The visual ecology of fiddler crabs

With their eyes on long vertical stalks, their panoramic visual field and their pronounced equatorial acute zone for vertical resolving power, the visual system of fiddler crabs is exquisitely tuned to the geometry of vision in the flat world of inter-tidal mudflats. The crabs live as burrow-centred grazers in dense, mixed-sex, mixed-age and mixed-species colonies, with the active space of an individual rarely exceeding 1 m². The full behavioural repertoire of fiddler crabs can thus be monitored over extended periods of time on a moment to moment basis together with the visual information they have available to guide their actions. These attributes make the crabs superb subjects for analysing visual tasks and the design of visual processing mechanisms under natural conditions, a prerequisite for understanding the evolution of visual systems. In this review we show, on the one hand, how deeply embedded fiddler crab vision is in the behavioural and the physical ecology of these animals and, on the other hand, how their behavioural options are constrained by their perceptual limitations. Studying vision in fiddler crabs reminds us that vision has a topography, that it is context-dependent and pragmatic and that there are perceptual limits to what animals can know and therefore care about. For Mike Land     

The visual control of behaviour in fiddler crabs

Male fiddler crabs (Uca pugilator Bosc) have visual control systems that enable them to track other crabs in front or behind, and to keep potential predators to the side, where escape is easiest. The system for tracking conspecifics appears to be double, with a low-gain velocity-sensitive mechanism operating over about a 90° range, backed up by a position-sensitive mechanism at the ends of this range which is responsible for recentring the target. This system has separate front and rear ranges, with a gap in the direction of the claw. The crabs separately fixate the burrow entrance, keeping it in the direction opposite the claw. Predator evasion employs two systems simultaneously. An openloop mechanism directs the crab's translatory movements directly away from the stimulus, and a rotational mechanism using continuous feedback turns the crab so that the stimulus is kept at near 90° to the body axis. Both systems are sensitive to the angular position of the stimulus, not its velocity. Eye movements have little or no role in object tracking. An attempt is made to list Uca's known visual control systems.     

Synchronous waving in two species of fiddler crabs

In the fiddler crabs Uca saltitanta and Uca perplexa, males attract mates by waving their enlarged claws. We show that in both species waving is closely synchronised between neighbouring males in clusters, both in the presence of mate-searching females and in their absence. Wandering females visit those males in the cluster that produce more waves at faster wave rates. In U. perplexa, they also selectively visit those males that produce the greatest number of leading waves. Synchronous waving may be the result of a precedence effect causing male competition to produce leading signals.     

Synchronous waving in fiddler crabs: a review

Many animals that use acoustic communication synchronize their mate attract!on signals: individuals precisely time their calls to overlap those of their neighbors. In contrast, synchrony in the mate attraction displays of species with visual/motion-based signals is rare. It has only been documented in five species of fiddler crabs. In all of them, small groups of males wave their single large claw in close synchrony. Here, I review what we know about synchrony in fiddler crabs, comparing the five species with each other to determine whether similar mechanisms and functions are comm on to all. I also propose future research questions that, if answered, would shed light on synchronous behavior in both visual and acoustic signallers.     

Interspecific assistance: fiddler crabs help heterospecific neighbours in territory defence

Theory predicts that territory owners will help established neighbours to repel intruders, when doing so is less costly than renegotiating boundaries with successful usurpers of neighbouring territories. Here, we show for the first time, to our knowledge, cooperative territory defence between heterospecific male neighbours in the fiddler crabs Uca elegans and Uca mjoebergi. We show experimentally that resident U. elegans were equally likely to help a smaller U. mjoebergi or U. elegans neighbour during simulated intrusions by intermediate sized U. elegans males (50% of cases for both). Helping was, however, significantly less likely to occur when the intruder was a U. mjoebergi male (only 15% of cases).     

Male versus female mate searching in fiddler crabs: a comparative analysis

We present a comparative analysis of mate searching in fiddlercrabs, genus Uca. Several ecological factors determine whichsex will search for mates and how complex male signaling willbe. Female searching is most tightly correlated with matingin male burrows. Female searching is associated with high burrowdensity, small body size, and large soil size. These factors explain variation in a female's need for male-defended incubationsites. Female searching also is correlated with short eyestalks.In species in which females search for mates, males use a morecomplex mate attraction signal than in species in which malessearch.     

Keeping up appearances: male fiddler crabs wave faster in a crowd

Courtship displays are often energetically and temporally costly as well as highly conspicuous to predators. Selection should therefore favour signalling tactics that minimize courtship costs while maintaining or increasing signal attractiveness. In fiddler crabs, males court females by waving their one greatly enlarged claw in a highly conspicuous and costly display. Here, we investigate whether courting males adjust their wave rate, and therefore the cost of courtship, to the current level of competition. We show that display rate increases as competition increases and that when competition is removed, males reduce their display rate by 30 per cent. These results suggest that male fiddler crabs actively reduce the cost of courtship by adjusting their wave rate in response to the immediate level of competition.     

Stimulation of feeding behavior in three species of fiddler crabs by hexose sugars

Feeding responses to five hexoses were examined in three closelyrelated species of fiddler crabs, Uca minax, U.pugnax and U.pugilator.Hexoses tested were glucose, galactose, sorbose, fructose andmannose. Intact crabs and eyestalk-ablated crabs were tested.Responses to sugars were species specific. Eyestalks are directlyinvolved in vision and overall neural integration as well aswith chemosensory and metabolic pathways associated with feeding.Overall, eyestalk-ablated crabs were more sensitive uian intactcrabs. Studies of responses of individuals within a populationto hexoses showed tiiere are individual U.pugnax that respondto galactose and others that do not. Similarly, there were U.pugilatorindividuals that were mannose responsive and others that didnot respond to mannose. An additional study of differences inpopulation responses to hexoses would provide valuable toolsin studying geographic relationships between fiddler crab populations.     

Semi-monthly reproductive cycles in male and female fiddler crabs, Uca pugnax

In the fiddler crab, Uca pugnax, numbers of displaying males, numbers of burrows with half-dome superstructures (hoods), numbers of females with decalcified vulvar opercula, and numbers of females hatching their eggs, all follow a semi-monthly cycle with peaks near the time of the spring tides. These are all aspects of reproduction in Uca. Decalcification of female opercula, although necessary for mating and egg deposition, is neither necessary nor sufficient for behavioural response to courting males. In fact, behavioural responsiveness to males probably precedes decalcification in most cases. The different aspects of the reproductive cycle are coordinated so that females and males are ready to mate at about the same time near one spring tide, and the resulting eggs will be ready to hatch on the next spring tide. Two decalcified females which were kept isolated from males produced fertile clutches, suggesting that stored sperm from a previous mating may fertilize eggs in a later cycle. This may be one reason for the relative rarity of observed matings or precopulatory behaviour in Uca.     

Male-like mudballing behavior of some female fiddler crabs (Uca tangeri)

At each low tide, male and female Uca tangeri remove mudballs from inside their burrows and place them on the surface. Previous studies have shown clear intersexual differences in mudball arrangements. However, we noticed that some females placed their mudballs in an arrangement similar to that of males. In this study, we investigated several factors that may have been responsible for this change in female mudballing behavior. We found no significant effect of the lunar cycle, female size and reproductive state, or burrow features. We briefly discuss the avoidance of sexual coercion or parasite modification of host behavior as possible factors. Our study shows that intersexual differences in mudballing behavior are more complex than previously thought. Received: October 18, 2000 / Accepted: May 7, 2001     

Habitat and host specificity of trematode metacercariae in fiddler crabs from mangrove habitats in Florida

Fiddler crabs (Uca spp.) are common inhabitants of temperate and tropical coastal communities throughout the world, often occupying specific microenvironments within mangrove and salt marsh habitats. As second intermediate hosts for trematodes, we investigated patterns of host distribution and parasitism for 3 species of sympatric fiddler crabs in mangrove habitats adjacent to the Indian River Lagoon, Florida. Fiddler crab distribution varied among species, with Uca speciosa dominating the low and mid intertidal regions of mangrove banks. This species also exhibited higher prevalence and abundance of Probolocoryphe lanceolata metacercariae compared with Uca rapax, which is relatively more abundant in the high intertidal zone. We conducted a field experiment to test whether U. speciosa was more heavily parasitized by P. lanceolata as a result of its habitat distribution by raising U. speciosa and U. rapax under identical environmental conditions. After exposure to shedding cercariae under the same field conditions, all individuals of U. speciosa became parasitized by P. lanceolata, whereas no U. rapax were parasitized, suggesting that differences in parasitism were driven by host selection.