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Observation of the sac-spawning euphausiid Nematoscelis difficilis Hansen during shipboard laboratory incubations showed that its embryos usually hatch as pseudometanauplius (PMN) or metanauplius (MN). The eggs, which have a minute perivitelline space, are spherical at spawning and become elliptical after the nauplius 1 develops. When ready to hatch, the PMN or MN embryos extend and contract their first and second antennae in a swimming movement, breaking the chorion into almost equal halves joined by one small section in the anterior part of the chorion. The mandibles play a secondary role in cutting the chorion. Then the embryo pushes itself backwards with the first and second antennae to escape from the chorion. This is known as ‘push-off’ hatching. The embryos always hatch progressively from the distal end towards the proximal end of the ovigerous sac. The time between hatching of the first and last embryo may reflect the time the females require to lay a clutch of eggs (&lt;2.1 h). Development time to the PMN stage at 10°C was ∼55–60 h and to MN stage ∼84 h. Eggs of one brood of N. difficilis hatched backwards at 47 h as nauplius 2 (N2) rather than as PMN or MN. This is the second observation of early hatching by any sac-spawning euphausiid species. Therefore, a morphological description is provided of the free-swimming N. difficilis N2. It is expected that N. difficilis N2 could be found in the ocean, a taxonomic complication for the identification of nauplii, and therefore a comparison is made with N2 stages of three of the most abundant broadcast-spawning euphausiid species (Euphausia pacifica, Thysanoessa spinifera and Thysanoessa inspinata) in the same geographical range. This is also the first description of the N2 for T. inspinata. Both sac-spawning and broadcast-spawning strategies in euphausiids have shown high flexibility in the hatching schedule. However, early hatching in sac-spawners and late hatching in broadcast-spawners are usually associated with low embryo hatching success.

Hatching mechanism and accelerated hatching of the eggs of a sac-spawning euphausiid Nematoscelis difficilis