Structure of a hinge joint with textured sliding surfaces in terrestrial isopods (Crustacea: Isopoda: Oniscidea)

Abstract

Background The study of joints in terrestrial arthropods can provide insights into the evolutionary optimization of contacting surfaces that slide without lubrication. This work reports on the structure of the joint between the propodus and the dactylus in terrestrial isopods, the most successful group of crustaceans on land, focusing on the woodlouse Porcellio scaber. Methods The joints were studied using fluorescence microscopy, 3D reconstruction, scanning electron microscopy and transmission electron microscopy. The obtained results were functionally interpreted using high-speed video recordings by analyzing the use of the joint during locomotion. Results In the joint, which allows the dactylus to move in a single plain, a semicircular process on the propodus fits into a groove on the dactylus and guides its movement. The sliding surfaces of the propodal process are textured in the form of parallel epicuticular ridges a few hundred nanometers thick. This texturing is selective: while the less heavily loaded surfaces are textured, the surfaces that support the isopod during standing and walking are smooth. In contrast, the groove on the dactylus is completely smooth. We found a similar surface texture in several other species of terrestrial isopods and one aquatic isopod. Conclusions The selective texturing of the joint may reduce wear by eliminating small particles. This effect of the ridges was confirmed using electron microscopy. The absence of ridges on heavily loaded surfaces may enhance the dissipation of forces in these regions.

Cuticle structure of Carpathian endemic species: Trachelipus trilobatus (Crustacea, Isopoda, Oniscidea) described with the scanning electron microscope

“…The SEM analysis revealed morphological aspects which now are much clearer than in the previous studies realized with classic light microscopy (Tomescu et al, 2015). It is the case of the genital parts, which now could be observed distinctly, or the case of the appendages, in which the dactylus is extremely similar to the one recently described in another terrestrial isopod species (Vittori, 2021). This highlights the utility of SEM in helping to understand the adaptations and the way of life of T. trilobatus, an endemic species with a very small distribution range (Tomescu et al, 2015;Ferenți et al, 2020).…”

“…This highlights the utility of SEM in helping to understand the adaptations and the way of life of T. trilobatus, an endemic species with a very small distribution range (Tomescu et al, 2015;Ferenți et al, 2020). Thus, 59 studies on species from different environments are necessary, and also studies on other endemic species, as most previous studies aimed a common species such as Porcellio scaber (Holdich and Lincoln, 1974;Ziegler and Altner, 1995;Csonka et al, 2018;Vittori, 2021).…”

Structural diversity of crustacean exoskeletons and its implications for biomimetics

“…Even though two crustacean skeletal elements are linked by a mobile joint, the cuticle between them is continuous. The flexible region of the cuticle that enables the movement of a joint is the arthrodial membrane, which is thin and flexible [ 4 , 5 ]. The cuticle covering structures that participate in breathing and osmoregulation is also thin and non-mineralized [ 6 ].…”

“…The organic fibres in the leg cuticle are organized in the typical Bouligand pattern but they all align in the axial direction in the claw cuticle [ 31 ]. This is also the direction of loading when the animal stands or walks on the tips of its claws [ 5 ].…”

“… Structures that withstand predictable loads incorporate parallel fibres oriented in the direction of the predominant loading. This was demonstrated in the isopod claws in which chitin–protein fibres are oriented longitudinally [ 5 , 31 ] as well as in the incisor processes of shrimps and isopods [ 39 , 45 , 46 ]. In surface layers of structures that are exposed to impacts and pressure, chitin–protein fibres are oriented towards the surface.…”