Naupliar and Metanaupliar Development of Thysanoessa raschii (Malacostraca, Euphausiacea) from Godthåbsfjord, Greenland, with a Reinstatement of the Ancestral Status of the Free-Living Nauplius in Malacostracan Evolution.

Hasna Akther,Jørgen Olesen,Mette Dalgaard Agersted

doi:10.1371/journal.pone.0141955

Hasna Akther, Jørgen Olesen + Show 1 more

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https://doi.org/10.1371/journal.pone.0141955

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Abstract

The presence of a characteristic crustacean larval type, the nauplius, in many crustacean taxa has often been considered one of the few uniting characters of the Crustacea. Within Malacostraca, the largest crustacean group, nauplii are only present in two taxa, Euphauciacea (krill) and Decapoda Dendrobranchiata. The presence of nauplii in these two taxa has traditionally been considered a retained primitive characteristic, but free-living nauplii have also been suggested to have reappeared a couple of times from direct developing ancestors during malacostracan evolution. Based on a re-study of Thysanoessa raschii (Euphausiacea) using preserved material collected in Greenland, we readdress this important controversy in crustacean evolution, and, in the process, redescribe the naupliar and metanaupliar development of T. raschii. In contrast to most previous studies of euphausiid development, we recognize three (not two) naupliar (= ortho-naupliar) stages (N1-N3) followed by a metanauplius (MN). While there are many morphological changes between nauplius 1 and 2 (e.g., appearance of long caudal setae), the changes between nauplius 2 and 3 are few but distinct. They involve the size of some caudal spines (largest in N3) and the setation of the antennal endopod (an extra seta in N3). A wider comparison between free-living nauplii of both Malacostraca and non-Malacostraca revealed similarities between nauplii in many taxa both at the general level (e.g., the gradual development and number of appendages) and at the more detailed level (e.g., unclear segmentation of naupliar appendages, caudal setation, presence of frontal filaments). We recognize these similarities as homologies and therefore suggest that free-living nauplii were part of the ancestral malacostracan type of development. The derived morphology (e.g., lack of feeding structures, no fully formed gut, high content of yolk) of both euphausiid and dendrobranchiate nauplii is evidently related to their non-feeding (lecithotrophic) status.

Highlights

It has traditionally been difficult to identify characters shared by all or at least most of the morphologically diverse Crustacea
A Tukey posthoc test revealed no significant difference between N1, N2 and N3, whereas all three nauplii stages were significantly different from the metanauplius stage
We recognize three nauplius stages (N1-N3) and one metanauplius stage (MN) of Thysanoessa raschii, which is in contrast the two nauplius stages previously identified for this species [43], and in contrast to most other broadcast-spawning euphausiaceans

Summary

Introduction

It has traditionally been difficult to identify characters shared by all or at least most of the morphologically diverse Crustacea. This may not be surprising considering the great age of Crustacea (e.g., [1]). One of the classical candidates for a uniting set of characters for Crustacea is the ‘nauplius larvae’, a characteristic larval type with only three pairs of appendages: first antennae, second antennae, and mandibles (see reviews in [4, 5]). Despite the likely paraphyly of Crustacea, shared larval types such as the nauplius still potentially holds important evolutionary information, e.g., for phylogeny, or at least for explaining the evolution of the Crustacea, in which heterochrony (= evolution caused by developmental changes in timing of events) has played an important role

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