Fiddler crabs are referred to as ecosystem engineers because they actively shape the environment they live in (Zeil et al, 2006). U. vomeris are found on intertidal mudflats in parts of the Indo-Pacific, particularly prominent throughout Queensland, Australia (see figure 5 for a typical U. vomeris environment). They engineer their environment by digging burrows, which oxygenize and bioturbate the substratum, affecting nutrient availability, chemical exchange rates and the composition of the microbial community as mud is filtered and moved around between different parts of the substratum and the surface (Peer et al, 2015; Zeil et al, 2006). This is particularly important to mangrove trees and the microbial benthic and soil communities found in the area (Peer et al, 2015; Zeil et al, 2006).

Another important role that the U. vomeris perform is the recycling of nutrients (Zeil et al, 2006). As they filter through the mud using their specialized mouthparts (see “Diet and Feeding”), they are able to increase the rate of decomposition and utilization of resources that would otherwise be unavailable (Peer et al, 2015). Living on tidal flats also means that the area sees constant flushing, providing a nutrient retention service and recycling is therefore very important for the whole mudflat community as less nutrients goes to waste (Peer etal, 2015). 

U. vomeris are deposit feeders feeding mainly on different types of detritus such as plant matter, microphytobenthos and microheterotrophs that are found on the intertidal mudflats (Crane et al, 1975; Peer et al, 2015). Fiddler crabs have maxilliped setation that show specialization depending on their historical environment, with sand-dwelling species (U.perplexa) having spoon-tipped setae and mud-dwelling species (U. vomeris) having more plumose setae (Lim & Kalpana, 2011). U. vomeris also have specialized mouthparts with fine feathery setae that filter the mud for specific nutritious detritus and micro-organisms living in the mud (Lim & Kalpana, 2011; Peer et al, 2015). The filtered mud is then spat out as chunks of “cleaned” mud (Mokhlesi et al, 2011).

The enlarged claw of the males is too big to be used for feeding so it is only the small claw that is used for gathering up mud (Rosenberg, 2001). Compensations for this feeding handicap seem to vary between species where some will move their feeding appendage faster, some grab larger scoops of mud and some have a higher assimilation efficiency of ingested food (Lim & Kalpana, 2011). Not all species have or utilize any compensation for their feeding disability, instead they simply spend more time feeding (Lim & Kalpana, 2011; Peer et al, 2015). Mokhlesi et al. (2011) explains that the size of the crab also affects feeding rate.

Although most species of fiddler crabs are considered detritivores like the U. vomeris, there are some that have been shown to actively hunt shrimp (Uca annulipes) (Peer et al, 2015). Fiddler crabs in general seem to be very opportunistic feeders and will eat more or less anything they are fed (including other crabs) (Peer et al, 2015). It is not know if U. vomeris occasionally hunt for food or would eat whatever they are given but because of their specialized mouthparts it is unlikely that they would be able to eat food that is very different in texture compared to what they normally eat.

The mating display of the male fiddler crabs is probably what they are most famous for, and it has earned them many of their common names such as “waving” and “calling” crabs. The second common name of U. vomeris “southern calling fiddler crab” is also a reference to this behavior (Rosenberg, 2001). U. vomeris mating display, like most fiddler crabs, consists primarily of waving their enlarged claw (Zeil & Hofmann, 2001). Something that is not like most other fiddler crabs, however, is that the U. vomeris follow the female back to her burrow, where courtship and mating might occur should she accept him into her burrow (Zeil & Hofmann, 2001). For most species of fiddler crab it is the female that will wander around and the male’s burrow that they will retire to.

The mating display, which looks quite simple, has more to it than first meets the eye. Because of the special vision of the fiddler crabs (see “Vision” section), anything above the horizon is generally perceived as a threat (How et al, 2012; Jordão et al, 2007). The male’s claw is above the horizon in the female’s view when he performs his waving display so doing it in a way that attracts instead of frightens is therefore very important (Zeil & Hofmann, 2001). The angle of the waving might be a way to overcome this as their eyes are sensitive to polarized light, and changing the claws color or luminance in certain ways might also be a way to make the display more pleasing to the females (Hemmi, 2005).

A thing to note about fiddler crab waving is that females can also be seen doing it (Zeil et al, 2006).This is, however, not a mating display but rather one of the many social and communication displays seen in fiddler crabs (Bywater et al, 2014; Zeil et al, 2006).

Once a female and male U. vomeris have retired to the female’s burrow they will copulate (Zeil & Hofmann, 2001). The male will then return to the surface and guard the burrow from potential competitors (Peer et al, 2015; Zeil & Hofmann, 2001). After about 25 hours (for some Uca spp.) the female will lay her eggs rhythmically (Peer et al, 2015). The eggs will be released by the female when she decides the environmental conditions are right, which is often in response to certain tidal and lunar conditions (Peer et al, 2015). After spending a few months as planktonic larvae they will do their last larval moult and will settle on a mudflat where they will reach sexual maturity about a year later (Mohktari et al, 2006; Peer et al, 2015). This is when the enlarged claw of the males will start to develop and when the females will do their “moult of puberty” which will spark their ovulation and make them sexually mature (Peer et al, 2015).

The courtship and reproduction in most fiddler crabs seem to coincide with spring tides or often have a semilunar cycle (Crane, 1975; Peer et al, 2015).

See figure 6 for a representation of the general life cycle of fiddler crabs created by Peer et al. (2015).

Predator avoidance and fiddler crab hiding behavior is an interesting, complex and well-studied topic (Hemmi, 2004; Hemmi, 2005; Jennions et al, 2005; Raderschall et al, 2011). Size, sex, predator size, angle of approach and stage of tidal cycle have all been shown to be important factors affecting fiddler crab hiding behavior (Hemmi, 2004; Hemmi, 2005; Jennions et al, 2007). Hiding decreases the amount of time the individual can spend foraging for food so it is important to not bee too careful while still avoid being eaten (Jennions et al, 2005). See figure 7 for a partially hidden fiddler crab.

Experiment:

-Background

For the course Biol3207 (Animal Behaviour) at the University of Queensland I was part of a group conducting an experiment looking at how long fiddler crabs stay in their burrows after an induced predator threat. We looked at a total of 135 different fiddler crabs for three different sites and took note of the relative size of the crabs (small, medium, large), how long they stayed in their burrows and the distance before flight initiation. We did not distinguish between genders, despite previous studies showing gender to significantly affect hiding behavior (Hemmi, 2005) because of temporal restraints set by the course. Catching each crab to determine gender was therefore not possible (male juvenile fiddler crabs do not have the enlarged cheliped so we could not quickly assess gender) (Mohktari et al, 2006; Peer et al, 2015).

-Methods

We used a 20m transect and chose three random 1x1m quadrats 1m from the transect. One person walked along the transect, as the induced predator threat, while another student observed the crab and took note of its relative size. Once the crab entered the burrow (at least 50% of the crab hidden) the crab-watcher started their timer and the “predator” dropped a stick (so we could measure the distance before flight initiation) before retreating to a safe distance (about 10-15m away) and stood still. When the crab had fully re-emerged we stopped the timer and took note of time and distance. This was repeated for five different crabs in each quadrat and we did three transects for each site, each transect being on a different day.

We tested the data for normality using the Shapiro-Wilk method in R and created a Two-way ANOVA once normality had been confirmed. The Two-way ANOVA compared time in burrow to size of the crabs and site. A post-hoc test was then done to determine which site and size showed significant differences.

A linear regression analysis was done comparing the distance before flight initiation and time spent in the burrow.

-Results

For the Two-way ANOVA we got significant p-values for both size and site (1.340e-09 and 6.776e-09, respectively). The post-hoc test showed a significant difference between sizes large vs small (p-value = 2.3e-06) and medium vs small (p-value = 1.6e-05). No significant difference was found between large and medium sized crabs (p-value = 0.66). Between the sites we found significant differences for Lota vs Manly (p-value = 4.3e-14) and Lota vs Wellington Point (p-value =7.8e-11) while Manly vs Wellington Point showed no significant difference (p-value = 0.17). See figure 8 for a graph showing the size and site vs time spent in burrows. 

The regression analysis of the distance before flight initiation compared with time spent in burrows gave us a positive relationship for each site, with the more skittish fiddler crabs also spending more time in the burrows after the induced predator threat (see figure 9).

-Discussion

Our time vs size results are consistent with previous studies such as Hemmi (2004 & 2005) and Jennions et al. (2011) which all found that larger crabs spend more time in their burrows. This is potentially due to these crabs being easier to spot by predators. This could be a learned trait that they have developed as they have grown or it could be that the more careful of the small crabs are more likely to live to reach large sizes.

A reason why we saw such differences in hiding behavior between the different sites might be because of differences in the texture of the mud. Sound and vibrations travel more easily through hard packed (dense) substances compared to less dense ones, so wetter mud could be “masking” our approach. Fiddler crabs are known to be highly sensitive to vibrations and it is used for communication (Zeil et al, 2006). Hemmi (2005) disputes this theory, however, and claims that fiddler crabs rely solely on visual cues for their predator avoidance decisions. This makes some sense as fiddler crabs main predator are birds, which are unlikely to cause much ground vibrations when hunting for crabs. Relying on vibration could therefore potentially lower the crab’s ability to detect a predator as it would be focusing on different detection methods and thereby might be lose some accuracy/speed at picking up an a predator approaching from the air. It would be interesting to study this further to determine the effect of substances on fiddler crab hiding behavior.

Habituation is often seen in animals and could also be a reason for the differences in the time spent in burrows between different sites, as different levels of human interaction would be present at each site. However, a study by Raderschall et al. (2011) makes this unlikely as fiddler crabs showed little ability to learn and seemed to be more affected by the relative size of the threat and the angle of approach.

Our regression analysis, which showed flight initiation being positively correlated with time spent in burrows, is not consistent with previous studies. Hemmi (2004 & 2005) reported that individuals that ran for their burrows earlier also re-emerged sooner. Reasons for this could be differences in methods such as different definitions of a re-emerged crab, angles of approach and differences in the size of the induced predator threat, as it has been shown to be an important factor in hiding behavior (Hemmi, 2004; Hemmi, 2005; Raderschall et al, 2011). This skittishness could also be an indicator of differences in personality, as fiddler crabs are also known to show different levels of aggressiveness from individual to individual (Blackwell et al, 2007). Trying to look for this specifically would be an interesting next step in studying the socially complex fiddler crabs.

The enlarged claw found in the mature males of fiddler crabs has been widely studied in both behavioral and sexual selection context (Gerald & Thiesen, 2014). It is primarily used in mating displays and dominance fighting between males and there is generally a 50/50 relationship between left-clawed and right-clawed individuals for different species of fiddler crab (Backwell et al, 2007; Zeil et al, 2006). For U. vomeris, however, numbers as low as 1.4% of the male population have been observed as left-clawed, and it has been shown to be a significant disadvantage when fighting right-clawed individuals (Backwell et al, 2007). This is surprising because being left-handed is often an advantage in many species (humans included) due to the individuals having more experience fighting right-handed opponents than right-handed individuals have of fighting left-handed opponents (Backwell et al, 2007).

The reason for this is not fully understood but Backwell et al.(2007) suggests that certain mechanisms in the dominance fights between males might favor being right-clawed and that left-clawed individuals also act more timid. Differences in heritability is also likely to be present (Backwell et al, 2007).

Fiddler crabs have something called “apposition compound eyes” (How et al, 2012) situated on top of long stalks, which gives them a 360° field of vision (How et al, 2012: Jordão et al, 2007). Their eyes, when fully erect, are the highest point of the crab, allowing them to stay partially hidden in their burrows or in shallow, muddy waters while still observing the surrounding environment for threats (see figure 7 in the "Predator Avoidance" section) (Alkaladi & Zeil, 2014; Jordão et al, 2007). They are aligned “equatorially” which allows them to use the horizon as comparison when determining the relative proximity of an object or potential threat, and is used when estimating the severity of this potential threat (How et al, 2012; Jordão et al, 2007).

Despite the importance of vision for predator avoidance, the U. vomeris have only one class of photoreceptors (with maximum absorption peak at 502 nm), which means that they are colorblind (although there is still some debate on this) (Jordão et al, 2007). Instead they have specializations such as lots of small and heavily coated color pigment granules around their rhabdoms (a special structure of the compound eyes of arthropods) which is an adaptation that helps vision in brightly lit areas while also sharpening and shifting their vision towards the longer wavelengths (orange-red) (Jordão et al, 2007). Zeil & Hemmi (2006) reports that the U.vomeris’ color of the screening pigment is different between different parts of the eyes. This means that the spectral sensitivity is different depending on the visual field (i.e. above the horizon vs. below the horizon).

Although the eyes of fiddler crabs are not as accurate and show the world as clear as our eyes does, they still rely heavily on vision as their main predator-detection method, both by looking for predators and looking at how nearby crabs are behaving (Hemmi, 2005; Jordão et al, 2007). Individuals have been shown to be able to detect a threat from about 100 meters away (Zeil & Hemmi, 2006) and they are especially sensitive to movement from above, which is of vital importance for detecting birds (their main predators) (Hemmi, 2005).

An interesting thing to note is that U. vomeris have been found to be most sensitive to polarized light of all crustaceans, although the reason for this is not fully understood (How et al, 2012). Polarized light is abundant on mudflats and being sensitive to it (which we humans are not) might allow the crabs more accurate orientation and navigation skills and thereby better predator avoidance (How et al, 2012). It can also be considered as a substitute to color vision (as seen with cephalopods) which could be important for mating display behavior (How et al, 2012).  

All fiddler crabs breathe using gills (found in the epibranchial chambers), but the extent that water is required seems to be species-specific (Jimenez & Bennett, 2005). Some species have been found to rely almost exclusively on breathing air (Uca pugilator) and they require bubbles of air in their burrows during high tide to survive (Jimenez & Bennett, 2005). The specific breathing requirements for two-toned fiddler crabs is not known but they are most likely still largely dependent on water as they need to return to their burrows every now and then to re-humidify their gills (Hemmi, 2005). 

Domain: Eukarya

Kingdom: Animalia

Phylum: Arthropoda

Subphylum: Crustacea

Class: Malacostraca

Order: Decapoda

Infraorder: Brachyura

Family: Ocypodidae

Subfamily: Ucinae

Genus: Uca

Species: Uca vomeris

Rosenberg (2001)

Fossil records show evidence of crustaceans existing since the Cambrian period over 500 million years ago (VanHook & Patel, 2008). As a brachyuran crab, U. vomeris is recognized as a “true crab” (having a reduced abdomen) and are among the most recent marine animals to move on to land (Zeil et al, 2006). They follow many of the typical characteristics of the arthropods such as moulting between stages in their life, an exoskeleton and segmentation of their body (although modified) (Peer et al, 2015; VanHook & Patel, 2008). 

The first descriptions of fiddler crabs were by a German naturalist called Marcgrave who described coloration and habitat of two Brazilian species in 1648 (Peer et al, 2015). Complications in the systematics of fiddler crabs is a topic pointed to in the paper by Rosenberg (2001), where he explains some of the issues in naming and classifying different fiddler crab species due to their multitude, polymorphism, color-changing ability and wide dispersal around the globe. The lack of effective communication methods between scientists distributed around the world prior to the middle/late 20^th century is also one of the main reasons for the challenges with classifying fiddler crabs (Peer et al, 2015). Although communication is not an issue anymore, there are still challenges and changes occurring due, in most part, to the historical issues of naming species of the Uca genus (Peer e al, 2015).

Uca vomeris, seems to be fairly well accepted in the phylogeny and it has not seen any major changes since being named by McNeill in 1920 (Hemmi et al, 2006; Rosenberg, 2001). There are, however, alternative classification for the species such as Uca (Thalassuca) vocans vomeris, Ucamarionis var. vomeris and Uca (Gelasimus) vomeris (Rosenberg, 2001) but they seem to be due to temporary accepted attempts at fixing the historical issues in the systematics of fiddler crab naming and not because of disagreements to the validity of the term Uca vomeris.