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Abstract Phenology, or seasonal variation in life cycle events, is poorly described for many macroalgal species. We describe the phenology of a non‐native population ofGracilaria vermiculophyllawhose thalli are free‐living or anchored by decorating polychaetes to tube caps. At a site in South Carolina, USA, we sampled 100 thalli approximately every month from January 2014 to January 2015. We assessed the reproductive state and measured thallus size based on wet weight, thallus length, and thallus surface area from herbarium mounts. Because life cycle stage cannot be assigned using morphology, we implemented a PCR assay to determine the life cycle stage—tetrasporophyte, female gametophyte, or male gametophyte—of each thallus. Tetrasporophytes dominated throughout the year, making up 81%–100% of thalli sampled per month. Reproductive tetrasporophytes varied between 0% and 65% of monthly samples and were most common in warm summer months (July through September) when thalli also tended to be larger. The vast majority of the reproductive thalli were worm‐anchored and not fixed to hard substratum via a holdfast. Thus, free‐living thalli can be reproductive and potentially seed new non‐native populations. GivenG. vermiculophyllareproduction seems tied closely to temperature, our work suggests phenology may change with climate‐related changes in seawater temperatures. We also highlight the importance of understanding the natural history of macroalgae to better understand the consequence of range expansions on population dynamics. 

Abstract AimAs within‐species genomic data have been shown useful in interpreting broader biogeographic trends, we analysed the mode of population genomic isolation involved in a well‐studied intertidal genomic cline to better understand the mechanisms maintaining it. These results were interpreted in the context of spatial variation in habitat use and availability as well as likely fitness consequences for hybridization between the two lineages. LocationPacific coast of North America. TaxonArthropods (Class Maxillopoda, Order Sessilia, Family Balanidae;Balanus glandula). MethodsGenotype‐by‐sequencing approaches were used to generate single‐nucleotide polymorphism markers across sites sampled between southern Alaska and Southern California. Inference using standard population genomic methods, including analysis of population structure, inbreeding and linkage disequilibrium, was used to identify the steepest transitions across the largest number of loci examined. These data were put in the context of observed population density and habitat availability. ResultsWe show that the majority of markers analysed show strong clinal transitions in a very narrow portion of the California coast. Patterns of linkage disequilibrium among markers, along with prior evidence of variation in reproductive potential by latitude and by mitochondrial lineage, suggest some reproductive isolation among the northern and southern lineages ofB. glandulathat are concordant with the drop in population density and habitat availability in central California. Main ConclusionsA significant clinal transition in genomic diversity is stronger and more localized than previously recognized and exhibits statistical patterns suggesting that the lineages are reproductively and phenotypically distinct in ways that may be ecologically important. As this species has been used to infer process in coastal biogeography, further study of concordant patterns will be important for advancing our understanding of this region.