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  1. Abstract

    Northern sand lance (Ammodytes dubius) are essential forage fish in most offshore, temperate-to-polar waters on the Northwest Atlantic shelf (NWA), but their population structure and genetic separation from the American sand lance (A. americanus) remain unresolved. We assembled a reference genome for A. dubius (first in the Ammodytidae) and then used low-coverage whole genome sequencing on 262 specimens collected across the species distribution (Mid-Atlantic Bight to Greenland) to quantify genetic differentiation between geographic regions based on single nucleotide polymorphisms. We found strong separation between A. dubius from locations north and south of the Scotian Shelf, largely due to massive genetic differentiation spanning most of chromosomes 21 and 24. Genetic distance increased with geographic distance in the smaller southern cluster but not in the larger northern cluster, where genetic homogeneity appeared across large geographic distances (>103 km). The two genetic clusters coincide with a clear break in winter sea surface temperature, suggesting that differential offspring survival, rather than limited transport, causes a break in realized connectivity. Nuclear and mitochondrial DNA both clearly delineated A. dubius from A. americanus, thereby confirming a species boundary through spatial niche partitioning into inshore (A. americanus) and offshore (A. dubius) sand lance species on the NWA.

     
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  2. Browman, Howard (Ed.)
    Abstract Experiments examining fish sensitivities to future oceanic CO2 levels have greatly expanded over past decades and identified many potentially affected traits. Curiously, data on reproductive trait responses to high CO2 are still scarce, despite their strong link to Darwinian fitness and thus to population vulnerability to ocean acidification. We conducted two rearing experiments on the first broadcast-spawning marine fish model (Atlantic silverside, Menidia menidia) to examine how long-term and novel whole life-cycle exposures to predicted future CO2 levels (∼2,000 µatm) affect laboratory spawning, temperature-specific reproductive investment, fecundity, and size distributions of maturing oocytes. At low temperatures (17°C), female body size and therefore potential fecundity (FPot, oocytes/female) slightly increased with CO2, while relative fecundity (FRel, oocytes/g female) remained unaffected. At high temperatures (24°C), high CO2 substantially reduced both FPot (−19%) and FRel (−28%) relative to control treatments. Irrespective of CO2, females at 24°C grew larger and heavier than those at 17°C, and although larger females produced larger oocytes at some developmental stages, they also had lower gonadosomatic indices and lower FRel. Our findings contrast with most previous studies and thus highlight the need to investigate reproductive impacts of increasing CO2 on multiple fish species with contrasting life history strategies. 
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