Views from 'crabworld': the spatial distribution of light in a tropical mudflat

Natural scene analysis has been extensively used to understand how the invariant structure of the visual environment may have shaped biological image processing strategies. This paper deals with four crucial, but hitherto largely neglected aspects of natural scenes: (1) the viewpoint of specific animals; (2) the fact that image statistics are not independent of the position within the visual field; (3) the influence of the direction of illumination on luminance, spectral and polarization contrast in a scene; and (4) the biologically relevant information content of natural scenes. To address these issues, I recorded the spatial distribution of light in a tropical mudflat with a spectrographic imager equipped with a polarizing filter in an attempt to describe quantitatively the visual environment of fiddler crabs. The environment viewed by the crabs has a distinct structure. Depending on the position of the sun, the luminance, the spectral composition, and the polarization characteristics of horizontal light distribution are not uniform. This is true for both skylight and for reflections from the mudflat surface. The high-contrast feature of the line of horizon dominates the vertical distribution of light and is a discontinuity in terms of luminance, spectral distribution and of image statistics. On a clear day, skylight intensity increases towards the horizon due to multiple scattering, and its spectral composition increasingly resembles that of sunlight. Sky-substratum contrast is highest at short wavelengths. I discuss the consequences of this extreme example of the topography of vision for extracting biologically relevant information from natural scenes.

Allostasis revisited: A perception, variation, and risk framework

The framework of allostasis, allostatic load and overload (i.e., stability through change) attempts to combine homeostasis processes in day-to-day responses of physiology and behavior. These include predictive changes in environment such as seasons, and facultative responses to perturbations. The latter can be severe, occur at any time, and may present considerable additional challenges to homeostasis. Hormonal cascades, such as the hypothalamo-pituitary-adrenal cortex (HPA) axis, play a key role in responses to perturbations across vertebrate taxa. Glucocorticoids have been implicated in these processes in relation to energy balance that plays a role in determining responses to energetic demand (allostatic load) and influencing subsequent physiology and behavior associated with coping. Circulating glucocorticoid levels are likely regulated in part based on an individual’s proximity to energetic crisis, identified as the perturbation resistance potential (PRP). In the model of allostatic load, PRP is quantified as the difference between available resources and all energetic costs of allostatic load such as daily routines, life history stages (breeding, migration, molt and so on), and the impact of environmental perturbations. PRP can change gradually or abruptly and may be reflected by spikes in blood hormone levels. The pattern of individual responsiveness to PRP may vary and has specific implications for the activation of mineralocorticoid vs glucocorticoid-type receptors, hormone metabolizing enzymes and other downstream factors in target tissues. However, PRP is a difficult metric to measure. Here, we examine the variety of cues that animals may use to inform them about the status of their PRP and probability of energetic crisis. We consider (1) elevation in glucocorticoids as an endocrine “decision,” and (2) error management strategies in evaluating responsiveness to cues that may reflect or predict an impending energetic crisis. The potential for differential receptor activation as well as further integrative “decisions” to determine the diverse and sometimes contradictory effects of receptor activation and its downstream actions are important to the consideration of error management. This perspective offers insight into the basis of intra- and inter-individual variability in responsiveness and opens an avenue toward improving compatibility of the allostasis model with more classical views on “stress”.

Behavioural and neural responses of crabs show evidence for selective attention in predator avoidance

Selective attention, the ability to focus on a specific stimulus and suppress distractions, plays a fundamental role for animals in many contexts, such as mating, feeding, and predation. Within natural environments, animals are often confronted with multiple stimuli of potential importance. Such a situation significantly complicates the decision-making process and imposes conflicting information on neural systems. In the context of predation, selectively attending to one of multiple threats is one possible solution. However, how animals make such escape decisions is rarely studied. A previous field study on the fiddler crab, Gelasimus dampieri, provided evidence of selective attention in the context of escape decisions. To identify the underlying mechanisms that guide their escape decisions, we measured the crabs' behavioural and neural responses to either a single, or two simultaneously approaching looming stimuli. The two stimuli were either identical or differed in contrast to represent different levels of threat certainty. Although our behavioural data provides some evidence that crabs perceive signals from both stimuli, we show that both the crabs and their looming-sensitive neurons almost exclusively respond to only one of two simultaneous threats. The crabs' body orientation played an important role in their decision about which stimulus to run away from. When faced with two stimuli of differing contrasts, both neurons and crabs were much more likely to respond to the stimulus with the higher contrast. Our data provides evidence that the crabs' looming-sensitive neurons play an important part in the mechanism that drives their selective attention in the context of predation. Our results support previous suggestions that the crabs' escape direction is calculated downstream of their looming-sensitive neurons by means of a population vector of the looming sensitive neuronal ensemble.

Fiddler crab electroretinograms reveal vast circadian shifts in visual sensitivity and temporal summation in dim light

Many animals with compound eyes undergo major optical changes to adjust visual sensitivity from day to night, often under control of a circadian clock. In fiddler crabs, this presents most conspicuously in the huge volume increase of photopigment-packed rhabdoms and the widening of crystalline cone apertures at night. These changes are hypothesised to adjust the light flux to the photoreceptors and to alter optical sensitivity as the eye moves between light- and dark-adapted states. Here, we compare optical sensitivity in fiddler crab eyes (Gelasimus dampieri) during daytime and night via three electroretinogram (ERG) experiments performed on light- and dark-adapted crabs.

1) Light intensity required to elicit a threshold ERG response varied over six orders of magnitude, allowing more sensitive vision for discriminating small contrasts in dim light after dusk. During daytime, the eyes remained relatively insensitive, which would allow effective vision on bright mudflats, even after prolonged dark adaptation.

2) Flicker fusion frequency (FFF) experiments indicated that temporal summation is employed in dim light to increase light-gathering integration times and enhance visual sensitivity during both night and day.

3) ERG responses to flickering lights during 60 mins of dark adaptation increased at a faster rate and greater extent after sunset compared to daytime. However, even brief, dim and intermittent light exposure strongly disrupted dark-adaptation processes.

Together, these findings demonstrate effective light adaptation to optimise vision over the large range of light intensities that these animals experience.

Evidence of predictive selective attention in fiddler crabs during escape in the natural environment

Selective attention is of fundamental relevance to animals for performing a diversity of tasks such as mating, feeding, predation and avoiding predators. Within natural environments, prey animals are often exposed to multiple, simultaneous threats, which significantly complicates the decision-making process. However, selective attention is rarely studied in complex, natural environments or in the context of escape responses. We therefore asked how relatively simple animals integrate the information from multiple, concurrent threatening events. Do they identify and respond only to what they perceive as the most dangerous threat, or do they respond to multiple stimuli at the same time? Do simultaneous threats evoke an earlier or stronger response than single threats? We investigated these questions by conducting field experiments and compared escape responses of the fiddler crab Gelasimus dampieri when faced with either a single or two simultaneously approaching dummy predators. We used the dummies' approach trajectories to manipulate the threat level; a directly approaching dummy indicated higher risk while a tangentially approaching dummy that passed the crabs at a distance represented a lower risk. The crabs responded later, but on average more often, when approached more directly. However, when confronted with the two dummies simultaneously, the crabs responded as if approached only by the directly approaching dummy. This suggests that the crabs are able to predict how close the dummy's trajectory is to a collision course and selectively suppress their normally earlier response to the less dangerous dummy. We thus provide evidence of predictive selective attention within a natural environment.

The predator and prey behaviors of crabs: from ecology to neural adaptations

Predator avoidance and prey capture are among the most vital of animal behaviors. They require fast reactions controlled by comparatively straightforward neural circuits often containing giant neurons, which facilitates their study with electrophysiological techniques. Naturally occurring avoidance behaviors, in particular, can be easily and reliably evoked in the laboratory, enabling their neurophysiological investigation. Studies in the laboratory alone, however, can lead to a biased interpretation of an animal's behavior in its natural environment. In this Review, we describe current knowledge – acquired through both laboratory and field studies – on the visually guided escape behavior of the crab Neohelice granulata. Analyses of the behavioral responses to visual stimuli in the laboratory have revealed the main characteristics of the crab's performance, such as the continuous regulation of the speed and direction of the escape run, or the enduring changes in the strength of escape induced by learning and memory. This work, in combination with neuroanatomical and electrophysiological studies, has allowed the identification of various giant neurons, the activity of which reflects most essential aspects of the crabs' avoidance performance. In addition, behavioral analyses performed in the natural environment reveal a more complex picture: crabs make use of much more information than is usually available in laboratory studies. Moreover, field studies have led to the discovery of a robust visually guided chasing behavior in Neohelice. Here, we describe similarities and differences in the results obtained between the field and the laboratory, discuss the sources of any differences and highlight the importance of combining the two approaches.

Differences in the escape response of a grapsid crab in the field and in the laboratory

Escape behaviours of prey animals are frequently used to study the neural control of behaviour. Escape responses are robust, fast, and can be reliably evoked under both field and laboratory conditions. Many escape responses are not as simple as previously suggested, however, and are often modulated by a range of contextual factors. To date it has been unclear to what extent behaviours studied in controlled laboratory experiments are actually representative of the behaviours that occur under more natural conditions. Here we have used the model species, Neohelice granulata, a grapsid crab, to show that there are significant differences between the crabs' escape responses in the field compared to those previously documented in laboratory experiments. These differences are consistent with contextual adjustments such as the availability of a refuge and have clear consequences for understanding the crabs' neural control of behaviour. Furthermore, the methodology used in this study mirrors the methodology previously used in fiddler crab research, allowing us to show that the previously documented differences in escape responses between these grapsid species are real and substantial. Neohelice's responses are delayed and more controlled. Overall, the results highlight the adaptability and flexibility of escape behaviours and provide further evidence that the neural control of behaviour needs to be address in both the laboratory and field context.

Ocean acidification slows retinal function in a damselfish through interference with GABAA receptors

Vision is one of the most efficient senses used by animals to catch prey and avoid predators. Therefore, any deficiency in the visual system could have important consequences for individual performance. We examined the effect of CO2 levels projected to occur by the end of this century on retinal responses in a damselfish, by determining the threshold of its flicker electroretinogram (fERG). The maximal flicker frequency of the retina was reduced by continuous exposure to elevated CO2, potentially impairing the capacity of fish to react to fast events. This effect was rapidly counteracted by treatment with a GABA antagonist (gabazine), indicating that GABAA receptor function is disrupted by elevated CO2. In addition to demonstrating the effects of elevated CO2 on fast flicker fusion of marine fishes, our results show that the fish retina could be a model system to study the effects of high CO2 on neural processing.

Null point of discrimination in crustacean polarisation vision

The polarisation of light is used by many species of cephalopods and crustaceans to discriminate objects or to communicate. Most visual systems with this ability, such as that of the fiddler crab, include receptors with photopigments that are oriented horizontally and vertically relative to the outside world. Photoreceptors in such an orthogonal array are maximally sensitive to polarised light with the same fixed e-vector orientation. Using opponent neural connections, this two-channel system may produce a single value of polarisation contrast and, consequently, it may suffer from null points of discrimination. Stomatopod crustaceans use a different system for polarisation vision, comprising at least four types of polarisation-sensitive photoreceptor arranged at 0°, 45°, 90° and 135° relative to each other, in conjunction with extensive rotational eye movements. This anatomical arrangement should not suffer from equivalent null points of discrimination. To test whether these two systems were vulnerable to null points, we presented the fiddler crab Uca heteropleura and the stomatopod Haptosquilla trispinosa with polarised looming stimuli on a modified LCD monitor. The fiddler crab was less sensitive to differences in the degree of polarised light when the e-vector was at -45°, than when the e-vector was horizontal. In comparison, stomatopods showed no difference in sensitivity between the two stimulus types. The results suggest that fiddler crabs suffer from a null point of sensitivity, while stomatopods do not.