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ABSTRACT Eggshell characteristics can be important components of fitness, providing protection to the developing embryo against environmental stressors. Many invertebrates, however, produce multiple egg types as part of their reproductive strategies. In monogonont rotifers, for example, reproduction occurs via cyclical parthenogenesis-a process involving extended periods of asexual reproduction interrupted by brief phases of sexuality. Asexual reproduction yields diploid subitaneous (amictic) eggs or haploid eggs that develop into males, while sexual reproduction produces diploid diapausing (resting) eggs. To date, only the eggshells of diapausing eggs have received substantial ultrastructural investigation. Here, we examine the ultrastructure of subitaneous eggshells in monogonont rotifers across diverse taxa and lifestyles, using light and electron microscopy. Our results reveal considerable variation in eggshell thickness, layering, and staining properties among taxa. Generally, sessile rotifers exhibited thinner eggshells with fewer layers and limited morphological diversity, whereas species that brood or oviposit eggs on substrates had thicker, more layered shells with more complex staining profiles. These findings indicate that subitaneous eggshell ultrastructure is more diverse than previously recognized and may hold value for future ecological and evolutionary studies.
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Integrative microscopy to explore physical and nanomechanical properties of eggshells of diapausing embryos in Rotifera: a proof-of-concept study
Diapausing embryos of invertebrates represent investments in future populations. Thus, these embryos must be capable of withstanding a variety of environmental assaults. Consequently, their eggshells should be adapted to resist injuries from predators, sediments, or excessive shrinkage if desiccated. To date, there have been no direct nanomechanical measurements of the eggshells of most diapausing invertebrates. Here, we used three approaches to understand how eggshells of two rotifers, a freshwater species (Brachionus calyciflorus) and a brackish water species (B. plicatilis), tolerate harsh conditions: (1) atomic force microscopy to measure elasticity and hardness; (2) transmission electron microscopy to study ultrastructure; (3) scanning electron microscopy to examine surface features. We compare these values to measurements of brine shrimp (Artemia salina) cysts and mosquito (Aedes aegypti) overwintering eggs. Our results revealed that rotifer eggshells are structurally similar and have comparable nanomechanical values. While rotifer eggshells had lower Young’s moduli (ca. 13–16 MPa) and hardness values (1.84–1.85x10-2 GPa) than eggshells of Artemia and Aedes, eggshells of all species were relatively elastic and not particularly resistant to deformation. Pliancy of shells that form egg banks (i.e., Artemia, Brachionus) may be an adaptation to resist cracking under the physical forces of buried sediments. Though there are no obvious relationship between eggshell thickness, ultrastructure, ornamentation, or nanomechanical values in rotifer eggshells, we hypothesize that eggshell chemistry may play an important role in determining elasticity and hardness. Future studies should consider an integrative approach to understand importance of eggshell structure, chemistry, and mechanics in protecting diapausing embryos.
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- PAR ID:
- 10494576
- Publisher / Repository:
- Journal of Natural History
- Date Published:
- Journal Name:
- Journal of Natural History
- Volume:
- 57
- Issue:
- 45-48
- ISSN:
- 0022-2933
- Page Range / eLocation ID:
- 1984 to 2005
- Subject(s) / Keyword(s):
- atomic force microscopy egg mechanics zooplankton, diapause
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Species of Monogononta produce at least three kinds of eggs, each with a different shell: amictic, male, and mictic (diapause). As coverings, eggshells function to protect the embryo from the external environment but may have other functions including to facilitate transport (i.e., hydrochory, zoochory, or anemochory). The fine structure of rotifer eggshells is poorly known despite their important functions. Here, we investigate rotifers from 10 families that live in different environments (marine and freshwater) and have varied lifestyles (planktonic and sessile). We use transmission electron microscopy to characterize eggshell layers and determine whether there are commonalities among eggshells within a species (amictic vs. mictic) and among related species and/or species from different environments and lifestyles. Our findings indicate that sessile species that live in protective tubes possess the thinnest amictic eggshells with minimal complexity. Planktonic species tend to have thicker amictic eggshells, but their thickness is not always related to having more shell layers. Resting eggshells tend to have the largest number of shell layers and are often the thickest shells. Many shells also possess spaces between layers that are interpreted as adaptations for floatation or anemochory.more » « less
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1080/00222933.2023.2279255
