Locking of the operculum in a water snail: Theoretical modeling and applications for mechanical sealing
Introduction
Most water snails possess a distinct appendage, the operculum, which is secreted by specific gland cells on their posterior foot (Houbrick, 1980). When a water snail crawls, the shell rests on the outer surface of the operculum, whereas the soft body and operculum are pulled into shell by the contraction of the columellar muscle when disturbed (Thompson et al., 1998). The essential function of the operculum is to seal the shell aperture and protect the soft body from predators and dry weather (Páll-Gergely et al., 2016). In addition, the micro-grooves of the operculum may generate hydrodynamic lubrication which can effectively protect the structure of the snail's operculum and shell (Xu et al., 2018). Some species of water snails use the operculum as an offensive weapon or as an anchor for locomotion on the substrate (Checa and Jiménez-Jiménez, 1998). For Pomacea canaliculata, the diameter of operculum is slightly smaller than the shell aperture, such that the operculum can be retracted into the shell deeply when locking the entrance of the shell.
Regulation of water balance is the primary component of homeostasis in all organisms (Chown et al., 2011). As a result, organisms have some different sealing strategies to deal with water loss. In the mammals, water loss is prevented by keratinization and glandular secretions, on the skin surface (Findlay, 1970). The air pores of liverwort are equipped with hydrophobic ledges to prevent entry of water (Schönherr and Ziegler, 1975). For land plants, cuticular wax serves the function of limiting nonstomatal water loss (Samuels et al., 2008). There are some animals that use sealed air to achieve specific functions. For example, in true bugs aquatic (Heteroptera), the hemelytra-locking mechanism takes part in sealing the space under the wings, thus keeping air stored with the option to easily unlock hemelytra prior to flight (Perez Goodwyn and Gorb, 2003). The apterous dung beetle possess a subelytral cavity sealed by hairs and microtrichia, which contributes to gas exchange (Duncan et al., 2010).
The operculum of the water snail (P. canaliculata) can be locked in its shell, which provides a unique locking mechanism. The superimposed rings distributed on the surface of the operculum which may contribute to locking and sealing. However, the combined microstructures of the operculum with its locking mechanism has not been analyzed previously. The aim of the study is to explore the locking mechanism of the operculum of P. canaliculata.
Section snippets
Samples and filming of operculum closing
The water snail (P. canaliculata) specimens were collected in Quanzhou, Fujian province, China (24.93°N, 118.58°E). Twenty snails were fed in a glass aquarium (200 mm × 140 mm × 170 mm) which was equipped with an artificial heater to maintain the freshwater temperature at 26 °C. Water snails were fed with leaves of cabbage and the water was changed twice a week. As shown in Fig. 1(a), water snail was attached to a glass slide using double-sided adhesive tape and was illuminated using LED lamp.
Movement and deformation of the operculum
Our observation indicates that the operculum is located between the foot and underside shell when the snail is crawling, we define this state as the initial state. However, the operculum can be retracted into the shell rapidly to plug the entrance of the shell when the snail is threatened, and the state when the operculum seals the shell aperture completely is defined as the locking state. A locking process of the operculum from the initial state to the locking state can be categorized into two
Discussion
The stiffness K, maximum normal stress σ, and the contact area S were calculated using Matlab. We assume the operculum edge has different layers but same total thickness. Fig. 6 shows the influence of the layer number on the stiffness and maximum normal stress. It can be seen that the stiffness K decreases sharply at first until layer number up to 4, and then drops slightly with further increases of layer number. This curve suggests that the laminated microstructure of the operculum edge can
Conclusions and implications
The water snail P. canaliculata, is well adapted to the environment in alternating wetland and dryland habitats, because these snails have both gills and lungs (Halwart, 1994). It is worth noting that, when habitats are dry, the animal retracts its soft body into the shell until water is available. Apple snails can survive for months or even years by closing the operculum in dry weather, but cannot survive more than 10 days when the operculum is removed that exposes the soft parts to
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant no. 51475258); the Fundamental Research Funds for the Central Universities (grant no. 2652017063); and the Research Project of the State Key Laboratory of Tribology under contract SKLT2016B03.
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Cited by (0)
- 1
Xiaoyan Xu: one of the first authors who wrote the manuscript, performed the experiments and analyzed the data.
- 2
Jianing Wu: one of the first authors who found the operculum's special micro-morphology, designed the experiments and revised the manuscript.
- 3
Kai Wang: the general author who performed the analysis with constructive discussions.
- 4
Yunqiang Yang: a corresponding author who conceived the project and designed the experiments.
- 5
Shaoze Yan: a corresponding author who conceived the project and designed the experiments.
- 6
These authors contributed equally to this study.