Deep genetic divergences and geographic distribution of the red algal genus Caulacanthus (Gigartinales)

An increasing number of studies have demonstrated that genetic differentiation and cryptic diversity in the sea occur over considerably smaller spatial scales than previously comprehended, considering the wide distribution range of many morphologically defined macroalgal species. However, knowledge of the turf-forming red algaCaulacanthusis incomplete regarding its species diversity, as well as genetic differentiation within the genus. We analyzedCaulacanthusspecimens from the NW Pacific, NE Pacific, Central Pacific, SW Pacific, SE Indian, NE Atlantic, and SE Atlantic Ocean using mitochondrial cytochrome oxidase subunit I (COI-5P), plastid ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL), and Rubisco spacer (rbcL-S). The objectives of this study were to 1) determine the number of species that exhibit the morphology ofC. ustulatus, 2) investigate the present distribution pattern ofCaulacanthusspecies, and 3) estimate the degree of genetic connectivity between the populations ofCaulacanthusspecies from different regions. Our results revealed molecular evidence that the genusCaulacanthuscomprises of at least seven species with deep genetic divergence, which is indicative of not only a strong geographical subdivision but also a relatively long temporal discontinuity. Most species exhibited limited geographic distribution, showing considerable genetic divergence in the populations isolated by distance. Our study provides evidence of a greater evolutionary independence ofCaulacanthuspopulations, which have undergone a series of allopatric diversification events.