Phylogeography and genetic connectivity of the marine macro-alga Sargassum ilicifolium (Phaeophyceae, Ochrophyta) in the northwestern Pacific1

Multigene phylogenetics of Sargassum (Phaeophyceae) revealed low molecular diversity in contrast to high morphological variability in the NE Atlantic Ocean

Sargassum species play a key role in habitat formation in tropical and subtropical regions; however, species identification has been hampered by the phenological plasticity exhibited in response to environmental conditions and life history. Molecular phylogenetics has challenged taxa circumscriptions and proven critical in delimiting species in this genus. Yet, the Atlantic species of Sargassum remain poorly understood, and recent studies have shown low molecular diversity between the species in the NW Atlantic. Here, we expand the taxon sampling to the NE Atlantic to assess the diversity of Sargassum in the Atlantic basin, based on a comprehensive morphological and multigene approach. We selected genes commonly used in delineating species in this genus (ITS2, rbcLS, cox3, mtsp) and explored additional markers (cox2, nad6, psbC, clpC, atpB) to infer the phylogenetic relationships between the morphospecies observed in the NE Atlantic. Phylogenetic analyses using single-gene and multigene alignments including 185 new sequences confirmed the low molecular diversity and supported the distinction of a single clade in Sargassum section Sargassum of N Atlantic benthic species. In contrast, morphological analysis resulted in the identification of 10 species and three new morphospecies that we described here but opt not to equate to species until further molecular evidence is available. Our results were congruent with previous findings from the NW Atlantic and highlight the morphological and ecological diversity of Sargassum in the Atlantic. These results suggest a recent colonization and incipient speciation of Sargassum in the Atlantic basin and showcase the need of further high-throughput analyses.

Systematics and Biogeography of the Red Algal Genus Yonagunia (Halymeniaceae, Rhodophyta) from the Indo-Pacific Including the Description of Two New Species from Taiwan

Carpopeltis maillardii has been regarded as a widely distributed species in the Indo-Pacific region. In this study, we analyzed the genetic diversity of C. maillardii and related species collected from Taiwan and the Indian Ocean based on rbcL sequences, in order to elucidate species boundaries, diversity, and biogeographic patterns. Our analyses show that C. maillardii specimens are only distantly related to the genus Carpopeltis (type: C. phyllophora) but instead form a clade together with species of Yonagunia. We therefore propose the new combination Yonagunia maillardii comb. nov. In addition, two new species (Yonagunia palmata sp. nov. and Yonagunia taiwani-borealis sp. nov.) are described from Taiwan. The close relationship of Yonagunia to Grateloupia is corroborated by detailed observations of the female reproductive structures, which demonstrate that the development of auxiliary cell ampullae before and after diploidization is similar to that of Grateloupia sensu stricto. Namely, the ampullae are composed of only two orders of unbranched filaments in which only a few ampullar cells are incorporated into a basal fusion cell after diploidization of the auxiliary cell and the pericarp consists almost entirely of secondary medullary filaments. Of all Yonagunia species, Y. maillardii has the widest distribution in the Indo-Pacific, and can be identified in the field by its relatively thin, feathery, and highly branched morphology. Most other species, including those that occur in Taiwan, are seemingly more range-restricted. Our phylogenetic analyses resulted in a well-resolved phylogeny of Yonagunia, with an origin estimated in the Eocene-Oligocene, and diversification of species mainly in the Miocene.

Limited Genetic Connectivity Among Sargassum horneri (Phaeophyceae) Populations in the Chinese Marginal Seas Despite Their high Dispersal Capacity

Sargassum horneri is a habitat-forming species in the Northwest Pacific and an important contributor to seaweed rafts. In this study, 131 benthic samples and 156 floating samples were collected in the Yellow Sea and East China Sea (ECS) to test the effects of seaweed rafts on population structure and connectivity. Our results revealed high levels of genetic diversity in both benthic and floating samples based on concatenated mitochondrial markers (rpl5-rps3, rnl-atp9, and cob-cox2). Phylogenetic analyses consistently supported the existence of two lineages (lineages I and II), with divergence dating to c. 0.692 Mya (95% HPD: 0.255-1.841 Mya), indicating that long-term isolation may have occurred during the mid-Pleistocene (0.126-0.781 Mya). Extended Bayesian skyline plots demonstrated a constant population size over time in lineage I and slight demographic expansion in lineage II. Both lineages were found in each marginal sea (including both benthic and floating samples), but PCoA, FST , and AMOVA analyses consistently revealed deep genetic variation between regions. Highly structured phylogeographic pattern supports limited genetic connectivity between regions. IMA analyses demonstrated that asymmetric gene flow between benthic populations in the North Yellow Sea (NYS) and ECS was extremely low (ECS→NYS, 2Nm = 0.6), implying that high dispersal capacity cannot be assumed to lead to widespread population connectivity, even without dispersal barriers. In addition, there were only a few shared haplotypes between benthic and floating samples, suggesting the existence of hidden donors for the floating masses in the Chinese marginal seas.

Asymmetrical gene flow in five co-distributed syngnathids explained by ocean currents and rafting propensity

Ocean circulation driving macro-algal rafting is believed to serve as an important mode of dispersal for many marine organisms, leading to predictions on population-level genetic connectivity and the directionality of effective dispersal. Here, we use genome-wide single nucleotide polymorphism data to investigate whether gene flow directionality in two seahorses (Hippocampus) and three pipefishes (Syngnathus) follows the predominant ocean circulation patterns in the Gulf of Mexico and northwestern Atlantic. In addition, we explore whether gene flow magnitudes are predicted by traits related to active dispersal ability and habitat preference. We inferred demographic histories of these co-distributed syngnathid species, and coalescent model-based estimates indicate that gene flow directionality is in agreement with ocean circulation data that predicts eastward and northward macro-algal transport. However, the magnitude to which ocean currents influence this pattern appears strongly dependent on the species-specific traits related to rafting propensity and habitat preferences. Higher levels of gene flow and stronger directionality are observed in Hippocampus erectus, Syngnathus floridae and Syngnathus louisianae, which closely associated with the pelagic macro-algae Sargassum spp., compared to Hippocampus zosterae and the Syngnathus scovelli/Syngnathus fuscus sister-species pair, which prefer near shore habitats and are weakly associated with pelagic Sargassum. This study highlights how the combination of population genomic inference together with ocean circulation data can help explain patterns of population structure and diversity in marine ecosystems.

Contributions to the knowledge of Pseudolevinseniella (Trematoda: Digenea) and temnocephalans from alien crayfish in natural freshwaters of Thailand

Redclaw crayfish (Cherax quadricarinatus) is a decapod species originating from Australian freshwater. For more than two decades, these crayfish have been re-distributing to environments in many countries, including Thailand. Moreover, they can carry endosymbionts and/or ectosymbionts into new environments. The aim of this study was to introduce a morphological description of Pseudolevinseniella anenteron as a metacercaria of the endoparasites of redclaw crayfish collected from natural water sources in Thailand. The occurrence of two ectosymbiotic temnocephalans (Diceratocephala boschmai and Temnosewellia sp.) in C. quadricarinatus was also reported. The internal morphology of P. anenteron, D. boschmai and Temnosewellia were described and discussed. The surface ultrastructure of the multidentate spines on the body and the metacercarial cyst wall of P. anenteron was investigated by scanning electron microscopy (SEM). By performing a search of the GenBank nucleotide database of partial sequences of 18S, 28S rDNA and cytochrome c oxidase subunit I (cox1), P. anenteron was found to be related to Maritrema, and Temnosewellia was found to be related to T. fasciata. However, according to the cox1 gene, Temnosewellia was found to be similar to T. minor. These results reveal that redclaw crayfish that inhabit natural freshwaters in Thailand may harbour endoparasites and ecto- and endosymbionts. Furthermore, these findings may be able to monitor invasive or non-invasive species in an ecosystem.