Eggshell morphology, structure, and nanomechanical properties of diapausing embryos in sessile rotifers

Abstract Diapausing embryos of sessile rotifers remain poorly characterized, limiting comparative analyses of diapause strategies across Monogononta. We examined the diapausing embryos of four sessile species— Collotheca monastica , Ptygura crystallina , Ptygura ctenoida, and Ptygura thalenoiensis —using light microscopy, electron-based imaging, and atomic force measurements to characterize eggshell morphology, layering, and mechanical properties. These approaches reveal either a three-layered, single-piece eggshell or a two-layered bivalved configuration, together with marked differences in stiffness, adhesion, and deformation modes. Comparisons with planktonic rotifers indicate that sessile species generally produce more elongated diapausing embryos with thinner eggshell layers, whereas planktonic taxa tend to retain thicker and more ornamented eggshells. These differences may reflect the distinct protective contexts in which embryos develop. Our observations also suggest a possible correspondence between the eggshell layers of sessile and planktonic lineages, although confirming such homology would require broader taxonomic sampling and ultrastructural comparisons. Variation in tube geometry and firmness probably parallels differences in embryo shape. Ornamentation is presumably less developed in species whose tubes or substrates provide stronger protection. Together, these results provide a comparative framework linking eggshell structure, mechanical performance, and protective environment across sessile and planktonic lineages.