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The reproductive biology and early life phases of tropical broadcast spawning krill are largely unknown worldwide. This investigation provides the first published data on the reproductive period, brood size, embryo and nauplius-to-metanauplius morphology, biometry, development, and hatching success rates of two of the smallest krill species known (Euphausia lamelligera, <11 mm and E. distinguenda, <14.5 mm total length), endemic to the Eastern Tropical Pacific. Embryos were obtained from gravid females collected on the Jalisco continental shelf (Mexico) every 2 wk from July 2011 to June 2012, and incubated under laboratory conditions. Both species spawned throughout the year (with higher brood sizes between Jan and Jun), showing similar mean interspecific brood sizes: E. lamelligera, 34 eggs female −1 (range: 4−95) and E. distinguenda, 36 eggs female −1 (range: 14−72). E. distinguenda spawned larger eggs (chorion 0.700, embryo 0.329 mean diameters, perivitelline space [PVS] 0.185 mm) than E. lamelligera (chorion 0.405, embryo 0.291, PVS 0.057 mm). Both species had high hatching success (> 66%) with the shortest hatching times (9 to 14 h) known so far for any species of the Order Euphausiacea. E. distinguenda was significantly larger than E. lamelligera at each early larval stage. A pseudometanauplius stage (molting between nauplius and metanauplius stages), previously thought to be an exclusive stage of sac-spawning species, was observed for both broadcast spawning species. Our results support the hypothesis that both species exhibit a continuous but seasonally variant spawning reproductive strategy associated with female body size and seasonal coastal upwelling dynamics, and show brood sizes within the low range of variability known for temperate krill species.

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