The timing of reproduction influences key evolutionary and ecological processes in wild populations. Variation in reproductive timing may be an especially important evolutionary driver in the marine environment, where the high mobility of many species and few physical barriers to migration provide limited opportunities for spatial divergence to arise. Using genomic data collected from spawning aggregations of Pacific herring ( Clupea pallasii ) across 1600 km of coastline, we show that reproductive timing drives population structure in these pelagic fish. Within a specific spawning season, we observed isolation by distance, indicating that gene flow is also geographically limited over our study area. These results emphasize the importance of considering both seasonal and spatial variation in spawning when delineating management units for herring. On several chromosomes, we detected linkage disequilibrium extending over multiple Mb, suggesting the presence of chromosomal rearrangements. Spawning phenology was highly correlated with polymorphisms in several genes, in particular SYNE2 , which influences the development of retinal photoreceptors in vertebrates. SYNE2 is probably within a chromosomal rearrangement in Pacific herring and is also associated with spawn timing in Atlantic herring ( Clupea harengus ). The observed genetic diversity probably underlies resource waves provided by spawning herring. Given the ecological, economic and cultural significance of herring, our results support that conserving intraspecific genetic diversity is important for maintaining current and future ecosystem processes.

Invasions by shell-boring polychaetes such as Polydora websteriHartman have resulted in the collapse of oyster aquaculture industries in Australia, New Zealand, and Hawaii. These worms burrow into bivalve shells, creating unsightly mud blisters that are unappealing to consumers and, when nicked during shucking, release mud and detritus that can foul oyster meats. Recent findings of mud blisters on the shells of Pacific oysters (Crassostrea gigas Thunberg) in Washington State suggest a new spionid polychaete outbreak. To determine the identity of the polychaete causing these blisters, we obtained Pacific oysters from two locations in Puget Sound and examined them for blisters and burrows caused by polychaete worms. Specimens were also obtained from eastern oysters (Crassostrea virginica Gmelin) collected in New York for morphological and molecular comparison. We compared polychaete morphology to original descriptions, extracted DNA and sequenced mitochondrial (cytochrome c oxidase I [mtCOI]) and nuclear (small subunit 18S rRNA [18S rRNA]) genes to determine a species-level molecular identification for these worms. Our data show that Polydora websteri are present in the mud blisters from oysters grown in Puget Sound, constituting the first confirmed record of this species in Washington State. The presence of this notorious invader could threaten the sustainability of oyster aquaculture in Washington, which currently produces more farmed bivalves than any other US state. open Scientific RepoRtS | (2020) 10:3961 | https://doi.org/10.1038/s41598-020-60805-w www.nature.com/scientificreports www.nature.com/scientificreports/ a brittle layer of nacre that walls off the burrow 4,10,32-34 . The worm continues to expand this burrow beneath the thin, calcareous layer produced by its host; as this space fills with detritus, mud, and worm feces, a "mud blister" is formed 33,35 . Blisters can be irregular in shape and darkly colored, compromising the presentation of oysters served on the half-shell (only the cupped or left valve is used for serving the oyster) 36 . Moreover, if a blister is nicked during oyster shucking, the mud and feces will foul the oyster meat, rendering it inedible 3 . This is particularly problematic for oyster-growing areas where a large proportion of production goes to the half-shell market.In addition to their detrimental impact on aquaculture production, heavy mud worm infestations can also impact shell integrity, growth, and survivorship of mollusc hosts 37 . When infested with Polydora ciliata Johnston, the gastropod Littorina littorea (Linnaeus) has significantly reduced shell strength relative to uninfested individuals, making the infested gastropods more vulnerable to predation 38 . Pacific oysters (Crassostrea gigas) infested by the polydorids Polydora hoplura Claparède, Polydora cornuta Bosc, and Boccardia semibranchiata Radashevsky grow more slowly and have poorer body condition than do uninfested oysters 39 . Glycogen, protein, and lipid content relative to the shell cavity volume are lower i...

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