Evolutionary variation in the mechanics of fiddler crab claws

Exoskeletal Trade-off between Claws and Carapace in Deep-sea Hydrothermal Vent Decapod Crustaceans

Limitations on energetic resources create evolutionary trade-offs, prompting us to investigate if investment in claw strength remains consistent across crustaceans living in diverse habitats. Decapod crustaceans living in deep-sea hydrothermal vents are ideal for this study due to their extreme environment. In this study, we investigated whether decapods (blind crab Austinograea sp. and the squat lobster Munidopsis lauensis) living in deep-sea hydrothermal vents prioritize investing in strong claws compared to the carapace, like coastal decapods. We analyzed exoskeleton morphology, mechanical properties, structures, and elemental composition in both the carapace and claws of four Decapoda species (two each from Brachyura and Anomura infraorders) in vent and coastal habitats. Coastal decapods had ∼4-9 times more teeth on their claw cutting edge than the vent species. Further, only the coastal species exhibited higher firmness in their claws than in their carapaces. Each infraorder controlled exoskeletal hardness differently: Brachyura changed the stacking height of the Bouligand structure, while Anomura regulated magnesium content in the exoskeleton. The vent decapods may prioritize strengthening their carapace over developing robust claws, allocating resources to adapt to the harsh conditions of deep-sea hydrothermal vents. This choice might enhance their survival in the extreme environment, where carapace strength is crucial for protecting internal organs from environmental factors, rather than relying on the powerful claws seen in coastal decapods for a competitive advantage.

Influence of environmental patterns on the population structure and secondary production of the fiddler crab Uca maracoani (Latreille) in the Amazon mangroves

Uca maracoani is a fiddler crab found in estuaries along the western Atlantic coast, with a notable preference for euhaline environments. This study aimed to analyze the population structure and dynamics of this species in an estuary on the North Coast of Brazil, specifically in an area of the upper estuary where seasonal rainfall fluctuations result in significant changes in salinity. Monthly crab samples were taken from December 2013 to November 2015, together with measurements of environmental variables, such as water and climate parameters. The population maintains a balanced sex ratio; however, males are generally larger, with lower mortality rates and longer lifespans than females. Reproduction is continuous but mainly takes place in the dry season when salinity levels are higher (above 12‰). Higher crab densities have been observed during the rainy season when, despite lower salinity levels (below 10‰), the conditions for survival (food availability and milder climate) seem to be more favorable. The estimated average annual biomass and production for the population were 2.62 g AFDM m-2 and 5.43 g AFDM m-2 year-1, respectively, characterized by a high turnover rate (P/B = 2.10 year-1). Our results suggest that U. maracoani has thriving populations in the Amazon coast's mangroves, benefiting from the vast muddy intertidal zone and the high organic content delivered by the estuaries.

Exploring the preservation of a parasitic trace in decapod crustaceans using finite elements analysis

The fossil record of parasitism is poorly understood, due largely to the scarcity of strong fossil evidence of parasites. Understanding the preservation potential for fossil parasitic evidence is critical to contextualizing the fossil record of parasitism. Here, we present the first use of X-ray computed tomography (CT) scanning and finite elements analysis (FEA) to analyze the impact of a parasite-induced fossil trace on host preservation. Four fossil and three modern decapod crustacean specimens with branchial swellings attributed to an epicaridean isopod parasite were CT scanned and examined with FEA to assess differences in the magnitude and distribution of stress between normal and swollen branchial chambers. The results of the FEA show highly localized stress peaks in reaction to point forces, with higher peak stress on the swollen branchial chamber for nearly all specimens and different forces applied, suggesting a possible shape-related decrease in the preservation potential of these parasitic swellings. Broader application of these methods as well as advances in the application of 3D data analysis in paleontology are critical to understanding the fossil record of parasitism and other poorly represented fossil groups.

Population structure and fecundity of the Xanthid crab Leptodius exaratus (H. Milne Edwards, 1834) on the rocky shore of Gujarat state, India

Background

The population structure and breeding biology of the Xanthid crab, Leptodius exaratus (H. Milne Edwards, 1834), on the rocky intertidal region of Shivrajpur in Saurashtra coast, Gujarat state, were examined.

Method

From March 2021 to February 2022, monthly sampling was conducted during low tide using catch per unit effort in the 500 m2 area. The sampled specimens were categorised into male, non-ovigerous female or ovigerous female. In order to estimate fecundity, the morphology of the crab specimens (carapace width and body weight) as well as the size of eggs, number of eggs and weight of egg mass were recorded.

Results

A total of 1,215 individuals were sampled of which 558 individuals were males and 657 individuals were females. The size (carapace width) of males ranges from 5.15 to 29.98 mm, while females ranges from 5.26 to 28.63 mm which shows that the average size of male and female individuals did not differ significantly. The overall as well as monthly sex ratio was skewed towards males with a bimodal distribution while unimodal in females. The population breeds year-round, which was indicated by the occurrence of ovigerous females throughout the year. However, the maximum percentage occurrence of ovigerous females was observed from December to April which indicates the peak breeding season. The size of eggs, number of eggs and weight of egg mass were shown to positively correlate with the morphology of ovigerous females (carapace width and wet weight).

Evolution of horn length and lifting strength in the Japanese rhinoceros beetle Trypoxylus dichotomus

What limits the size of nature's most extreme structures? For weapons like beetle horns, one possibility is a tradeoff associated with mechanical levers: as the output arm of the lever system-the beetle horn-gets longer, it also gets weaker. This "paradox of the weakening combatant" could offset reproductive advantages of additional increases in weapon size. However, in contemporary populations of most heavily weaponed species, males with the longest weapons also tend to be the strongest, presumably because selection drove the evolution of compensatory changes to these lever systems that ameliorated the force reductions of increased weapon size. Therefore, we test for biomechanical limits by reconstructing the stages of weapon evolution, exploring whether initial increases in weapon length first led to reductions in weapon force generation that were later ameliorated through the evolution of mechanisms of mechanical compensation. We describe phylogeographic relationships among populations of a rhinoceros beetle and show that the "pitchfork" shaped head horn likely increased in length independently in the northern and southern radiations of beetles. Both increases in horn length were associated with dramatic reductions to horn lifting strength-compelling evidence for the paradox of the weakening combatant-and these initial reductions to horn strength were later ameliorated in some populations through reductions to horn length or through increases in head height (the input arm for the horn lever system). Our results reveal an exciting geographic mosaic of weapon size, weapon force, and mechanical compensation, shedding light on larger questions pertaining to the evolution of extreme structures.

Electrocommunication signals and aggressive behavior vary among male morphs in an apteronotid fish, Compsaraia samueli

Within-species variation in male morphology is common among vertebrates and is often characterized by dramatic differences in behavior and hormonal profiles. Males with divergent morphs also often use communication signals in a status-dependent way. Weakly electric knifefish are an excellent system for studying variation in male morphology and communication and its hormonal control. Knifefish transiently modulate the frequency of their electric organ discharge (EOD) during social encounters to produce chirps and rises. In the knifefish Compsaraia samueli, males vary extensively in jaw length. EODs and their modulations (chirps and rises) have never been investigated in this species, so it is unclear whether jaw length is related to the function of these signals. We used three behavioral assays to analyze EOD modulations in male C. samueli: (1) artificial playbacks, (2) relatively brief, live agonistic dyadic encounters, and (3) long-term overnight recordings. We also measured circulating levels of two androgens, 11-ketotestosterone and testosterone. Chirp structure varied within and across individuals in response to artificial playback, but was unrelated to jaw length. Males with longer jaws were more often dominant in dyadic interactions. Chirps and rises were correlated with and preceded attacks regardless of status, suggesting these signals function in aggression. In longer-term interactions, chirp rate declined after one week of pairing, but was unrelated to male morphology. Levels of circulating androgens were low and not predictive of jaw length or EOD signal parameters. These results suggest that communication signals and variation in male morphology are linked to outcomes of non-breeding agonistic contests.

From Telson to Attack in Mantis Shrimp: Bridging Biomechanics and Behavior in Crustacean Contests

In the spirit of this symposium on the physical mechanisms of behavior, we review mantis shrimp ritualized fighting, from the telson to the attack, as an inspiring example of how the integration of biomechanics and behavioral research can yield a penetrating narrative for how animals accomplish important activities, including agonistic actions. Resolving conflicts with conspecifics over valuable resources is an essential task for animals, and this takes an unusual form in mantis shrimp due to their powerful raptorial appendages. Decades of field and laboratory research have provided key insights into the natural agonistic interactions of diverse mantis shrimp species, including how they use their raptorial weapons against one another in telson sparring matches over cavities. These insights provided the foundation for functional morphologists, biomechanists, and engineers to work through different levels of organization: from the kinematics of how the appendages move to the elastic mechanisms that power the strike, and down to the structure, composition, and material properties that transmit and protect against high-impact forces. Completing this narrative are studies on the defensive telson and how this structure is biomechanically matched to the weapon and the role it plays in ritualized fighting. The biomechanical understanding of the weapon and defense in mantis shrimp has, in turn, enabled a better understanding of whether mantis shrimp assess one another during contests and encouraged questions of evolutionary drivers on both the arsenal and behavior. Altogether, the body of research focused on mantis shrimp has presented perhaps the most comprehensive understanding of fighting, weapons, and defenses among crustaceans, from morphology and biomechanics to behavior and evolution. While this multi-level analysis of ritualized fighting in mantis shrimp is comprehensive, we implore the need to include additional levels of analysis to obtain a truly holistic understanding of this and other crustacean agonistic interactions. Specifically, both molting and environmental conditions are often missing from the narrative, yet they greatly affect crustacean weapons, defenses, and behavior. Applying this approach more broadly would generate a similarly profound understanding of how crustaceans carry out a variety of important tasks in diverse habitats.

How to become a crab: Phenotypic constraints on a recurring body plan

A fundamental question in biology is whether phenotypes can be predicted by ecological or genomic rules. At least five cases of convergent evolution of the crab-like body plan (with a wide and flattened shape, and a bent abdomen) are known in decapod crustaceans, and have, for over 140 years, been known as "carcinization." The repeated loss of this body plan has been identified as "decarcinization." In reviewing the field, we offer phylogenetic strategies to include poorly known groups, and direct evidence from fossils, that will resolve the history of crab evolution and the degree of phenotypic variation within crabs. Proposed ecological advantages of the crab body are summarized into a hypothesis of phenotypic integration suggesting correlated evolution of the carapace shape and abdomen. Our premise provides fertile ground for future studies of the genomic and developmental basis, and the predictability, of the crab-like body form.

Weapon performance drives weapon evolution

Many sexually selected traits function as weapons, and these weapons can be incredibly diverse. However, the factors underlying weapon diversity among species remain poorly understood, and a fundamental hypothesis to explain this diversity remains untested. Although weapons can serve multiple functions, an undeniably important function is their role in fights. Thus, a crucial hypothesis is that weapon diversification is driven by the evolution of weapon modifications that provide an advantage in combat (e.g. causing more damage). Here, we test this fighting-advantage hypothesis using data from 17 species of coreid bugs. We utilize the fact that male-male combat in coreids often results in detectable damage, allowing us to link different weapon morphologies to different levels of damage among species. We find that certain weapon morphologies inflict much more damage than others, strongly supporting the fighting-advantage hypothesis. Moreover, very different weapon morphologies can inflict similarly severe amounts of damage, leading to a weapon performance landscape with multiple performance peaks. This multi-peak pattern could potentially drive different lineages towards divergent weapon forms, further increasing weapon diversity among species. Overall, our results may help explain how sexually selected weapons have evolved into the diversity of forms seen today.

Harder, better, faster, stronger: Weapon size is more sexually dimorphic than weapon biomechanical components in two freshwater anomuran species

Sexual selection influences the evolution of morphological traits that increase the likelihood of monopolizing scarce resources. When such traits are used during contests, they are termed weapons. Given that resources are typically linked to monopolizing mating partners, theory expects only males to bear weapons. In some species, however, females also bear weapons, although typically smaller than male weapons. Understanding why females bear smaller weapons can thus help us understand the selective pressures behind weapon evolution. However, most of our knowledge comes from studies on weapon size, while the biomechanics of weapons, such as the size of the muscles, efficiency, and shape are seldom studied. Our goal was to test if the theoretical expectations for weapon size sexual dimorphism also occur for weapon biomechanics using two aeglid crab species. Males of both species had larger claws which were also stronger than female claws. Male claws were also more efficient than females' claws (although we used only one species in this analysis). For weapon shape, though, only one species differed in the mean claw shape. Regarding scaling differences, in both species, male claws had higher size scaling than females, while only one species had a higher shape scaling. However, male weapons did not have higher scaling regarding strength and efficiency than females. Thus, males apparently allocate more resources in weapons than females, but once allocated, muscle and efficiency follow a similar developmental pathway in both sexes. Taken together, our results show that sexual dimorphism in weapons involves more than differences in size. Shape differences are especially intriguing because we cannot fully understand its causes. Yet, we highlight that such subtle differences can only be detected by measuring and analysing weapon shape and biomechanical components. Only then we might better understand how weapons are forged.

Variation in an Extreme Weapon: Horn Performance Differences across Rhinoceros Beetle (Trypoxylus dichotomus) Populations

Japanese rhinoceros beetle (Trypoxylus dichotomus) males have exaggerated head horns that they use as weapons in combat over reproductive opportunities. In these contests, there is an advantage to having a longer horn, and there seems to be little cost to horn exaggeration. However, populations vary in the amount of horn exaggeration across this widespread species. Here, we examine four populations and quantify scaling and functional morphology of the horn. We then measure force production by the horn system in a combat-relevant movement. We find that not only does horn length vary among populations, but allometry of lever mechanics and force production varies in a complex way. For instance, some beetle populations make relatively long horns, but exert relatively low forces. Other populations make shorter horns and produce higher forces during fights. We suggest that this performance variation could be associated with differences in the intensity or type of sexual selection across the species.

Size, shape, and sex-dependent variation in force production by crayfish chelae

The ability to generate large closing forces is important for many animals. Several studies have demonstrated that bite or pinching force capacity is usually related to the linear dimensions of the closing apparatus. However, relatively few studies have applied geometric morphometrics to examine the effects of size-independent shape on force production, particularly in studies of crustacean pinching force. In this study, we utilized traditional and geometric morphometric techniques to compare the pinching force of Procambarus clarkii crayfish to their chela morphology. We found that males possessed larger chelae and pinched harder than females, but that their chela shape and size were weak predictors of strength. Female pinching force was significantly affected by both chela size and shape, with shape variation along the short axis of the claw contributing most to pinching force. We discuss our results in the context of reliable signaling of strength by males and females, and the different selective forces acting on chela shape in the two sexes.

The benefits of expanding studies of trait exaggeration to hemimetabolous insects and beyond morphology

Trait exaggeration, well known to naturalists and evolutionary biologists, has recently become a prominent research subject in the modern field of Evolutionary Developmental Biology. A large number of traits that can be considered as cases of exaggeration exist in nature. Yet, the field has almost exclusively focused on the study of growth-related exaggerated traits in a selection of holometabolous insects. The absence of the hemimetabola from studies of exaggeration leaves a significant gap in our understanding of the development and evolution of such traits. Here we argue that efforts to understand the mechanisms of trait exaggeration would benefit from expanding the study subjects to include other kinds of exaggeration and other model species.

Chela asymmetry in a durophagous crab: predominance of right-handedness and handedness reversal is linked to chela size and closing force

The swimming crab Portunus trituberculatus is a durophagous brachyuran. Right-handed crabs are predominant, but left-handed crabs are also found in nature. Left-handedness may arise from loss of the right crusher. We examined whether heterochely (morphology) was correlated with differences in closing force (physical property) and handedness (behaviour). The closing force was stronger in larger chela with greater apodeme height and handedness resided in the chela with stronger closing force. With loss of the right chela (autotomy), handedness transitioned from the right to left chela, and all crabs were left-handed thereafter. Reversed handedness was accompanied with a reduction of size and closing force in the regenerated right chela, and growth of the original left chela. After handedness reversal, dentition on the left dactylus of the newly-converted crusher was close to that of the original right crusher, but did not attain the same shape, even after 10 moults. Left-handed crabs were significantly worse than right-handed crabs at crushing hard-shelled prey. Chela formation was symmetrical in the zoea, and heterochely and right-handedness started in the megalopa, regardless of maternal handedness. Since the left chela is capable of being the crusher, heterochely may be caused by differences in morphogenetic velocity between the right and left chelae, under a signal discriminating right from left. Right-handedness is an attribute of P. trituberculatus, that would be inheritable across generations. It is probable that right-handedness was used in the earliest durophagous crabs, and this trend has been succeeded to extant species.