Uca Vomeris Research Articles

Meiofauna and crabs in mangroves and adjoining sandflats: Is the interaction physical or trophic?

Meiofauna distribute widely in most soft substrates in the marine and freshwater realms. Given their small body size (63 to 500μm) and high density, meiofauna are potential food items for predators such as deposit-feeding brachyuran crabs. Crab bioturbation may also affect meiofaunal assemblages through effects such as translocation to unsuitable microhabitats. This study aimed to investigate the significance and nature of top-down control on the density of meiofauna based on their interactions with deposit-feeding crabs in a mangrove and adjoining sandflat; specifically, whether the interaction is primarily physical or trophic. Field manipulative experiments were conducted within the aggregation zones of soldier crabs (Mictyris longicarpus) and fiddler crabs (Uca vomeris) in a mangrove-lined creek in Southeast Queensland, Australia. Meiofaunal density in five experimental cage treatments (Exclusion, Inclusion with complete crab (‘Inclusion’), Inclusion with ‘disabled’ crab (feeding claw removed, ‘Disabled’), Half-cage, and Ambient) was compared. Removal of soldier crabs from the cages (Exclusion) increased meiofaunal density (426±46 ind./10cm2; mean±SE) by 50% over that in the Inclusion treatment (283±22). The nature of the interactions was further investigated by comparing meiofaunal density in the Inclusion treatment (with both physical and trophic effects present) with that in the Disabled treatment (with physical but no trophic effect present). Removal of trophic effect by ‘disabling’ the crab increased meiofaunal density by 30% compared to that in the Inclusion treatment, but at a similar density to the Exclusion treatment. This pattern suggests that the top-down control by soldier crabs on the meiofauna is fundamentally trophic, i.e. predation. In the experiment with fiddler crabs, meiofaunal densities in the inclusion treatments (Inclusion and Disabled) were not significantly different from each other, but density was reduced by more than 50% in the Exclusion treatment. Fiddler crabs significantly impact the meiofauna through their bioturbation activities such as sediment turnover and burrowing, but their trophic activities did not significantly reduce meiofaunal density. Different crab species at different habitats, therefore, may influence meiofaunal density through different processes on sub-tropical soft shores.

Confessions of an ambidextrous fiddler crab

Confessions of an ambidextrous fiddler crab

Building a dishonest signal: the functional basis of unreliable signals of strength in males of the two-toned fiddler crab, Uca vomeris.

Males of many species use signals during aggressive contests to communicate their fighting capacity. These signals are usually reliable indicators of an individual's underlying quality; however, in several crustacean species, displays of weapons do not always accurately reflect the attribute being advertised. Male fiddler crabs possess one enlarged claw that is used to attract females and to intimidate opponents during territorial contests. After the loss of their major claw, males can regenerate a replacement claw that is similar in size but considerably weaker. As this inferior weapon can still be used to successfully intimidate rivals, it represents one of the clearest cases of unreliable signalling of strength during territorial contests. We investigated the functional mechanisms that govern signal reliability in the two-toned fiddler crab, Uca vomeris. Male U. vomeris exhibit both reliable and unreliable signals of strength via the expression of original and regenerated claw morphs. We examined the morphological, biomechanical and biochemical characteristics of original and regenerated claws to establish the best predictors of variation in claw strength. For a given claw size, regenerated claws have less muscle mass than original claws, and for a given muscle mass, regenerated claws were significantly weaker than original claws. The mechanical advantage was also lower in regenerated claws compared with original claws. However, the activity of three catabolic enzymes did not differ between claw types. We conclude that the structural and physiological predictors of force production influence the frequencies of reliable and unreliable signals of strength in U. vomeris. This study furthers our understanding of the proliferation of unreliable signals in natural populations.

Functional anatomy of the fiddler crab compound eye (Uca vomeris: Ocypodidae, Brachyura, Decapoda)

We describe the structural organization of the ommatidium in the compound eye of the fiddler crab, Uca vomeris, at both the light- and the electron-microscopy levels. We pay particular attention to the organization of the optical system, the retinular cells, the rhabdom, and of pigment cells. Although the fiddler crab compound eye is of the apposition type, typical for Brachyuran crabs, we identify a number of novel, functionally relevant aspects of ommatidial organization that have not previously been described. The flat corneal facet lenses provide the main focusing power and therefore must contain a gradient of refractive index. Each ommatidium has the typical set of eight retinular cells, with a distal retinular cell R8 lying close to the proximal tip of the crystalline cone. R8 is shaped into four lobes, which are separated by proximal extensions of the four crystalline cone cells and of distal extensions of retinular cells R1-R7. The microvilli in the R8 rhabdom are not aligned in a uniform direction, while the microvilli of the main rhabdom show the typical crustacean pattern of alternating bands of horizontally (R3, R4, R7) and vertically aligned microvilli (R1, R2, R5, R6). We describe in detail the distribution and structural properties of screening pigment granules in the two types of pigment cells and in the retinular cells in the equatorial eye. We discuss the functional significance of this fine-structural organization of the fiddler crab compound eye in relation to visual processing and visual ecology.

Metabolic incentives for dishonest signals of strength in the fiddler crab Uca vomeris.

To reduce the potential costs of combat, animals may rely upon signals to resolve territorial disputes. Signals also provide a means for individuals to appear better than they actually are, deceiving opponents and gaining access to resources that would otherwise be unattainable. However, other than resource gains, incentives for dishonest signalling remain unexplored. In this study, we tested the idea that unreliable signallers pay lower metabolic costs for their signals, and that energetic savings could represent an incentive for cheating. We focused on two-toned fiddler crabs (Uca vomeris), a species that frequently uses its enlarged claws as signals of dominance to opponents. Previously, we found that regenerated U. vomeris claws are often large but weak (i.e. unreliable). Here, we found that the original claws of male U. vomeris consumed 43% more oxygen than weaker, regenerated claws, suggesting that muscle quantity drives variation in metabolic costs. Therefore, it seems that metabolic savings could provide a powerful incentive for dishonesty within fiddler crabs.

High e-vector acuity in the polarisation vision system of the fiddler crabUca vomeris

Polarisation vision is used by a variety of species in many important tasks, including navigation and orientation (e.g. desert ant), communication and signalling (e.g. stomatopod crustaceans), and as a possible substitute for colour vision (e.g. cephalopod molluscs). Fiddler crabs are thought to possess the anatomical structures necessary to detect polarised light, and occupy environments rich in polarisation cues. Yet little is known about the capabilities of their polarisation sense. A modified polarisation-only liquid crystal display and a spherical rotating treadmill were combined to test the responses of fiddler crabs to moving polarisation stimuli. The species Uca vomeris was found to be highly sensitive to polarised light and detected stimuli differing in e-vector angle by as little as 3.2 deg. This represents the most acute behavioural sensitivity to polarised light yet measured for a crustacean. The occurrence of null points in their discrimination curve indicates that this species employs an orthogonal (horizontal/vertical) receptor array for the detection of polarised light.

Is honesty the best policy? Testing signal reliability in fiddler crabs when receiver‐dependent costs are high

Summary1.Contests between conspecific males are an important method of establishing mating rights or territories, yet the potential costs of injuries are high. To reduce potential risks, males can use signals to convey information about their underlying strength to competitors, and although individuals could signal deceptively to gain an advantage, most signals seem to be reliable.2.Theory suggests that signal reliability is maintained because individuals that signal unreliably may be punished (i.e. receiver‐imposed costs). Manipulative studies support this idea, showing that low‐quality individuals that are given high‐quality signals bear substantial costs.3.Here, we explore the importance of receiver‐imposed costs in natural, un‐manipulated populations. Specifically, we show how the likelihood of being exposed (i.e. potential receiver density) and the potential severity of punishment (i.e. average receiver size) affect the predominance of unreliable signalling among populations.4.Male fiddler crabs, Uca vomeris, use their enlarged claws as signals and weapons in combat, and the relationship between claw size (signal) and strength (quality) determines whether or not a male is a reliable signaller. We predicted that the prevalence of unreliable signalling among individual males would increase in populations where the receiver‐imposed costs of deception were low. That is, males should produce unreliable (large but weak) claws.5.We show that individual crabs produce more reliable signals of strength in populations with a high biomass, where there are both higher densities and larger average body sizes of potential receivers. Our study provides evidence that receiver‐imposed costs can maintain signal reliability in natural un‐manipulated populations and supports contemporary models of aggressive signalling.

The role of polarized light in prey capture in an aquatic predator

The role of polarized light in prey capture in an aquatic predator

Fiddler crab burrow defence: A robot simulation

Fiddler crab burrow defence: A robot simulation

Flicker is part of a multi-cue response criterion in fiddler crab predator avoidance

Predator avoidance behaviour costs time, energy and opportunities, and prey animals need to balance these costs with the risk of predation. The decisions necessary to strike this balance are often based on information that is inherently imperfect and incomplete because of the limited sensory capabilities of prey animals. Our knowledge, however, about how prey animals solve the challenging task of restricting their responses to the most dangerous stimuli in their environment is very limited. Using dummy predators, we examined the contribution of visual flicker to the predator avoidance response of the fiddler crab Uca vomeris. The results illustrate that crabs let purely black or purely white dummies approach significantly closer than black-and-white flickering dummies. We show that this effect complements other factors that modulate escape timing such as retinal speed and the crab's distance to its burrow, and is therefore not due exclusively to an earlier detection of the flickering signal. By combining and adjusting a range of imperfect response criteria in a way that relates to actual threats in their natural environment, prey animals may be able to measure risk and adjust their responses more efficiently, even under difficult or noisy sensory conditions.

Habituation under natural conditions: model predators are distinguished by approach direction

Habituation is an active process that allows animals to learn to identify repeated, harmless events, and so could help individuals deal with the trade-off between reducing the risk of predation and minimizing escape costs. Safe habituation requires an accurate distinction between dangerous and harmless events, but in natural environments such an assessment is challenging because sensory information is often noisy and limited. What, then, comprises the information animals use to recognize objects that they have previously learned to be harmless? We tested whether the fiddler crab Uca vomeris distinguishes objects purely by their sensory signature or whether identification also involves more complex attributes such as the direction from which an object approaches. We found that crabs habituated their escape responses after repeated presentations of a dummy predator consistently approaching from the same compass direction. Females habituated both movement towards the burrow and descent into the burrow, whereas males only habituated descent into the burrow. The crabs were more likely to respond again when a physically identical dummy approached them from a new compass direction. The crabs distinguished between the two dummies even though both dummies were visible for the entire duration of the experiment and there was no difference in the timing of the dummies' movements. Thus, the position or approach direction of a dummy encodes important information that allows animals to identify an event and habituate to it. These results argue against the traditional notion that habituation is a simple, non-associative learning process, and instead suggest that habituation is very selective and uses information to distinguish between objects that is not available from the sensory signature of the object itself.

Burrow-centered neural model for burrow surveillance in fiddler crabs

A fiddler crab can estimate how close an intruder is from its burrow [1]. It is believed that crabs use visual information as well as path integration to process that estimation. The fiddler crab Uca vomeris defend their burrows against intruders and it seems they use a burrow-centered reference coordinate. The crabs respond when an intruder approaches the burrow within some distance of the burrow. There are several possible cues to estimate the distance between the intruder and the burrow entrance by visual information such as disparity, optical flow, image size and the retinal elevation. However, experiments show only specific visual information is related to the burrow surveillance [1]. The retinal position, azimuth and elevation angle of the intruder in view is related to the distance between the intruder and the observer, following the geometric function. The former parameters can determine the intruder-crab distance depending on the crab-burrow distance. Fiddler crabs on leaving home accumulate the distance and direction of the exploring path can determine the homing distance and direction from the current position to the burrow [2]. To determine the distance between an intruder crab and the burrow entrance, we need to use the crab-burrow distance by the path integration as well as the azimuth and elevation angle of an intruder. There has been a suggestion that the burrow surveillance in fiddler crabs may be connected with directionally sensitive neurons and distance neurons [1]. Our experiments with a neural model for a mapping function to determine the intruder-burrow distance shows large errors are observed when the intruder is at the opposite side of the crab, since the far vicinity has a low resolution. Thus, we suggest that the fiddler crab might estimate the boundary distance of the response to defend its burrow rather than accurately calculate the distance of the intruder from the burrow. In addition, the crab can determine how directly an intruder is moving toward its burrow. We can thus build a burrow-centered frame of reference with a circular array of directionally sensitive neurons. The firings of the neurons can be mapped from the retinal position of the intruder, that is, the azimuth and elevation angle of the intruder. From that, the fiddler crab can easily determine whether or not the intruder approaches the burrow. As a result, we can build a model of the crab’s response when an intruder approaches the burrow within some distance of the burrow.

Topography of vision and behaviour

Given the great range of visual systems, tasks and habitats, there is surprisingly little experimental evidence of how visual limitations affect behavioural strategies under natural conditions. Analysing this relationship will require an experimental system that allows for the synchronous measurement of visual cues and visually guided behaviour. The first step in quantifying visual cues from an animal's perspective is to understand the filter properties of its visual system. We examined the first stage of visual processing - sampling by the ommatidial array - in the compound eye of the fiddler crab Uca vomeris. Using an in vivo pseudopupil method we determined sizes and viewing directions of ommatidia and created a complete eye map of optical and sampling resolution across the visual field. Our results reveal five distinct eye regions (ventral, dorsal, frontal, lateral and medial) which exhibit clear differences in the organisation of the local sampling array, in particular with respect to the balance of resolution and contrast sensitivity. We argue that, under global eye space constraints, these regional optimisations reflect the information content and behavioural relevance of the corresponding parts of the visual field. In demonstrating the tight link between visual sampling, visual cues and behavioural strategies, our analysis highlights how the study of natural behaviour and natural stimuli is essential to our understanding and interpretation of the evolution and ecology of animal behaviour and the design of sensory systems.

High stimulus specificity characterizes anti-predator habituation under natural conditions

Habituation is one of the most fundamental learning processes that allow animals to adapt to dynamic environments. It is ubiquitous and often thought of as a simple form of non-associative learning. Very little is known, though, about the rules that govern habituation and their significance under natural conditions. Questions about how animals incorporate habituation into their daily behaviour and how they can assure only to habituate to non-relevant stimuli are still unanswered. Animals under threat of predation should be particularly selective about which stimuli they habituate to, since ignoring a real threat could be fatal. In this study, we tested the response of fiddler crabs, Uca vomeris, to repeatedly approaching dummy predators to find out whether these animals habituate to potential predators and to test the selectivity of the habituation process. The crabs habituated to model predators, even though they were confronted with real predators during the same habituation process. They showed remarkable selectivity towards the stimulus: a simple change in the approach distance of the stimulus led to a recovery in their responses. The results strongly indicate that in the context of predator avoidance, habituation under natural conditions is highly selective and a stimulus is not defined just by its current sensory signature, but also its spatio-temporal history.

The variable colours of the fiddler crabUca vomerisand their relation to background and predation

Colour changes in fiddler crabs have long been noted, but a functional interpretation is still lacking. Here we report that neighbouring populations of Uca vomeris in Australia exhibit different degrees of carapace colours, which range from dull mottled to brilliant blue and white. We determined the spectral characteristics of the mud substratum and of the carapace colours of U. vomeris and found that the mottled colours of crabs are cryptic against this background, while display colours provide strong colour contrast for both birds and crabs, but luminance contrast only for a crab visual system. We tested whether crab populations may become cryptic under the influence of bird predation by counting birds overflying or feeding on differently coloured colonies. Colonies with cryptically coloured crabs indeed experience a much higher level of bird presence, compared to colourful colonies. We show in addition that colourful crab individuals subjected to dummy bird predation do change their body colouration over a matter of days. The crabs thus appear to modify their social signalling system depending on their assessment of predation risk.

Predator avoidance in fiddler crabs: 1. Escape decisions in relation to the risk of predation

The risk of predation is a strong force shaping many aspects of animal behaviour. Early detection and efficient avoidance strategies not only help prey to survive, but also limit the negative impact predation has on other aspects of their lives. The type of antipredator strategy an animal uses must depend on its ability to collect accurate information on the risks to which it is exposed. However, models attempting to predict when a prey animal should escape from an approaching predator often assume that the prey has accurate information on a predator's distance and direction of approach. To test whether such models could be applied to a prey animal with restricted sensory capabilities, I explored the predator avoidance behaviour of the fiddler crab Uca vomeris by approaching crabs with small dummies intended to mimic a hunting tern. The crabs responded strongly and reliably to the simple dummies. They were clearly sensitive to risk and responded more often and earlier, the further away they were from their refuge. The probability of response was most strongly influenced by how directly and therefore how closely the dummies approached the crabs and reached 100% for very direct approaches. Surprisingly, however, the crabs responded later when the dummies approached them more directly. I argue that this counterintuitive result reflects a lack of reliable information on the predator's distance and movement relative to the crab and its refuge, because like many small animals, crabs cannot measure the distance to a predator. I conclude that general models attempting to predict when animals should respond to an approaching predator need to incorporate the information that prey animals have available at the time of response.

Predator avoidance in fiddler crabs: 2. The visual cues

The efficiency of predator avoidance strategies depends on the availability and accuracy of sensory information. Although vision can in principle provide instant information on a predator's position, direction of approach and identity, prey animals that face fast predators have to respond so early (close to the limits of detection) that visual cues are unreliable predictors of actual risk. This is a major problem for prey animals that have to balance predation risk with the cost of antipredator action. I investigated the visual cues fiddler crabs Uca vomeris use to decide when to run towards their burrow in response to an approaching (dummy) predator by running towards their burrow. The crabs did not always escape immediately when they first detected the dummy, but continued to monitor and assess its approach. The crabs relied on retinal image speed to trigger a home run. Retinal speed did not correlate well with the actual risk of predation because it confounded a predator's direction of approach with its speed and proximity. In an attempt to reconcile what is known about predator avoidance in semiterrestrial crabs, I propose a two-tier antipredator response system and discuss the crabs' multistage response strategy in the context of the associated costs and the availability of information. Multistage predator avoidance, which is common among prey animals, appears to be designed to improve the quality of information in an attempt to minimize the costs associated with predator avoidance responses.

IN DEFENSE OF A BURROW

![Figure][1] With real estate in high demand, foraging is risky for fiddler crabs, Uca vomeris . Ever vigilant for burrow thieves, the crabs stay within a meter of the entrance and rush back to defend the burrow when another crab gets too close. To do this effectively, they must be able to

Signals from 'crabworld': cuticular reflections in a fiddler crab colony.

Fiddler crabs inhabit intertidal sand- and mudflats, where they live in dense colonies and are active on the surface during low tide. They exhibit a rich behavioural repertoire, with frequent interactions between animals in the context of territorial and mating activities. Male fiddler crabs have one massively enlarged and conspicuously coloured claw, which they use in waving displays and in fights with other males. The crabs carry their eyes on long, vertically oriented stalks high above the body and, as a consequence, see the bodies of conspecifics in the ventral visual field, below the local visual horizon, and against the mudflat surface as background. We filmed events in a colony of Uca vomeris with a normal video camera and an ultraviolet-sensitive camera placed at the eye height of an average crab, approximately 2-3 cm above ground. We also used a spectrographic imager and linear polarized filters to analyse the cues potentially available to the animals for detecting, monitoring and possibly identifying each other. Areas of high contrast in mudflat scenes include specular reflections on the wet cuticle of crabs that are horizontally polarised. Besides specular reflections, some parts of the cuticle generate high-contrast signals against the mudflat background, both at wavelengths between 400 and 700 nm, and in the ultraviolet region between 300 and 400 nm. Uca vomeris can be very colourful: the different parts of the large claw of the male are white, orange or red. The carapace colours of both males and females can range from a mottled yellowish green brown, to a brilliant light blue. White and blue colours contrast starkly with the mudflat background, especially in the ultraviolet wavelengths. Under stress, the blue and white colours can change within minutes to a duller and darker blue or to a dull white.

Homing in fiddler crabs ( Uca lactea annulipes and Uca vomeris : Ocypodidae)

Fiddler crabs emerge from burrows on intertidal sand- and mudflats to feed during low tide. In the species studied here (Uca lactea annulipes, Uca vomeris) a crab normally wanders no more than about 1 m away from its burrow and, when frightened, dashes back along a straight line to take cover. Feeding crabs tend to move sideways, without changing orientation, along paths radiating from the burrow. When they move along circumferential paths they adjust their orientation so that one side continues to point towards the burrow. The crabs do not need to see the burrow in order to stay aligned with the home vector, and they are not misled by a dummy hole close to their own burrow unless they have come to within about 10 cm of it. The home runs of crabs end within a few centimeters of a burrow that is covered with a sheet of sandpaper and then give way to search runs, centred upon a position slightly short of the burrow location. Feeding crabs can be displaced on sandpapers and their subsequent home runs end at a position where the burrow would be, had there been no displacement. Landmarks close to the burrow do not influence the home runs of displaced crabs. Crabs that are rotated on a sheet of sandpaper, counter-turn to keep their original orientation constant. Fiddler crabs thus employ path integration with external compass information and close range visual guidance for homing.