Naupliar development of Facetotecta (Crustacea: Thecostraca) and the nature of the first nauplius instar in the Crustacea-pro et contra the Hexanauplia concept

From a number of partial moult series obtained by laboratory culture of plankton-caught specimens, we described all naupliar instars of the facetotectan species, Hansenocaris itoi. It reveals seven naupliar instars, instead of the five that were previously supposed for the Facetotecta. This number of naupliar instars is unique not only for Facetotecta, but also for Thecostraca and Hexanauplia as well. We studied the external morphology of each naupliar instar in detail with light microscopy and SEM. Nauplius 1 is entirely non-feeding and differs from subsequent instars in having a smooth, unsculptured cuticle and undeveloped armament of the labrum and the limbs. The subsequent naupliar development is characterized by an increase in size and in the number of cuticular plates and by the appearance of different armament of the limbs. We compare nauplius 1 across all Thecostraca and discuss its nature in relation to the larval development of Crustacea. We also discuss the presence of seven naupliar instars in Facetotecta and the concept of Hexanauplia.

Automated summary

Through laboratory culture of plankton-caught specimens, the study revealed seven naupliar instars for the facetotectan species Hansenocaris itoi, instead of the previously assumed five. The naupliar development is characterized by an increase in size, cuticular plates and armament of the limbs. The presence of seven naupliar instars in Facetotecta challenges previous notions and is also unique in Thecostraca and Hexanauplia.