Scent of death: Evolution from sea to land of an extreme collective attraction to conspecific death

The plastic homes of hermit crabs in the Anthropocene

Plastic is the most pervasive element of marine waste, with harmful impact on wildlife. By using iEcology (i.e., internet Ecology, use of online data sources as a new tool in ecological research), we report on the emergence of a novel behaviour in hermit crabs related to the use of plastic or other anthropogenic materials as protective shells. We analysed images posted on social media to identify 386 individuals with artificial shells - mainly plastic caps (85 %). We report that 10 of the world's 16 terrestrial hermit crabs use artificial shells, a behaviour observed on all of the Earth's tropical coasts. Four non-exclusive mechanisms may drive individual choice for artificial shells: sexual signaling, lightness of artificial shells, odour cues, and camouflage in a polluted environment. Further research is needed to determine the impact of this behaviour on hermit crab evolutionary trajectories.

Doors to the Homes: Signal Potential of Red Coloration of Claws in Social Hermit Crabs

Red coloration on a signaler's body may be an informative signal in many animals. For species that inhabit architecture (e.g., burrows, nests, or other structures), certain parts of the body are more exposed than others, potentially serving as superior platforms for signaling via coloration. Yet whether animals differentially advertise red coloration on body parts that are more versus less exposed from their architecture remains to be tested. Here, we systematically quantified red coloration in social hermit crabs (Coenobita compressus). These crabs inhabit architecturally remodeled shells and have claws that visibly block the shell entrance, like doors to their homes. We hypothesized that red coloration of claws may be a signal of resource-holding potential (RHP). Consistent with this RHP signaling hypothesis, we found that within the same individuals' bodies, exposed claws showed significantly greater red coloration than unexposed carapaces. Furthermore, larger body size predicted greater red coloration of claws. Competing hypotheses (e.g., interspecific signaling, camouflage, and UV protection), while not explicitly tested, nevertheless appear unlikely based on natural history. Red claw coloration may therefore function as a signal to conspecifics, and experiments are now needed to test recipient responses. Broadly, relative to surrounding architecture, exposed body surfaces offer rich potential as signaling platforms for coloration.

Wild social behavior differs following experimental loss of vision in social hermit crabs

Even for animals with multiple senses at their disposal, there may be a strong reliance on a single sense, like vision, for social behavior. Experimentally blocking or eliminating vision offers a powerful means of testing impacts on social behavior, though few studies have followed experimentally blinded individuals in the wild to test potential changes in social behavior in natural settings. Here we conducted experiments with social hermit crabs (Coenobita compressus), applying opaque material overtop their eyes to temporarily blind individuals. We then released these experimentally blinded individuals and non-blinded control individuals into the wild as well as into captive social settings. Compared to control individuals, experimentally blinded individuals initiated significantly fewer social contacts with conspecifics in the wild. These experimentally blinded individuals were not, however, differentially targeted by conspecifics. Interestingly, unlike the wild experiments, the captive experiments showed no differences in social behavior between experimentally blinded and non-blinded control individuals, suggesting that experiments in natural settings in the wild may be essential to fully unraveling impacts of blindness on social behavior. Broadly, for social animals that are highly reliant on the visual modality, social behavior may change dramatically if they lose their vision.

Cross-sensory interference assessment after exposure to noise shows different effects in the blue crab olfactory and sound sensing capabilities

Underwater noise pollution is an increasing threat to marine ecosystems. Marine animals use sound in communication and orientation processes. The introduction of anthropogenic noise in their habitat can interfere with sound production and reception as well as with the acquisition of vital information through other sensory systems. In the blue crab (Callinectes sapidus), the statocyst is responsible for acoustic perception, and it is housed at the base of its first pair of antennae (antennule). The sensilla of the distal part of these antennule hosts the olfactory system, which is key for foraging. Given the anatomical proximity of the two sensory regions, we evaluated the possible interference of sound exposure with the crab ability to find food, by using an aquatic maze, and looked at the potential impairment of the righting reflex as well as at ultrastructural damages in statocysts. Although a significant effect was observed when looking at the time used by the animal to recover its habitual position ("righting reflex"), which was associated to lesions in the statocyst sensory epithelia, the time required to find food did not increase after the exposure to sound. When the crabs were exposed to natural sounds (marine background noise and sounds of their predators: Micropogonias undulates and Sciaenops ocellatus) they did not show significant differences in foraging behaviour. Although we found no unequivocal evidence of a negative impact of sound on olfactory capabilities, the study showed a clear righting reflex impairment correlated with ultrastructural damages of the statocysts. We argue that crab populations that cannot easily avoid noise sources due to their specific coastal distributions may incur in significant direct fitness costs (e.g. impairment of complex reflexes). This integrated approach to sound effect assessment could be used as a model for other invertebrate species to effectively monitor noise impact in marine environments.

Individualism versus collective movement during travel

Collective movement may emerge if coordinating one’s movement with others produces a greater benefit to oneself than can be achieved alone. Experimentally, the capacity to manoeuvre simulated groups in the wild could enable powerful tests of the impact of collective movement on individual decisions. Yet such experiments are currently lacking due to the inherent difficulty of controlling whole collectives. Here we used a novel technique of experimentally simulating the movement of collectives of social hermit crabs (Coenobita compressus) in the wild. Using large architectural arrays of shells dragged across the beach, we generated synchronous collective movement and systematically varied the simulated collective’s travel direction as well as the context (i.e., danger level). With drone video from above, we then tested whether focal individuals were biased in their movement by the collective. We found that, despite considerable engagement with the collective, individuals’ direction was not significantly biased. Instead, individuals expressed substantial variability across all stimulus directions and contexts. Notably, individuals typically achieved shorter displacements in the presence of the collective versus in the presence of the control stimulus, suggesting an impact of traffic. The absence of a directional bias in individual movement due to the collective suggests that social hermit crabs are individualists, which move with a high level of opportunistic independence, likely thanks to the personal architecture and armour they carry in the form of a protective shell. Future studies can manipulate this level of armour to test its role in autonomy of movement, including the consequences of shell architecture for social decisions. Our novel experimental approach can be used to ask many further questions about how and why collective and individual movement interact.

The Architecture of Cooperation Among Non-kin: Coalitions to Move Up in Nature’s Housing Market

The evolution of cooperation among non-kin poses a major theoretical puzzle: why should natural selection favor individuals who help unrelated conspecifics at a cost to themselves? The relevance of architecture to this question has rarely been considered. Here I report cooperation among non-kin in social hermit crabs (Coenobita compressus), where unrelated conspecifics work together to evict larger individuals from a housing market of architecturally remodeled shells. I present (1) the first detailed description of natural coalitions in the wild and (2) a theoretical framework, which examines the evolutionary benefits to each coalition member and predicts when forming a coalition will be successful. In the wild, important ecological and social constraints exist, which are built into the model. Based on these constraints, I show that coalitions can be a successful strategy if several key criteria hold: the coalition is necessary, effective, stable dyadically, and stable polyadically. Notably, the “splitting the spoils” problem—which often undermines non-kin cooperation—is eliminated via architecture: a small individual (C) who helps a medium individual (B) to evict a large individual (A) will ultimately benefit, since C will get B’s left behind shell after B moves into A’s shell. Coalitions, however, can break down due to added layers of social complexity involving third-party “free riders” and “cheaters,” which strategically butt in the architectural queue and thereby steal incentives from the smaller coalition member. Overall, therefore, substantial scope exists for both cooperation and conflict within nature’s housing market of architecture. Experiments are now needed to directly test the impact on coalitions of architecture, from the interior of homes up to whole housing markets.

Social conquest of land: Sea-to-land changes in shell architecture and body morphology, with consequences for social evolution

Architecture, like nests, burrows, and other types of fortresses, may have played an important role in the evolution of social life on land. However, few studies have examined architecture in organisms that transitioned from sea to land to test how and why architectural and morphological changes might have jointly impacted social evolution. Here I contrasted the shell architecture and body morphology of two of the phylogenetically most closely-related land versus sea species of hermit crab (the terrestrial hermit crab, Coenobita compressus, and the marine hermit crab, Calcinus obscurus), as well as the original builder of their shells (the gastropod, Nerita scabricosta). In contrast to the shells of gastropods and marine hermit crabs, only the shells of terrestrial hermit crabs were architecturally remodeled, with no columella inside for the occupants to grip upon to resist eviction. The bodies of terrestrial hermit crabs were also significantly more exposed outside the enlarged openings of their remodeled shells, whereas the substantially smaller-bodied marine hermit crabs were safeguarded deep within the recesses of their unremodeled shells. Ultimately, these changes in shell architecture and body morphology likely had consequences for social evolution on land, making conspecifics not only more dependent upon one another for homes, but also potentially easier to evict. Further changes in claw shape on land (with the claws of terrestrial hermit crabs becoming shorter, wider, and thicker) may have evolved to help offset their heightened danger of social eviction, acting as a more effective door against conspecifics.

More than one way to smell ashore - Evolution of the olfactory pathway in terrestrial malacostracan crustaceans

Crustaceans provide a fascinating opportunity for studying adaptations to a terrestrial lifestyle because within this group, the conquest of land has occurred at least ten times convergently. The evolutionary transition from water to land demands various morphological and physiological adaptations of tissues and organs including the sensory and nervous system. In this review, we aim to compare the brain architecture between selected terrestrial and closely related marine representatives of the crustacean taxa Amphipoda, Isopoda, Brachyura, and Anomala with an emphasis on the elements of the olfactory pathway including receptor molecules. Our comparison of neuroanatomical structures between terrestrial members and their close aquatic relatives suggests that during the convergent evolution of terrestrial life-styles, the elements of the olfactory pathway were subject to different morphological transformations. In terrestrial anomalans (Coenobitidae), the elements of the primary olfactory pathway (antennules and olfactory lobes) are in general considerably enlarged whereas they are smaller in terrestrial brachyurans compared to their aquatic relatives. Studies on the repertoire of receptor molecules in Coenobitidae do not point to specific terrestrial adaptations but suggest that perireceptor events - processes in the receptor environment before the stimuli bind - may play an important role for aerial olfaction in this group. In terrestrial members of amphipods (Amphipoda: Talitridae) as well as of isopods (Isopoda: Oniscidea), however, the antennules and olfactory sensilla (aesthetascs) are largely reduced and miniaturized. Consequently, their primary olfactory processing centers are suggested to have been lost during the evolution of a life on land. Nevertheless, in terrestrial Peracarida, the (second) antennae as well as their associated tritocerebral processing structures are presumed to compensate for this loss or rather considerable reduction of the (deutocerebral) primary olfactory pathway. We conclude that after the evolutionary transition from water to land, it is not trivial for arthropods to establish aerial olfaction. If we consider insects as an ingroup of Crustacea, then the Coenobitidae and Insecta may be seen as the most successful crustacean representatives in this respect.

Evolutionary loss of threat display in more social species: phylogenetic comparisons, natural interactions in the wild, and experiments with models

Fitness can be increased dramatically by communication. So why, given the benefits of communication, would displays ever be completely lost evolutionarily? Threat displays, in particular, are relatively cheap to produce and are precursors of attack, so should be especially hard for both senders and receivers to lose completely. Here we explore an evolutionary transition in sociality, testing whether complete evolutionary loss of threat display has occurred in ‘more social’ hermit crab species, which interact more frequently with conspecifics. First, we synthesised literature and observations on the presence versus absence of threat display across hermit crab species, mapping this information onto a phylogenetic tree. We found that all ‘less social’ species — marine and terrestrial — produce threat displays, consistent with threat display being the ancestral state. But ‘more social’ terrestrial species, which are highly derived, do not produce a threat display, suggesting an evolutionary loss. Next, we contrasted natural interactions in the wild within a less social species (Pagurus bernhardus) versus within a more social species (Coenobita compressus), finding that the less social species, despite a lower rate of social encounter, had a higher rate of display per encounter (24%). In contrast, the more social species’ rate of display per encounter was negligible (<1%), effectively indicating a loss in production. Finally, we experimentally reanimated threat display in the more social species, using postured models to test whether receivers retained any responsiveness to threat display. Starkly, receivers were not deterred by threat display, showing equal responsiveness across both threat and non-threat models, regardless of whether the models were stationary or dynamically moving. Our results thus reveal a case of complete collapse of communication involving threat display, implicating the social environment in this loss. In more social species, an extreme dependence on conspecific-derived shells likely drove a ‘desperado effect’, with threat displays being lost because they could not stop others from pursuing these valuable resources.

Shells as 'extended architecture': to escape isolation, social hermit crabs choose shells with the right external architecture

Animals’ cognitive abilities can be tested by allowing them to choose between alternatives, with only one alternative offering the correct solution to a novel problem. Hermit crabs are evolutionarily specialized to navigate while carrying a shell, with alternative shells representing different forms of ‘extended architecture’, which effectively change the extent of physical space an individual occupies in the world. It is unknown whether individuals can choose such architecture to solve novel navigational problems. Here, we designed an experiment in which social hermit crabs (Coenobita compressus) had to choose between two alternative shells to solve a novel problem: escaping solitary confinement. Using X-ray microtomography and 3D-printing, we copied preferred shell types and then made artificial alterations to their inner or outer shell architecture, designing only some shells to have the correct architectural fit for escaping the opening of an isolated crab’s enclosure. In our ‘escape artist’ experimental design, crabs had to choose an otherwise less preferred shell, since only this shell had the right external architecture to allow the crab to free itself from isolation. Across multiple experiments, crabs were willing to forgo preferred shells and choose less preferred shells that enabled them to escape, suggesting these animals can solve novel navigational problems with extended architecture. Yet, it remains unclear if individuals solved this problem through trial-and-error or were aware of the deeper connection between escape and exterior shell architecture. Our experiments offer a foundation for further explorations of physical, social, and spatial cognition within the context of extended architecture.

Anthropomorphism in comparative affective science: Advocating a mindful approach

Anthropomorphism is the attribution of human-like capacities and traits to non-human entities. Anthropomorphism is ubiquitous in everyday life and in scientific domains, operating both implicitly and explicitly as a function of the human lens through which we view the world. A rich history of work in psychology, animal behavior, cognitive science, and philosophy has highlighted the negative and, to a lesser degree, the positive implications of anthropomorphism. In this article, we aim to provide a nuanced perspective of how anthropomorphism impacts the work of comparative affective science. Specifically, we discuss three domains of empirical inquiry in which lessons can be drawn about the benefits and pitfalls of anthropomorphism: responses to death, inequity aversion, and prosocial behavior. On balance, we advocate a mindful approach to anthropomorphizing in comparative affective science, and comparative science more generally.

Entrapment in plastic debris endangers hermit crabs

Significant quantities of plastic debris pollute nearly all the world's ecosystems, where it persists for decades and poses a considerable threat to flora and fauna. Much of the focus has been on the marine environment, with little information on the hazard posed by debris accumulating on beaches and adjacent vegetated areas. Here we investigate the potential for beach debris to disrupt terrestrial species and ecosystems on two remote islands. The significant quantities of debris on the beaches, and throughout the coastal vegetation, create a significant barrier which strawberry hermit crabs (Coenobita perlatus) encounter during their daily activities. Around 61,000 (2.447 crabs/m²) and 508,000 crabs (1.117 crabs/m²) are estimated to become entrapped in debris and die each year on Henderson Island and the Cocos (Keeling) Islands, respectively. Globally, there is an urgent need to establish a clear link between debris interactions and population persistence, as loss of biodiversity contributes to ecosystem degradation. Our findings show accumulating debris on these islands has the potential to seriously impact hermit crab populations. This is important for countless other islands worldwide where crabs and debris overlap, as crabs play a crucial role in the maintenance of tropical ecosystems.

Finding a home in the noise: cross-modal impact of anthropogenic vibration on animal search behaviour

Chemical cues and signals enable animals to sense their surroundings over vast distances and find key resources, like food and shelter. However, the use of chemosensory information may be impaired in aquatic habitats by anthropogenic activities, which produce both water-borne sounds and substrate-borne vibrations, potentially affecting not only vibroacoustic sensing but other modalities as well. We attracted marine hermit crabs (Pagurus acadianus) in field experiments using a chemical cue indicative of a newly available shell home. We then quantified the number of crabs arriving in control versus impulsive noise conditions. Treatment (control or noise), time (before or after), and the interaction between the two significantly affected the numbers of crabs, with fewer crabs attracted to the chemical cue after noise exposure. The results indicate that noise can affect chemical information use in the marine environment, acting cross-modally to impact chemically-guided search behaviour in free-ranging animals. Broadly, anthropogenic noise and seabed vibration may have profound effects, even on behaviours mediated by other sensory modalities. Hence, the impact of noise should be investigated not only within, but also across sensory modalities.

This article has an associated First Person interview with the first author of the paper.

Summary: Chemical cues enable animals to sense their surroundings and find key resources. Here we show that anthropogenic noise affects a chemically-guided search behaviour by acting cross-modally.

Architectural modification of shells by terrestrial hermit crabs alters social dynamics in later generations

Organisms architecturally modify environments and these modifications may persist across generations, potentially strongly shaping social behavior. However, few experiments have directly tested the impact of architectural modifications from earlier generations on social behavior in later generations. Here, I report experiments using extremely durable resources, shells, which endure for decades to centuries in stable form. Terrestrial hermit crabs (Coenobita compressus) architecturally remodel shells and pass these modified shelters to subsequent generations, which reuse them long after the original architect's death. I conducted controlled field experiments in a population of these crabs in which shells have been individually marked and tracked for a decade. I examined the impact of architectural modifications by contrasting social behavior around introduced shells, either remodeled shells (whose internal architecture was modified by earlier generations) or unremodeled shells (whose architecture had never been modified). Remodeled shells generated radically different social dynamics than unremodeled shells, catalyzing vacancy chains in which shells were socially redistributed across the population. Social groups that formed around remodeled shells consisted of size-ordered queues, with precise timing and social coordination required if individuals were to acquire superior shells. Interestingly, comparative experiments in two non-architect species (Clibanarius albidigitus and Calcinus obscurus) failed to show any impact of architectural modifications on social behavior; such impacts were only found in the architect species (C. compressus). Broadly, architecture from earlier generations can thus play a major role in driving social dynamics among later generations.