Hatching mechanisms and death of euphausiid evolutionary reversal of the free-living embryos during hatching process: evidences for nauplius?

This essay attempts to summarize and hypothetically reconstruct the original condition for the reproductive strategy and hatching mechanisms of the family Euphausiidae (Order Euphausiacea). Comparison is made of the hatching mechanisms and hatching success rates among five broadcast-spawning (Euphausia pacifica, Euphausia eximia, Euphausia distinguenda, Thysanoessa spinifera, and Thysanoessa inspinata) and two sac-spawning euphausiid species (Nematoscelis difficilis and Nyctiphanes simplex) collected from the Oregon coast, Bahía Magdalena (west coast of Baja California peninsula), and Gulf of California. These along with the discovery of a novel source of embryo mortality during hatching for broadcast-spawning species, and recently published genetic and phylogenetic information of the euphausiids, support the hypothesis that hatching as a free-living nauplius is a reversed character within the Order Euphausiacea in comparison with species belonging to other orders in the Class Crustacea. The hatching of embryos at nauplius stage, with distinct hatching mechanisms, appears repeatedly and intermittently in the Euphausiidae family phylogeny both in euphausiids with broadcast and sac-spawning reproductive strategy. This may represent a condition re-emerging well back in crustacean phylogeny, even though it is not necessarily primitive among the Order Euphausiacea as a whole.