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

J Gómez-Gutiérrez

doi:10.37543/oceanides.v21i1-2.26

Abstract

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.

Highlights

The crustacean nauplius larva, characterized by a remarkably conserved morphology, has become the focus of studies on life-history evolution that investigate the interplay of animal structure, function, ecology and evolutionary history (Williams, 1994; Dahms, 2000)
The present study shows that the tropical euphausiids E. eximia and E. distinguenda hatch preferentially as nauplius, but it has been observed hatching as metanauplius (Table 3)
Broadcast euphausiid embryos infrequently hatch in later developmental stages using two unusual hatching mechanisms associated with relatively low Hatching success (HS): forward and flipping hatching mechanisms (Gómez-Gutiérrez, 2002)

Summary

Introduction

The crustacean nauplius larva, characterized by a remarkably conserved morphology, has become the focus of studies on life-history evolution that investigate the interplay of animal structure, function, ecology and evolutionary history (Williams, 1994; Dahms, 2000). In the Class Malacostraca, only in the Suborder Dendrobranchiata (Decapoda) and seven genera of the Order Euphausiacea (Euphausia, Meganyctiphanes, Nematoscelis, Nyctiphanes, Stylocheiron, Thysanoessa and Thysanopoda; ~91% of all known 86 valid species) is currently known to hatch as a nauplius, several sac-spawning euphausiid species usually hatch in older pseudometanauplius (PMN) or metanauplius (MN) larval stages (Gordon, 1955; Brinton, 1966; Ponomareva, 1969; Zilch, 1978; Gómez-Gutiérrez, 2003a,b; Gómez-Gutiérrez & Robinson, 2005). Gordon (1955), Brinton (1965), Maas & Waloszek (2001) and Casanova et al (2002) argued that the shared derived characters (synapomorphies) between Decapoda and Euphausiacea are: 1) the carapace attached to 7, not 8, thoracomeres and 2) that in adult stage their thoracic segments are open. Gordon (1955), Brinton (1965), Maas & Waloszek (2001) and Casanova et al (2002) argued that the shared derived characters (synapomorphies) between Decapoda and Euphausiacea are: 1) the carapace attached to 7, not 8, thoracomeres and 2) that in adult stage their thoracic segments are open. Alwes & Scholtz (2004) concluded that the early cleavage and gastrulation pattern of the euphausiid Meganyctiphanes norvegica and Dendrobranchiata are maybe homologous features indicating a closely phylogenetic relationship between both groups

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