Diversity of diapausing egg ornamentation in rotifer species: standardizing traits

Abstract The ultrastructure of rotifer diapausing eggs is poorly studied. The challenge of identifying clear morphological traits that distinguish species highlights the potential for developing taxonomic parameters to improve rotifer species identification. Therefore, this study aims to expand the database of diapausing eggs and standardize the definitions of the traits observed in the eggshells. This standardization will provide key taxonomic characteristics that can be useful in species identification. A total of 764 diapausing eggs from 18 rotifer species were analyzed from sediment and cultures, using light and scanning electron microscopy. We identified and described ten interspecific morphological traits, including grooves, wrinkles, pores, reticulations, pillars, corrugated surface, exogenous membrane, intertwined fibers, and spines. The miscellaneous combinations between traits of the 18 species studied also supported unique morphotypes and, therefore, reinforced species-specific hypotheses. However, there is a high number of rotifer species that have not been studied yet, including some complex cases that still require a deep review. Thus, the standardization of eggshell traits becomes crucial as a taxonomical tool, but also due to the fact that their structure is always connected with functionality, which is in turn linked to evolution and ecology.