Marianela Zanolla

Endemic or introduced? Phylogeography of Asparagopsis (Florideophyceae) in Australia reveals multiple introductions and a new mitochondrial lineage.

The red seaweed Asparagopsis taxiformis embodies five cryptic mitochondrial lineages (lineage 1–5) introduced worldwide as a consequence of human mediated transport and climate change. We compared globally collected mitochondrial cox2‐3 intergenic spacer sequences with sequences produced from multiple Australian locations and South Korea to identify Asparagopsis lineages and to reveal cryptic introductions. We report A. taxiformis lineage 4 from Cocos (Keeling) Islands, Australia, and the highly invasive Indo‐Pacific Mediterranean lineage 2 from South Korea and Lord Howe Island, Australia. Phylogeographic analysis showed a clear haplotype and geographic separation between western Australian and Great Barrier Reef (GBR) isolates belonging to the recently described lineage 5. The same lineage, however, was characterized by a substantial genetic and geographic break between the majority of Australian specimens and Asparagopsis collections from South Solitary Island, Southern GBR, Lord Howe Island, Kermadec Islands, Norfolk Island, New Caledonia and French Polynesia. The disjunct geographic distribution and sequence divergence between these two groups supports the recognition of a sixth cryptic A. taxiformis mitochondrial lineage. As climatic changes accelerate the relocation of biota and offer novel niches for colonization, periodic surveys for early detection of cryptic invasive seaweeds will be critical in determining whether eradication or effective containment of the aliens are feasible.

Assessing global range expansion in a cryptic species complex: insights from the red seaweed genus Asparagopsis (Florideophyceae)

The mitochondrial genetic diversity, distribution and invasive potential of multiple cryptic operational taxonomic units (OTUs) of the red invasive seaweed Asparagopsis were assessed by studying introduced Mediterranean and Hawaiian populations. Invasive behavior of each Asparagopsis OTU was inferred from phylogeographic reconstructions, past historical demographic dynamics, recent range expansion assessments and future distributional predictions obtained from demographic models. Genealogical networks resolved Asparagopsis gametophytes and tetrasporophytes into four A. taxiformis and one A. armata cryptic OTUs. Falkenbergia isolates of A. taxiformis L3 were recovered for the first time in the western Mediterranean Sea and represent a new introduction for this area. Neutrality statistics supported past range expansion for A. taxiformis L1 and L2 in Hawaii. On the other hand, extreme geographic expansion and an increase in effective population size were found only for A. taxiformis L2 in the western Mediterranean Sea. Distribution models predicted shifts of the climatically suitable areas and population expansion for A. armata L1 and A. taxiformis L1 and L2. Our integrated study confirms a high invasive risk for A. taxiformis L1 and L2 in temperate and tropical areas. Despite the differences in predictions among modelling approaches, a number of regions were identified as zones with high invasion risk for A. taxiformis L2. Since range shifts are likely climate‐driven phenomena, future invasive behavior cannot be excluded for the rest of the lineages.

Size structure and dynamics of an invasive population of lineage 2 of Asparagopsis taxiformis (Florideophyceae) in the Alboran Sea

In this study, we present basic population data of the red macroalga Asparagopsis taxiformis, widely recognized as invasive in the Mediterranean Sea. A 13‐month field study was carried out on a population located in southern Spain, addressing its phenology, population dynamics and demography. We further tested whether biomass variations were related to environmental variables at the study site. Gametophytes were present year‐round while tetrasporophytes were only found in spring and summer. Recruitment capacity and vegetative growth of the gametophytes are discussed as important modulators for the population structure and enhancers of its persistence. Thallus size‐time histograms revealed a high prevalence of small shoots that showed high mortality that was not related to self‐thinning. Biomass of A. taxiformis was higher from March to July. Vegetative growth was the main way the gametophyte population was maintained, although the presence of tetrasporophytes and fertile gametophytes in the field confirms that sexual reproduction also occurs. Its continuous and high recruitment, in terms of the number of smallest shoots, makes this southern population of A. taxiformis a source of future invasive populations due to the intensive maritime traffic in the region.

Towards an Integrative Phylogeography of Invasive Marine Seaweeds, Based on Multiple Lines of Evidence

Molecular phylogeography has for decades been a frequently used approach to delineate novel evolutionarily significant units (ESUs) and to study the dynamics of invasive species. Next-generation sequencing technology (NGS) and the use of environmental DNA (eDNA) have the potential to revolutionize our way of understanding biodiversity and to establish rapid protocols for early-stage detection of invasive species. In seaweeds, however, several years of research on iconic invasive taxa of ambiguous taxonomic status (e.g. Caulerpa, Codium, Asparagopsis) have suggested that an integrative approach, namely the combination of multiple lines of evidence (e.g. phylogeographic, ecological, physiological and predictive modelling), is necessary to accurately resolve the taxonomy and their invasive potential. At present, integrative approaches in these fields are often weak because of incongruences among species delineation, newly discovered ESUs which remain undescribed taxonomically, and because databases containing vouchers of barcoded specimens are incomplete. As relocations of marine biota accelerate and climatic changes offer new potential niches for invasive seaweeds, new, transferable and internationally adopted protocols are necessary for exploring, monitoring and managing marine biodiversity. This is particularly urgent in areas of intense maritime traffic, such as the Mediterranean Sea and the Hawaiian archipelago, in order to achieve sustainable socio-economic development without compromising the local marine resources.

Reproductive ecology of an invasive lineage 2 population of Asparagopsis taxiformis (Bonnemaisoniales, Rhodophyta) in the Alboran Sea (western Mediterranean Sea)

Abstract Can the reproductive traits of Asparagopsis taxiformis contribute to its success as an invader? We present the reproductive phenology of a population located in southern Spain and a quantitative study of gametophyte reproductive structures. Reproductive allocation (the proportion of biomass allocated to reproductive tissue) in different size classes was analysed to investigate whether the shoot size influences the reproductive output of this population. Gametophytes were found to be reproductive in spring, autumn and the beginning of winter, whereas fertile tetrasporophytes were never observed. High reproductive performance was recorded in July, when reproductive allocation was surprisingly low (12.74±4.79%). Reproductive allocation of each size class varied over the study period, ensuring a continuous reproductive yield. The shoot size for reproduction ranged between 4–6 cm and 24–26 cm. An analysis of reproductive and vegetative volume suggested that reproductive allocation decreases with shoot size, which, as intermediate size classes are the most abundant and most persistent in the field throughout the year, optimizes the reproductive output. Reproduction in gametophytes was independent of environmental conditions in the study area. The yearly persistence of gametophytes together with the abundance of tetrasporophytes makes this population a donor source for future invasive populations.