Neuromuscular correlates of rhythmical cheliped flexion behavior in hermit crabs

Journal of Comparative Physiology A -     

1/f noise in membranes.

The present situation of 1/f noise in the passage of ions across membranes is examined. A survey of biological and synthetic membranes is given at which a 1/f frequency dependence has been observed in the spectrum of voltage or current fluctuations. Empirical relations and theories of 1/f noise in membranes are critically discussed.     

Effects of chemical context on shell investigation behavior in hermit crabs

Specific chemicals in the environment evoke significant changes in the behavior of many aquatic organisms. We studied in the laboratory whether satiated individuals of the hermit crab, Pagurus longicarpus Say 1817, adjust their investigatory behavior towards an empty, optimal gastropod shell according to differences of chemical context. We also explored to what extent shell investigation by a crab in the same hunger state was affected by occupying an inadequately sized shell. Our results confirmed in part previous findings that crabs can discriminate the odor of freshly dead snails from the odor of freshly dead conspecifics. In the presence of the former odor, crabs inhabiting shells of inadequate size were more responsive and active than those in better-fitting shells. To the contrary, regardless of the quality of the inhabited shell, P. longicarpus remained practically motionless when presented with the odor of freshly dead conspecifics, possibly because the risks of incurring in predators would outweigh the benefits of acquiring a new shell. Unexpectedly, we found that crabs in both types of shell quality exhibited nearly the same behavior in control water, while crabs in adequate shells were more responsive in the presence of food odor. Individuals appeared insensitive to the odor of live snails; indeed, only one hermit crab species has been seen removing living snails from their shells. An intriguing result was that water conditioned by the odors of live conspecifics exerted a strong effect on all the individuals by inducing an intense shell investigation. Our study underlines the central role exerted by chemical detection in hermit crabs' behavior and demonstrates the existence of a complex interplay among chemical context, the physiological state of the animal, and the ecological pressures of the habitat.     

Protein dynamics and 1/f noise.

It has long been recognized that protein dynamical processes occur over a wide temporal range. However, the functionality of this spectrum of events remains unclear. In this work, a generalized noise function analysis is applied to a collection of diverse protein dynamical systems. It is shown that a power law model with an oscillatory component can adequately describe the time course of a variety of processes. These results suggest that under the appropriate conditions, proteins are in a metastable state. A microscopic, chemical kinetic model based on a Poisson distribution of activation energies is presented. From this model specific functional forms for the parameters of the generalized noise model can be derived. Additionally, a model is presented to described kinetic hole burning effects observed at low temperatures. Scaling laws are derived for these models that provide a connection with the generalized noise analysis.     

1/f noise in membranes

The present situation of 1/f noise in the passage of ions across membranes is examined. A survey of biological and synthetic membranes is given at which a l/f frequency dependence has been observed in the spectrum of voltage or current fluctuations. Empirical relations and theories of 1/f noise in membranes are critically discussed. Supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 38 “Membranforschung”     

Diffusion and 1/f noise

Summary The noise associated with ion transport through porous membranes is considered as a diffusion process. This is confirmed experimentally by measuring the noise spectra associated with pores of known dimension. It is then shown that one dimensional diffusion through pores of variable length can produce approximate 1/f noise spectra, if the distribution of lengths is proportional to (length)^–1.     

Auditory stimulation dishabituates anti-predator escape behavior in hermit crabs (Coenobita clypeatus)

Responses to innocuous stimuli often habituate with repeated stimulation, but the mechanisms involved in dishabituation are less well studied. Chan et al. (2010b) found that hermit crabs were quicker to perform an anti-predator withdrawal response in the presence of a short-duration white noise relative to a longer noise stimulus. In two experiments, we examined whether this effect could be explicable in terms of a non-associative learning process. We delivered repeated presentations of a simulated visual predator to hermit crabs, which initially caused the crabs to withdraw into their shells. After a number of trials, the visual stimulus lost the ability to elicit the withdrawal response. We then presented the crabs with an auditory stimulus prior to an additional presentation of the visual predator. In Experiment 1, the presentation of a 10-s, 89-dB SPL noise produced no significant dishabituation of the response. In Experiment 2 we increased the duration (50 s) and intensity (95 dB) of the noise, and found that the crabs recovered their withdrawal response to the visual predator. This finding illustrates dishabituation of an antipredator response and suggests two distinct processes—distraction and sensitization—are influenced by the same stimulus parameters, and interact to modulate the strength of the anti-predator response.     

Models for 1/f noise in nerve membranes.

A recently proposed model for 1/f(w-1) noise in nerve membrane (Clay and Schlesinger, 1976; Lundström and McQueen, 1974) is shown to be mathematically inconsistent in several respects. A self-consistent model based on similar membranes lipid orientation fluctuation effects is proposed.     

Response of hermit crabs to sinistral shells

Shell rotating behavior of the hermit crabPagurus geminus was investigated. In preliminary observations, hermit crabs motivated to change shells rotated presented shells, filled with sand, in a way that dislodged the inside material. In order to determine if this behavior is stereotyped, or flexible and dependent on shell type, hermit crabs were tested with ordinary dextral shells ofLatirulus nagasakiensis and sinistral shells ofAntiplanes contraria. Sinistral shells are not normally encountered by hermit crabs. Their rotation of the dextral shell to the left was adequate for sand discharge. Sinistral shells were rotated in both directions. Analysis of recorded videotapes showed that variation in rotation direction could be attributed to variation in the position of the crab relative to the shell. When the crab faced the shell aperture from the inner lip, it rotated the sinistral shell to the right, and to opposite direction when it faced from the outer lip side. The crab always pushed the upper side of the horizontally laid shell, regardless of shell type or its own position.     

Use of energy reserves in fighting hermit crabs

When animals engage in fights they face a series of decisions, which are based on the value of the contested resource and either their relative or their absolute fighting ability. Certain correlates of fighting ability or 'resource holding potential' such as body size are fixed but physiological correlates are expected to vary during the encounter. We examine the role of energy reserves in determining fight outcomes and parameters during 'shell fighting' in hermit crabs. During these fights, the two contestants perform very different roles of attacker and defender. We show that the balance of the total energy pool, in the form of glucose and glycogen, determines the ability of defenders to resist eviction from their shells. Low glucose in evicted defenders is not caused by depletion of energy reserves, rather mobilization of glycogen appears to be the result of a strategic decision about whether to resist effectively, based on the perceived fighting ability of the attacker. Attackers, however, always initiate the fight so such a decision for this role appears unlikely. In addition to influencing decisions and ability during fights, physiological correlates of fighting ability can in turn be influenced by strategic decisions.     

Vulnerability and reliable signaling in conflicts between hermit crabs

When interacting organisms have opposing genetic interests,the integrity of communication systems may be undermined. Forsignaling in such conflict circumstances to remain evolutionarilystable, cheaters must be handicapped. Agonistic threat signals,however, are not always constrained or costly to produce, andyet these signals occur in the severest of conflicts where strongincentives exist for dishonesty. A leading hypothesis for howreliability is stabilized under these conditions is that signalingentails a risk, making signalers vulnerable to injury. HereI experimentally alter vulnerability to show how risk can modifyorganisms' willingness to escalate disputes, affecting the useof threat signals. The vulnerability to injury of hermit crabs(Pagurus bernhardus) was manipulated by varying the exposureof their soft uncalcified abdomens. When faced with potentiallydamaging conspecific attacks, more vulnerable crabs were conflictaverse, showing reluctance to claim ownership over contestablefood, frequently retreating from threats, and refraining fromthreatening others. The risk an organism can bear in escalatedconflict can thus mediate its agonistic behavior and usage ofthreats. Postural nuances can consequently provide reliableinformation about aggressive intentions despite minimal productioncosts and opposing interests between communication parties.     

Frequencies of interspecific shell exchanges between hermit crabs

Frequencies of interspecific shell exchange due to shell fighting were determined for a number of species pairs of hermit crabs from several different locations. Frequencies were determined in the laboratory using a standardized experimental design. Results suggest that most individuals of most species are able to retain adequate or good quality shells in the presence of members of another species occupying poor quality shells. High frequencies of shell exchange always seem to be associated with very asymmetric relationships in which one member of the species pair is clearly dominant over the other. Dominant species usually attain larger sizes than subordinates, are found lower in the intertidal habitat, and are less abundant.     

Extinction risk and the 1/f family of noise models

In order to predict extinction risk in the presence of reddened, or correlated, environmental variability, fluctuating parameters may be represented by the family of 1/f noises, a series of stochastic models with different levels of variation acting on different timescales. We compare the process of parameter estimation for three 1/f models (white, pink and brown noise) with each other, and with autoregressive noise models (which are not 1/f noises), using data from a model time-series (length, T) of population. We then calculate the expected increase in variance and the expected extinction risk for each model, and we use these to explore the implication of assuming an incorrect noise model. When parameterising these models, it is necessary to do so in terms of the measured ("sample") parameters rather than fundamental ("population") parameters. This is because these models are non-stationary: their parameters need not stabilize on measurement over long periods of time and are uniquely defined only over a specified "window" of timescales defined by a measurement process. We find that extinction forecasts can differ greatly between models, depending on the length, T, and the coefficient of variability, CV, of the time series used to parameterise the models, and on the length of time into the future which is to be projected. For the simplest possible models, ones with population itself the 1/f noise process, it is possible to predict the extinction risk based on CV of the observed time series. Our predictions, based on explicit formulae and on simulations, indicate that (a) for very short projection times relative to T, brown and pink noise models are usually optimistic relative to equivalent white noise model; (b) for projection timescales equal to and substantially greater than T, an equivalent brown or pink noise model usually predicts a greater extinction risk, unless CV is very large; and (c) except for very small values of CV, for timescales very much greater than T, the brown and pink models present a more optimistic picture than the white noise model. In most cases, a pink noise is intermediate between white and brown models. Thus, while reddening of environmental noise may increase the long-term extinction probability for stationary processes, this is not generally true for non-stationary processes, such as pink or brown noises.     

Evolutionary palaeoecology of symbioses between bryozoans and hermit crabs

Modern hermit crabs form associations with many organisms which encrust, bore into, or cohabit the living chambers of gastropod shells occupied by the crabs. Among these hermit crab symbionts are bryozoan species which develop massive, commonly multilayered, colonies encrusting hermit crab shells. These colonies extend the living chamber of the crab through a characteristic process of helicospiral tubular growth originating from the shell aperture. The scant information available on the ecology of Recent bryozoan‐hermit crab symbioses is reviewed. Symbioses have been recorded from intertidal to upper slope environments, and from tropical to cold temperate zones. None of the hermit crab species are obligatory symbionts of bryozoans, and the majority of the modern bryozoan species involved are also not obligatory symbionts. Fossil examples always lack the hermit crabs, which have a poor fossilization potential; however, the distinctive tubular growth pattern and other features of the bryozoans enable recognition of ancient examples of the symbiosis. The earliest inferred associations between bryozoans and hermit crabs date from the Mid Jurassic, but associations remained uncommon until the Neogene. A remarkably wide taxonomic diversity of Recent and fossil bryozoans are known or inferred symbionts of hermit crabs. The broad evolutionary pattern of the association demonstrates multiple originations of the symbiosis by bryozoans belonging to at least 5 cyclostome and 12 cheilostome families. Only the Miocene‐Recent cheilostome family Hippoporidridae has an evolutionary history closely tied to symbiosis with hermit crabs. There is no evidence for coevolution.