The Influence of Personality on a Group‐Level Process: Shy Hermit Crabs Make Longer Vacancy Chains

Animal personality has been investigated in a variety of contexts but few studies have investigated the effects of individual‐level traits on processes that occur at the group or population level. Vacancy chains describe sequences of linked vacancy moves in systems where individuals use discrete and reuseable resource units. When a new resource unit becomes available, this can cause a cascade of moves between resource units, beyond the first individual that encounters the new unit. Thus, vacancy chains can drive the distribution of resources through a population. Here, I investigate the effect of variation in boldness on movements between vacant gastropod shells in the hermit crab Pagurus bernhardus. Boldness was quantified using the duration of a startle response, which varied consistently between individuals. In groups of five crabs, introduction of a single new shell led to aggregate benefits, in terms of improving shell quality, across all group members. Boldness did not influence the extent of these benefits or the extent to which they were distributed between crabs of different sizes. Initially, vacancy chains were longer, indicating that more individuals moved between shells, for bold groups compared with shy groups. After 24 h, however, the chain length was longer for shy groups. Thus, although participation in a vacancy chain may lead to similar benefits, these could be more widespread for groups comprising shy individuals that show long startle responses, low inquisitiveness and low aggression. Similar effects on resource acquisition could be present in other examples of animals that utilise reusable resource units and show consistent between‐individual differences in behaviour.     

Effects of Predation Threat on the Structure and Benefits from Vacancy Chains in the Hermit Crab Pagurus bernhardus

Vacancy chains occur when individuals occupy discrete re‐useable resource units, which once abandoned by the current owner can then be occupied by a new owner. In order to enter the newly vacated resource the new owner must first vacate its current resource unit, such that a vacancy chain consists of a series of linked moves between resource units of different value, equivalent to different ‘strata’ in the chain. Vacancy chains may represent an important route by which resources are distributed through populations. Indeed, the arrival of a new resource has the potential to initiate a series of moves propagating beyond the individual that encounters the new resource unit. Thus, the chain participants as a whole may experience ‘aggregate benefits’ from the arrival of the new resource unit. The extent of these benefits, however, may not necessarily be evenly distributed between all chain participants; some individuals could receive greater than average benefits by moving through more than one stratum (‘skipping’) and some individuals could experience a reduction in resource value by moving to a resource unit of lower quality than that occupied initially (a ‘backward move’). Such moves represent deviations from the ‘ideal’ vacancy chains assumed by theory. Here we analyse the aggregate benefits and benefits to individuals participating in vacancy chains of empty gastropod shells in the hermit crab Pagurus bernhardus. We also investigate the effect of predation risk on these two levels of benefits and on chain structure. Adding a new shell at the top of the chain causes an overall increase in shell quality after 24 h but the distribution of benefits between strata in the chain varies with the presence and absence of the predator cue. Although there was significant concordance between chain structure in the presence and absence of the predator cue, the structure was significantly different from an ideal vacancy chain in the absence but not the presence of the predator cue.     

Morphology and Histochemistry of the Aesthetasc-Associated Epidermal Glands in Terrestrial Hermit Crabs of the Genus Coenobita (Decapoda: Paguroidea)

Crustaceans have successfully adapted to a variety of environments including fresh- and saltwater as well as land. Transition from an aquatic to a terrestrial lifestyle required adaptations of the sensory equipment of an animal, particularly in olfaction, where the stimulus itself changes from hydrophilic to mainly hydrophobic, air-borne molecules. Hermit crabs Coenobita spp. (Anomura, Coenobitidae) have adapted to a fully terrestrial lifestyle as adults and have been shown to rely on olfaction in order to detect distant food items. We observed that the specialized olfactory sensilla in Coenobita, named aesthetascs, are immersed in a layer of mucous-like substance. We hypothesized that the mucous is produced by antennal glands and affects functioning of the aesthetascs.Using various microscopic and histochemical techniques we proved that the mucous is produced by aesthetasc-associated epidermal glands, which we consider to be modified rosette-type aesthetasc tegumental glands known from aquatic decapods. These epidermal glands in Coenobita are multicellular exocrine organs of the recto-canal type with tubulo-acinar arrangement of the secretory cells. Two distinct populations of secretory cells were clearly distinguishable with light and electron microscopy. At least part of the secretory cells contains specific enzymes, CUB-serine proteases, which are likely to be secreted on the surface of the aesthetasc pad and take part in antimicrobial defense. Proteomic analysis of the glandular tissue corroborates the idea that the secretions of the aesthetasc-associated epidermal glands are involved in immune responses.We propose that the mucous covering the aesthetascs in Coenobita takes part in antimicrobial defense and at the same time provides the moisture essential for odor perception in terrestrial hermit crabs. We conclude that the morphological modifications of the aesthetasc-associated epidermal glands as well as the functional characteristics of their secretions are important adaptations to a terrestrial lifestyle.