 1997). C. okamurae was described based on the morphological features of vegetative plants (Yamada, 1933). Since then, taxonomic position of these species has been under debate (Searles, 1968; West and Calumpong, 1990); however, the distinctness of C. okamurae has been accepted based on different cystocarpic development and gemination patterns of tetraspores (Kamura, 1963), as well as molecular data (Rueness and Rueness 2000; Zuccarello et al., 2002). This suggests the need to confirm the actual distribution range of C. ustulatus in different regions, comparing with specimens from type locality through molecular analyses.

C. okamurae is native to the NW Pacific, including Japan, Korea, China, and Taiwan, and has recently been reported in many regions, including France (Zuccarello et al., 2002; Burel et al., 2019), Washington (Zuccarello et al., 2002), California (Miller, 2012; Hartnell College Genomics Group et al., 2020), Gulf of California (Norris et al., 2017), Italy (Petrocelli et al., 2020), and Spain (Bárbara et al., 2019). Recently, after molecular examinations, the name of C. ustulatus has been changed to C. okamurae in several region (Zuccarello et al., 2002; Petrocelli et al., 2020). C. okamurae forms a dense turf on substrates, thereby providing shelter for other macroalgae and marine invertebrates (Petrocelli et al., 2020). Another taxonomically accepted species, C. salifugus, was described based on sterile material growing in a semi-marine cavern in Queensland, Australia (Cribb, 1965), and has not been recorded since then. Other species, such as C. spinellus (Hooker f. Harvey) Kützing from New Zealand, C. indicus Weber-van Bosse from Indonesia, and C. rigidus Kützing from Senegal, were previously accepted within the genus. However, they have been regarded as synonyms of C. ustulatus because of their similar morphologies (Searles, 1968; West and Calumpong, 1990; Adams, 1994).

Mitochondrial DNA sequences generally constitute a powerful tool for species delimitation, particularly in red algal groups that are difficult to resolve using morphological data (Yang et al., 2021a; Muangmai et al., 2022; Preuss et al., 2022). The advent of species delimitation methods based on DNA sequences has provided a useful method for validating previously described species and identifying new evolutionary units (Muangmai et al., 2022; Boo et al., 2022). The present study involves Caulacanthus samples from 14 countries and addresses species diversity within the genus. We collected the specimens from Cádiz in Spain, which is type locality of C. ustulatus, to determine the regional distribution to which C. ustulatus was distributed.

In this study, using mitochondrial COI-5P and rbcL data with a previously determined rbcL‐S spacer, we addressed several questions concerning taxa regarded as being C. ustulatus and C. okamurae. Specifically, we aimed to 1) determine the number of species that exhibit the morphology of C. ustulatus, 2) investigate the present distribution pattern of the genus Caulacanthus, and 3) estimate the degree of genetic connectivity between the populations of Caulacanthus species from different regions.

We suggest at least seven OTUs within the genus Caulacanthus, through a combination of phylogenetic analyses using the mitochondrial and plastid sequence data, molecular species delimitation methods, and haplotype network, although bPTP resolved two additional OTUs within the Lineages 1 and 6 ( Figures 1 , 2 ). The finding of cryptic species diversity was similar to that for other red algal taxa, particularly turf algae, which has typically been overlooked because of their small size, structural simplicity, and densely entangled nature (Díaz-Tapia et al., 2020). Together with a detailed morphological description, our study provides molecular evidence that the generitype C. ustulatus is not a species with a widespread distribution but rather confined to the E Atlantic Ocean ( Figures 5 , 6 ). Another previously recognized species, C. okamurae, was found in the Northern Hemisphere, including nine countries. This finding was consistent in terms of previous studies that reported that C. okamurae was introduced from several countries (Petrocelli et al., 2020). The other five lineages were distributed in Japan/Taiwan/Australia (Lineage 3), New Zealand (Lineage 4), Australia (Lineage 5), Hawaii (Lineage 6), and the Philippines (Lineage 7) ( Figure 4 ).

The COI-5P haplotype network of Caulacanthus clearly showed deep genetic divergence with considerable mutational steps between the lineages ( Figure 3 ). The pairwise level of sequence divergence reported herein provided a clear gap between maximum intra-lineage divergence of 3.62% and minimum inter-lineage divergence of 9.4%, which provided the evidence for an independent genetic lineage. The high level of divergence within the Caulacanthus complex is indicative of not only a strong geographical subdivision but also a long temporal discontinuity (Fraser et al., 2010). Deep genetic divergences detected among the highly divergent lineage requires taxonomic validation and formal description through sufficient observations of regional specimens. In the case of Australia and New Zealand, some species names have previously been misattributed owing to morphological similarity within the region (Cribb, 1965; Searles, 1968; Rueness, 1997). C. salifugus was first reported in Paradise Cave in the SE Queensland (Cribb, 1965), in the middle of distribution between the Lineages 3 and 5, therefore, molecular evidence is acquired to confirm its accurate taxonomic position. C. spinellus was originally described in New Zealand, after it was reduced to synonymy with C. ustulatus. In addition, further studies are needed in other regions, such as Indonesia and Senegal, where synonyms of C. ustulatus have been reported (Searles, 1968; West and Calumpong, 1990; Rueness, 1997).

Based on molecular evidence, C. okamurae showed broad trans-oceanic distribution in the Northern Hemisphere (including Korea, Japan, China, Taiwan, France, Spain, Italy, Canada, and the USA) ( Figure 4 ). C. okamurae has been typically misidentified as C. ustulatus because of its resemblance to the gross morphology (Petrocelli et al., 2020). As the distribution of two different species, C. ustulatus and C. okamurae, has been identified in Europe, molecular screening of Caulacanthus specimens is required to assess their detail distribution. Moreover, our study provides the first molecular evidence of C. okamurae in British Columbia, Canada, where two haplotypes (H01–H02) were found ( Figure 2 ). Populations from the NE Pacific Ocean with two haplotypes could serve as evidence that this population had been introduced from the NW Pacific Ocean earlier than the population from the NE Atlantic Ocean. The lack of shared haplotypes between the NW and NE Pacific oceans also supported this hypothesis. Non-native species have been increasingly reported in many regions, facilitated by both natural and anthropogenic stressors (Nelson et al., 2021). Climate change can be predicted to affect species range by encouraging the settlement of temperate non-native species (Miller et al., 2011). Drastic temperature shifts, including the series of El Niño/La Niña-Southern Oscillation events, along with increased sea temperature could have favored the settlement and spreading of Caulacanthus in several areas (Miller et al., 2011). Moreover, the spread of non-native seaweed C. okamurae could have facilitated increased abundance and diversity of other seaweeds and invertebrates by forming novel turf in intertidal zones (Smith et al., 2014; Petrocelli et al., 2020).

In the NW Pacific, two Caulacanthus species were found, namely C. okamurae and Lineage 3, of which Lineage 3 was found from tropical to subtropical regions, including Shimen and Okinawa, Cooktown, and Magnetic Island ( Figure 4 ). Populations in the NW Pacific Ocean might be geographically separated by restricted gene flow. For example, the haplotypes from Korea/China were not shared with that from the Pacific coast of mainland Japan affected by Kuroshio Current, and this genetic division was supported by high F _ST values ( Table 2 ). The distribution of the NW Pacific haplotypes is associated with ocean currents, which might mediate gene flow between populations. The gene flow by ocean currents was also shown in the distribution of haplotype H04, which occurred from Nanji Island to the coasts of Korea and Dailan ( Figure 5 ). The occurrence of H04 in the entire coast of Korea may be the result of high gene flow between the Korean populations due to complex oceanic currents, which has also been observed in red algal species of the genus Chondrus Stackhouse (Yang and Kim, 2022). The influence of ocean currents affects population distribution may be confirmed in future studies including populations along the coasts where the Tsushima Warm Current passes.

The distribution of the seven Caulacanthus species provides information about the processes that have shaped the evolutionary history of this group, which were previously obscured by treating it as a single species. All lineages included a well-supported group with a deep split from each other, indicating that speciation occurred on an ocean basin scale. Therefore, our study provides evidence of a greater evolutionary independence among the populations of Caulacanthus, as measured by mtDNA divergence. Based on our phylogenetic reconstructions, we interpreted that the Caulacanthus species complex have undergone a series of allopatric diversification events (Derycke et al., 2008), showing considerable genetic divergence in the resulting population isolated by distance (Díaz-Tapia et al., 2020). The case of two subclades with genetic divergence of 3.62% within Lineage 5, that is, populations between the western and east coast of Australia, establishes genetic divergence by distance.

Accurately recognizing species boundaries is key to biodiversity research and conservation, as species are operational units for biodiversity quantification and management (Boo et al., 2019; Yang et al., 2020; Preuss et al., 2022). Our results revealed seven Caulacanthus lineages, most of which had limited geographic distribution. The lack of gene flow among lineages suggests that they have attained geographic isolation and may deserve to be assigned as species (Fraser et al., 2010). Five lineages would require validation and formal description. Additional studies should involve an integrative approach that considers other characteristics, such as morphology and ecology. Although the sampling in this study may not be sufficiently large to conclusively rule out the presence of lineages in an area, which was not sampled, the pattern emerging from our data showed seven lineages with regional distributions rather than a single cosmopolitan species. Further studies on the evolutionary relationships and genetic variation of regional populations, along with population collection from other countries, are essential to understand and effectively conserve and manage species of the genus Caulacanthus.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/ Supplementary Material .

MYY and MSK conceived and designed the study and performed the collections. MYY performed laboratory work and the data analyses. MYY and MSK participated in the interpretation of the data and writing the manuscript. All authors contributed to the article and approved the submitted version.