Structural optimization and amorphous calcium phosphate mineralization in sensory setae of a terrestrial crustacean (Isopoda: Oniscidea)

Intraspecific Sensory Diversity and the Decapod Claw: Patterns of Sensillation Are Heterochelic and Sexually Dimorphic In Pagurus bernhardus

Information detection affects physiological performance and behaviour and is vital to survival and fitness. Despite the recognised importance of sensory adaptations in information acquisition and manipulation, many forms of sensory variation-from within individuals to between species-remain underexplored. To better understand the role of information in evolution, it is important to examine sensory variation as part of a cohesive framework of sensory diversity. Using the decapod claw, a structure well-recognised for its morphological variation, we investigated sensory diversity at the intraspecific level by assessing heterochely and sexual dimorphism in the chelar morphologies of Pagurus bernhardus hermit crabs. We employed a novel methodology using scanning electron microscopy (SEM) to assess moulted chelar tissue from both the major and minor claws. The shape, size, and sensillation (i.e., the distribution and abundance of sensilla) of both chelipeds were examined by geometric morphometric landmark analysis (GMLA), generalised Procrustes analysis (GPA), and linear mixed effects models. Hermit crabs exhibited heterochely and sexual dimorphism in both gross and sensory chelar morphologies. Sexual dimorphism was greater in the sensory morphology of the major claw, suggesting sex-based sensory specialisations, likely due to differences in mating roles and behaviours. In contrast, the minor claw's sensory morphology lacked sexual dimorphism, suggesting the sensory role of this appendage is equally important for both sexes. Our results highlight sensory variation as a fundamental aspect of functional morphology and emphasise the need to consider sexual dimorphism and body asymmetry in information acquisition. These findings contribute to a broader framework for studying sensory diversity, underscoring the importance of integrating sensory morphology, function, and ecology to fully understand the evolutionary implications of sensory specialisations.

Mineral Composition of Skeletal Elements in Dorid Nudibranchia Onchidoris muricata (Gastropoda, Mollusca)

Energy-dispersive X-ray spectrometry (EDX), a standard technique in mineralogy and criminalistics, has not yet been fully incorporated into the study of various biomineral structures of invertebrates, despite the growing popularity of this topic in the last few decades. This is partly due to EDX's limitations and data interpretation complexities. This study used EDX to analyze the spicules' elemental composition of nudibranch gastropod mollusk Onchidoris muricata prepared via two methods (sectioning and fracturing). Hierarchical clustering and compositional data analysis of the resulting elemental data revealed three distinct spicule populations with varying element ratios, suggesting spicule transformation pathways. Two of the three clusters had a uniform layered microstructure, yet they showed reliable differences in element ratios. Raman spectroscopy confirmed the spicules' calcite or magnesian-calcite composition. EDX analysis of spicule sections, coupled with other analytical techniques, revealed mineral structure transformations and provided insights into the biomineral nature. The sample preparation method with epoxy-embedding, preserving surrounding tissues in their active state, allowed for the analysis of tissue elemental composition and the elucidation of their role in mineralization.

Multicopper oxidase-2 mediated cuticle formation: Its contribution to evolution and success of insects as terrestrial organisms

The insect cuticle is a non-cellular matrix composed of polysaccharide chitins and proteins. The cuticle covers most of the body surface, including the trachea, foregut, and hindgut, and it is the body structure that separates the intraluminal environment from the external environment. The cuticle is essential to sustain their lives, both as a physical barrier to maintain homeostasis and as an exoskeleton that mechanically supports body shape and movement. Previously, we proposed a theory about the possibility that the cuticle-forming system contributes to the "evolution and success of insects." The main points of our theory are that 1) insects evolved an insect-specific system of cuticle formation and 2) the presence of this system may have provided insects with a competitive advantage in the early land ecosystems. The key to this theory is that insects utilize molecular oxygen abundant in the atmosphere, which differs from closely related crustaceans that form their cuticles with calcium ions. With newly obtained knowledge, this review revisits the significance of the insect-specific system for insects to adapt to terrestrial environments and also discusses the long-standing question in entomology as to why, despite their great success in terrestrial environments, they poorly adapt to marine environments.

Structural diversity of crustacean exoskeletons and its implications for biomimetics

The crustacean cuticle is a biological composite material consisting of chitin-protein fibres in a mineralized matrix. Recent research has revealed a surprising range of fibre architectures and mineral compositions of crustacean skeletal structures adapted to various mechanical demands. It is becoming increasingly clear that the organic fibres in the cuticle may be organized in patterns differing from the standard twisted plywood model. Observed fibre architectures in protruding skeletal structures include longitudinal and circular parallel fibre arrays. Skeletal minerals often include calcium phosphates in addition to calcium carbonates. Furthermore, skeletal properties are affected by protein cross-linking, which replaces mineralization as a stiffening mechanism in some structures. Several common structural motifs, such as the stiffening of the outer skeletal layers, the incorporation of non-mineralized cuticle in exposed structures, and interchanging layers of parallel fibres and the twisted plywood structure, can be identified in skeletal elements with similar functions. These evolutionary solutions have the potential for biomimetic applications, particularly as manufacturing technologies advance. To make use of this potential, we need to understand the processes behind the formation of the crustacean exoskeleton and determine which features are truly adaptive and worth mimicking.

Suspension feeding in Copepoda (Crustacea) - a numerical model of setae acting in concert

Suspension feeding via setae collecting particles is common within Crustacea. Even though the mechanisms behind it and the structures themselves have been studied for decades, the interplay between the different setae types and the parameters contributing to their particle collecting capacities remain partly enigmatic. Here, we provide a numerical modeling approach to understand the relationship among the mechanical property gradients, the mechanical behavior and the adhesion of setae, and the feeding efficiency of the system. In this context, we set-up a simple dynamic numerical model that takes all of these parameters into account and describes the interaction with food particles and their delivery into the mouth opening. By altering the parameters, it was unraveled that the system performs best when the long and short setae have different mechanical properties and different degrees of adhesion since the long setae generate the feeding current and the short ones establish the contact with the particle. This protocol can be applied to any system in the future as the parameters (i.e., properties and arrangement of particles and setae) can be easily altered. This will shed light on the biomechanical adaptations of these structures to suspension feeding and provide inspiration for biomimetics in the field of filtration technologies.

The epidermis cells of mandible teeth in the terrestrial isopod Porcellio scaber: Differentiations for mineralisation with calcium phosphate and carbonate

Generally, the mineralisation of the crustacean cuticle occurs when the cuticle has expanded after moulting. However, in the partes incisivae of Porcellio scaber, cuticle mineralisation with calcium phosphate already occurs before the moult. We investigated the ultrastructure and distribution of organelles within the epidermis cells and searched for calcium-containing organelles using EDX and EFTEM analysis. We found two different cell types. Calcium carbonate-secreting C-cells, which resemble the epithelial cells of the general integument, and the P-cells, which, as an unusual feature, have cell extensions up to 400 μm long. During secretion of the partes incisivae, these extensions end at the unmineralised tip and the phosphate-containing middle region. Their cell bodies contain most of the mitochondria located in basal folds and a high amount of endoplasmic reticulum. The cell extensions contain many microtubules, endoplasmic reticulum, large and small vesicles and densely stained rod-shaped cisternae. The rod-shaped cisternae and the endoplasmic reticulum contain calcium. During cuticle mineralisation, vesicles, which probably belong to the endo-lysosomal system, contain calcium and phosphorus. They occur at some distance and close to the cuticle. The mineral in these vesicles has a similar composition to that within the cuticle, suggesting that they play a role in cuticle mineralisation.

Unravelling the ultrastructure and mineralogical composition of fireworm stinging bristles

Amphinomid fireworms are notorious for their stinging dorsal bristles (notochaetae), but it is still unclear whether the irritation they cause is merely mechanical or if the notochaetae contain toxins. Furthermore, although fireworm chaetae have always been described as calcareous, their composition has never been investigated to date and strong debates are ongoing on their internal structure. Unravelling the native ultrastructure and composition of fireworm chaetae is the first crucial step to assess whether the hypothesis of toxin vehiculation could be fully considered. We examined for the first time the chemical and mineralogical composition, the ultrastructure and the external structure of the dorsal and ventral chaetae of the large species Hermodice carunculata. All the measurements were carried out on samples prepared without the use of chemical reagents, except for those targeted to investigate if decalcification altered the ultrastructure of the chaetae. A crystal-chemical strategy, combining chemical, diffraction and thermal analyses clearly showed the occurrence of crystalline calcium carbonate and clusters of phosphatic amorphous material. Scanning electron micrographs and energy dispersive X-ray measurements showed that the dorsal chaetae have an extremely shallow insertion point in the body respect to the ventral chaetae, that could facilitate the release of the notochaetae in the environment. Their proximal part is characterized by canals with a hexagonal pattern rich in Ca and P, followed by a large cavity upwards. The harpoon-shaped ends and the central canals of the notochaetae completely disappeared after exposure to EDTA. The notochaetae are hollow and may be able to vehicle toxins. The absence of the honeycomb pattern in the distal part of the notochaetae and their slenderness probably contribute to their brittleness and high sensitivity to breakage on contact. These observations constitute keystone understandings to shed light on fireworm defensive and offensive capacities and their ecological success.