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Three different egg hatching mechanisms were observed under laboratory conditions in Euphausia pacifica Hansen, Thysanoessa spinifera Holmes and Thysanoessa inspinata Nemoto: backward, forward and flipping. Like all broadcast spawning euphausiids, these species usually hatch as nauplius 1 (N1). Some hours before hatching the vitelline membrane breaks and the embryo is freely suspended within the chorion; later the embryo takes on a slightly oval shape. When ready to hatch, the N1 pushes against the chorion with the posterior part of the abdomen producing a protuberance. No spine or egg tooth is present to break the chorion. The pressure breaks the chorion, and the nauplius pushes itself backwards with the first and second antennae and mandible to slide from the chorion. After about three quarters of the body is outside, the nauplius brings all the appendages together to move backwards without becoming stuck in the chorion. This is the backward hatching mechanism. The vitelline membrane remains within the egg after the nauplius leaves the chorion. Hatching takes 5–20 s, and most of the eggs in a clutch hatch during &lt;2 h. Several eggs of E. pacifica hatched as meta-nauplii (MN) (&gt;200 h after spawning) or as calyptopis 1 (C1) stage (&gt;232 h), rather than as N1. Delayed hatching of embryos also was observed in T. spinifera as nauplius 2 (N2) (&gt;120 h) or as MN stage (&gt;180 h), and in T. inspinata as N2 (106 h) after spawning. Eggs with larvae in stages of development beyond N1 have not been observed from preserved zooplankton samples. However, eggs spawned in the field and incubated in the laboratory also had extended development and late hatching but with low frequency (&lt;0.06%). It is proposed that, if the backward hatching mechanism fails, alternate hatching mechanisms can be used by the euphausiid. There is high flexibility in their hatching modes. The N2 and MN break the chorion with the first and second antennae, hatching forwards, and the C1 breaks it with the telson spines and by flipping of the abdomen, resembling the decapod hatching mechanism. Delayed hatching using the forward and flipping mechanisms were associated with low hatching success in comparison with the backward hatching mechanism.

Hatching mechanism and delayed hatching of the eggs of three broadcast spawning euphausiid species under laboratory conditions