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Fiddler crab

"Uca" redirects here. For other uses, see UCA (disambiguation).

The fiddler crab or calling crab can be one of the hundred species of semiterrestrial marine crabs in the family Ocypodidae.[2] These crabs are well known for their extreme sexual dimorphism, where the male crabs have a major claw significantly larger than their minor claw, whilst females claws are both the same size.[3] The name fiddler crab comes from the appearance of their small and large claw together, looking similar to a fiddle.

Fiddler crab
Temporal range: Early Miocene-recent[1]
Red-jointed fiddler crab (Minuca minax)
Red-jointed fiddler crab (Minuca minax)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Superfamily: Ocypodoidea
Family: Ocypodidae
Groups included

A smaller number of ghost crab and mangrove crab species are also found in the family Ocypodidae. This entire group is composed of small crabs, the largest being Afruca tangeri which is slightly over two inches (5 cm) across. Fiddler crabs are found along sea beaches and brackish intertidal mud flats, lagoons, swamps, and various other types of brackish or salt-water wetlands. Whilst fiddler crabs are currently split into two subfamilies of Gelasiminae and Ucinae, there is still phylogenetic and taxonomical debate as to whether the movement from the overall genus of ‘’Uca’’ to these subfamilies and the separate 11 genera[2]

Like all crabs, fiddler crabs shed their shells as they grow. If they have lost legs or claws during their present growth cycle, a new one will be present when they molt. If the major claw is lost, males will regenerate one on the same side after their next molt.[4] Newly molted crabs are very vulnerable because of their soft shells. They are reclusive and hide until the new shell hardens.

In a controlled laboratory setting, fiddler crabs exhibit a constant circadian rhythm that mimics the ebb and flow of the tides: they turn dark during the day and light at night.[5]

Ecology and Life Cycle

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Fiddler crabs primarily exist upon mudflats, sandy or muddy beaches as well as salt marshes within mangroves. Fiddler crabs are found in West Africa, the Western Atlantic, the Eastern Pacific, Indo-Pacific and Algarve region of Portugal.

Whilst the fiddler crab is classified as an omnivore, it does present itself as an opportunist and will consume anything with nutritional value.[6] The crab will feed through bringing a chunk of sediment to its mouth and sifting through it to extract organic material. This crab will filter out algae, microbes, fungus or any form of detritus. Once finished consuming all the organic matter from the sediment, these crabs will then deposit them as small sand balls near their burrow.

Fiddler crabs are thought to potentially act as ecosystem engineers within their habitat due to the way they rework the sediment during feeding.[7] Whilst these crabs do rework the sediment around them, upturning the very top layer and depositing it nearby, there is still debate that exists as to whether this turnover of sediment has any proven difference regarding nutrients and aeration of the sediment.[8]

Fiddler crabs are a burrowing species, where within their territory they may possess several burrows. There are two types of burrows that the fiddler crabs can build, either breeding burrows or temporary burrows.[9]Temporary burrows are constructed by both males and females during high tide periods. These burrows are also constructed at night time when the crabs are no longer feeding and are hiding from predators. Breeding burrows are constructed by solely males, and will be constructed within the area that they have deemed their territory. These breeding burrows are constructed by male crabs so that the female and male crabs may copulate within the burrow, and the female may deposit and incubate her eggs within this area. Larger males who can more easily defend their territory will often have multiple suitable breeding burrows within their territory to enable them to mate with multiple female crabs.[10] Female crabs are found to prefer to mate with males that have the widest burrows, however, carapace width and claw size does correlate with the width of the burrow, so could be a potential size bias.[11]

Two types of fiddler crabs are found to exist within a given territory, a wandering female or male, and territory-holding male or females. [12] When in a wandering state, this means crabs do not currently occupy a burrow. They will wander in order to look for territory which contains a burrow, or to look for a mate. Wandering females will look for a mate to copulate with, usually preferring to mate with a male that currently possesses a burrow. The female fiddler carries her eggs in a mass on the underside of her body. She remains in her burrow during a two-week gestation period, after which she ventures out to release her eggs into the receding tide. The larvae remain planktonic for a further two weeks.

The mating system of fiddler crabs is thought to be mainly polygynous, where the male crabs will mate with multiple females if they have the opportunity to, however, female fiddler crabs such as the Austruca lactea are known to also mate with multiple males.[13]

As they are a species of crustacean, they perform ecdysis, which is the process of moulting. When crabs moult, they produce hormones which trigger the shedding of their exoskeleton and regeneration of limbs. Moulting is already an extremely stressful time for fiddler crabs, as their shell becomes extremely soft, leaving them vulnerable to predation.[14] When undergoing this moulting cycle, crabs will frequently hide within their burrows to avoid harm. When male crabs are undergoing the moulting process, if they are exposed to other male crabs in high grouping with consistent light, their ability to regenerate limbs will be impaired.[15]

Whilst the crabs major claw does function as a tool for fighting and competition, it also plays a role in thermoregulation. As the claw is so large, and these crabs live in generally hot territory, so require strategies to keep themselves cool, particularly for wandering males without burrows. The presence of the major claw upon the male helps them keep their body temperature regulated, and decreases the chance of them losing or gaining too much heat in a given time period. The large claw draws away excess body heat from the core of the fiddler crab and allows it to dissipate.[16] Heat is found to dissipate significantly faster when male crabs are performing waving at the same time.

Fiddler crabs come in many different colourations and patterns, and are known to be able to change their colour over time. Fiddler crabs such as the Tubuca capricornis are capable of changing their colour rapidly when placed under significant stress.[17] When fiddler crabs undergo moulting, they are seen to have reduced colouration after each sequential moult. Female fiddler crabs are traditionally more colourful then male fiddler crabs. Conspicuous colouring in fiddler crabs is dangerous as it increases predation rate, however, sexual selection argues for brightly coloured crabs.[18] Fiddler crabs have finely tuned visual systems that aid in detecting colours of importance, which aid in selecting coloured mates.[19] When given the choice, females prefer to pick males that are more brightly coloured in comparison to dull males


Behaviour, Competition, and Courtship

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General anatomy of a fiddler crab

Fiddler crabs live rather brief lives of no more than two years (up to three years in captivity). Male fiddler crabs use many signalling techniques and performances towards females to win over a female to mate.[20] Females choose their mate based on claw size and also quality of the waving display.[21]

It is very common for male fiddler crabs to be viewed fighting against one another. Male fiddler crabs fight primarily over females and territory. Whilst fights within fiddler crabs are commonly male against male fights, male fiddler crabs will also fight against female fiddler crabs when there is suitable territory with a burrow that the male wishes to obtain.[22] When fighting, male fiddler crabs can often have their major claw ripped off, or have it harmed to the point where male fiddler crabs must autotomize this claw. Whilst this claw can regrow when the crab next moults, the properties of the claw will not be the same as they were previously. Whilst the size of the claw will be the same or similar to how it was before, the claw will become significantly weaker.[23] Whilst this claw is now significantly weaker, other crabs cannot tell that this claw is weaker, so will assume the claw is at full size and strength. This is a form of dishonest signalling, where the appearance of the claw displayed to other fiddler crabs does not represent the true mechanics of the claw.[24]

In order for a male fiddler crab to help produce offspring, he must first attract a mate and convince her to mate with him. To win over females, male crabs will performed a waving display towards females. This waving display constitutes of raising the major claw upwards and then dropping it down towards itself in what appears as a 'come here' motion, like a beckoning sign.[25] Male crabs will exhibit two forms of waving towards females to attempt to court them.[26] Broadcast waving is a general wave the male crabs perform when a female crab is not within their field of view. This wave is at a slower pace, as to not use up energy reserves. Directed waving is performed by male crabs when they have spotted a female they wish to mate with. This wave is performed through the male crab facing towards the female, and increasing the pace of the wave towards the female.

 
Male lemon-yellow clawed fiddler crab (Austruca perplexa), waving his big claw in display

When males are waving at females, this is usually done in synchrony with other male crabs in the neighbouring area. [27] Synchronous waving does provide a general positive benefit for male crabs attempting to attract wandering females, as a form of cooperative behaviour. Synchrony however, does not provide an individual benefit, as females prefer to mate with the male that is leading the synchronous wave. Therefore, synchronous waving is thought to have evolved as an incidental byproduct of males competing to lead the wave[28]

Fiddler crabs are also known to build sedimentary pillars around their burrows out of mud and sand. 49 of the total species under the family Ocypodidae will construct sedimentary pillars outside of their burrows for the purposes of courtship and defence from other crabs.[29] These structures can be built by either male or female crabs, and will be one of the six known structures constructed by fiddler crabs (insert photo). Fiddler crabs can build either a chimney, hood, pillar, semidome, mudball or rim. These mud pillars have correlations with sediment type, genus and sex. Females are more likely to be attracted to a male if he has a sedimentary pillar outside of his burrow in comparison to a male crab without a pillar. When females are not actively being courted, they are more likely to move to an empty burrow which has a pillar present in comparison to an empty burrow without a pillar present[30]Fiddler crabs with any hood or dome formed pillar above their burrow are more likely to be shy crabs that take less risks.

Female crabs will choose their based upon the claw size of the male, as well as the quality of the waving display, if he was the leader of the synchronous waving, and if the male currently possesses territory with a burrow for them to copulate within.[21] Females will also prefer to mate with males who have the widest and largest burrows.

 
Fiddler Crabs Fighting in Belle Hall, Mount Pleasant, South Carolina on March 24, 2023

Fiddler crabs such as Austruca mjoebergi have been shown to bluff about their fighting ability. Upon regrowing a lost claw, a crab will occasionally regrow a weaker claw that nevertheless intimidates crabs with smaller but stronger claws.[31] This is an example of dishonest signalling.

The dual functionality of the major claw of fiddler crabs has presented an evolutionary conundrum in that the claw mechanics best suited for fighting do not match up with the mechanics best suited for a waving display.[32]

Genera and species

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More than 100 species of fiddler crabs make up 11 of the 13 genera in the crab family Ocypodidae. These were formerly members of the genus Uca. In 2016, most of the subgenera of Uca were elevated to genus rank, and the fiddler crabs now occupy 11 genera making up the subfamilies Gelasiminae and Ucinae.[33][2][34]

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Captivity

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Fiddler crabs are occasionally kept as pets.[37] The fiddler crabs sold in pet stores generally come from brackish water lagoons. Because they live in lower salinity water, pet stores may call them fresh-water crabs, but they cannot survive indefinitely in fresh water.[37] Fiddler crabs have been known to attack small fish in captivity, as opposed to their natural feeding habits.[38]

See also

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References

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  1. ^ "Uca Leach 1814 (fiddler crab)". PBDB.
  2. ^ a b c Rosenberg, M. S. (2019). A fresh look at the biodiversity lexicon for fiddler crabs (Decapoda: Brachyura: Ocypodidae). Part 1: Taxonomy. Journal of Crustacean Biology, 39(6), 729-738. doi: 10.1093/jcbiol/ruz057
  3. ^ Levinton, J. S., Judge, M. L. & Kurdziel, J. P. (1995) Functional differences between the major and minor claws of fiddler crabs (Uca, family Ocypodidae, order Decapoda, Subphylum Crustacea): A result of selection or developmental constraint? Journal of Experimental Marine Biology and Ecology, 193(1-2), 147-160. doi: 10.1016/0022-0981(95)00115-8
  4. ^ Weis, J. S. (2019). On the Other Hand: The Myth of Fiddler Crab Claw Reversal. BioScience, 69(4), 244-246. doi: 10.1093/biosci/biz011
  5. ^ Smith, T. & Smith, R. (2012). Elements of Ecology (8th ed.). San Francisco: Pearson Benjamin Cummings. p. 144. https://www.pearson.com/en-us/subject-catalog/p/elements-of-ecology/P200000006836/9780137502165
  6. ^ Zeil, J., Hemmi, J. M. & Backwell, P. R. Y. (2006). Fiddler crabs. Current Biology, 16(2), 40-41. doi: 10.1016/j.cub.2006.01.012
  7. ^ Crane, J. (2015). Fiddler crabs of the world: Ocypodidae: genus Uca (Vol. 1276). Princeton University Press.
  8. ^ Michaels, R. E. & Zieman, J. C. (2013). Fiddler crab (Uca spp.) burrows have little effect on surrounding sediment oxygen concentrations. Journal of Experimental Marine Biology and Ecology, 448, 104-113. doi: 10.1016/j.jembe.2013.06.020
  9. ^ Christy, J. H. (1982). Burrow structure and use in the sand fiddler crab, Uca pugilator (Bosc). Animal Behaviour, 30(3), 687-694. doi: 10.1016/S0003-3472(82)80139-5
  10. ^ Mautz, B., Detto, T., Wong, B. B. M., Kokko, H., Jennions, M. D. & Backwell, P. R. Y. (2011). Male fiddler crabs defend multiple burrows to attract additional females. Behavioral Ecology, 22(2), 261–267. doi: 10.1093/beheco/arq207
  11. ^ Reaney, L. T., & Backwell, P. R. Y. (2007). Temporal constraints and female preference for burrow width in the fiddler crab, Uca mjoebergi. Behavioral Ecology and Sociobiology, 61, 1515-1521. doi: 10.1007/s00265-007-0383-5
  12. ^ Peso, M., Curran, E., & Backwell, P. R. Y. (2016). Not what it looks like: mate-searching behaviour, mate preferences and clutch production in wandering and territory-holding female fiddler crabs. Royal Society Open Science, 3(8). doi: 10.1098/rsos.160339
  13. ^ Murai, M., Goshima, S. & Henmi, Y. (1987). Analysis of the mating system of the fiddler crab, Uca lactea. Animal behaviour, 35(5), 1334-1342. doi: 10.1016/S0003-3472(87)80005-2
  14. ^ Guyselman, J. B. (1953). An analysis of the molting process in the fiddler crab, Uca pugilator. The Biological Bulletin, 104(2). doi: 10.2307/1538787
  15. ^ Weiz, J. S. (1976). Effects of environmental factors on regeneration and molting in fiddler crabs. The Biological Bulletin, 150(1). Doi: 10.2307/1540596
  16. ^ Darnell, M. Z. & Munguia, P. (2011). Thermoregulation as an Alternate Function of the Sexually Dimorphic Fiddler Crab Claw. The American Naturalist, 178(3). doi: 10.1086/661239
  17. ^ Detto, T., Henmi, J. M. & Backwell, P. R. Y. (2008). Colouration and Colour Changes of the Fiddler Crab, ‘’Uca capricornis’’: A Descriptive Study. PLOS ONE, 3(2). doi: 10.1371/journal.pone.0001629
  18. ^ Detto, T. (2007). The fiddler crab Uca mjoebergi uses colour vision in mate choice. Proceedings of the Royal Society B, 274(1627). doi: 10.1098/rspb.2007.1059
  19. ^ Zeil, J. & Hemmi, J. M. (2006). The visual ecology of fiddler crabs. The Journal of Comparative Physiology A, 192, 1-25. doi:10.1007/s00359-005-0048-7
  20. ^ Pope, D. S. (2000) Testing function of fiddler crab claw waving by manipulating social context. Behavioral Ecology and Sociobiology, 47, 432-437. https://www.jstor.org/stable/4601768
  21. ^ a b Perez, D. M., Rosenberg, M. S. & Pie, M. R. (2012) The evolution of waving displays in fiddler crabs (Uca spp., Crustacea: Ocypodidae). Biological Journal of the Linnean Society, 106, 307-315. doi: 10.1111/j.1095-8312.2012.01860.x
  22. ^ Hyatt, G. W. & Salmon, M. (1978). Combat in the Fiddler Crabs Uca Pugilator and U. Pugnax: a Quantitative Analysis. ‘’Behaviour, 65’’(1-2). 182-211. doi: 10.1163/156853978X00602
  23. ^ Bywater, C. L., Seebacher, F. & Wilson, R. S. (2015). Building a dishonest signal: the functional basis of unreliable signals of strength in males of the two-toned fiddler crab, Uca vomeris. The Journal of Experimental Biology, 218(19), 3077-3082. doi: 10.1242/jeb.120857
  24. ^ Lailvaux, S. P., Reaney, L. T. & Backwell, P. R. Y. (2008). Regenerated claws dishonestly signal performance and fighting ability in the fiddler crab Uca mjoebergi. Functional Ecology, 23(2), 359. doi:10.1111/j.1365-2435.2008.01501.x
  25. ^ How, M. J., Zeil, J. & Hemmi, J. M. (2009). Variability of a dynamic visual signal: the fiddler crab claw-waving display. Journal of Comparative Physiology A, 195, 55-67. doi: 10.1007/s00359-008-0382-7
  26. ^ How, M. J., Zeil, J. & Hemmi, J. M. (2007). Differences in context and function of two distinct waving displays in the fiddler crab, Uca perplexa (Decapoda: Ocypodidae). Behavioural Ecology and Sociobiology, 62, 137-148. doi: 10.1007/s00265-007-0448-5
  27. ^ Backwell, P., Jennions, M., Wada, K., Murai, M. & Christy, J. (2006). Synchronous waving in two species of fiddler crabs. Acta ethologica, 9, 22–25. doi: 10.1007/s10211-005-0009-8
  28. ^ Backwell, P. R. Y. (2018). Synchronous waving in fiddler crabs: a review. Current Zoology, 65(1), 83-88. doi: 10.1093/cz/zoy053
  29. ^ Pardo, J. C. F., Stefanelli-Silva, G., Christy, J. H. & Costa, T. M. (2020). Fiddler crabs and their above-ground sedimentary structures: a review. Journal of Ethology, 38, 137-154, doi: 10.1007/s10164-020-00647-1
  30. ^ Christy, J. (1988). Pillar Function in the Fiddler Crab Uca beebei (II):Competitive Courtship Signaling. Ethology, 78(2), 89-176. doi: 10.1111/j.1439-0310.1988.tb00223.xopen_in_newISSN
  31. ^ British Ecological Society. (2008). Fiddler crabs reveal honesty is not always the best policy.
  32. ^ Swanson, B. O., George, M. N., Anderson, S. P. & Christy, J. H. (2013). Evolutionary variation in the mechanics of fiddler crab claws. Bmc Evolutionary Biology, 13, 137. doi: 10.1186/1471-2148-13-137
  33. ^ Shih, H. T. & Ng, P. K. L., Davie, P. J. F., Schubart, C. D., Türkay, M., Naderloo, R., Jones, D. & Liu, M. (2016). Systematics of the family Ocypodidae Rafinesque, 1815 (Crustacea: Brachyura), Based on phylogenetic relationships, With a reorganization of subfamily rankings and a review of the taxonomic status of Uca Leach, 1814, Sensu lato and its subgenera. The Raffles Bulletin of Zoology, 64, 139-175. doi: 10.5281/zenodo.5355087
  34. ^ Peter K. L. Ng; Danièle Guinot; Peter J. F. Davie (2008). "Systema Brachyurorum: Part I. An annotated checklist of extant Brachyuran crabs of the world" (PDF). Raffles Bulletin of Zoology. 17: 1–286. Archived from the original (PDF) on 2011-06-06.
  35. ^ Hsi-Te Shih; Tohru Naruse; Peter K. L. Ng (2010). "Uca jocelynae sp. nov., a new species of fiddler crab (Crustacea: Brachyura: Ocypodidae) from the Western Pacific" (PDF). Zootaxa. 2337: 47–62. doi:10.11646/zootaxa.2337.1.4.
  36. ^ Shih, H. T.; Chan, Benny K. K.; Ng, Peter K. L. (29 March 2018). "Tubuca alcocki, a new pseudocryptic species of fiddler crab from the Indian Ocean, sister to the southeastern African T. urvillei (H. Milne Edwards, 1852)". ZooKeys (747): 41–62. doi:10.3897/zookeys.747.23468. PMC 5904522. PMID 29674902. Retrieved 3 April 2018.
  37. ^ a b Lianne McLeod. "How to Set Up a Tank for Fiddler Crabs". About.com. Archived from the original on September 29, 2011. Retrieved January 13, 2010.
  38. ^ Robert (2019-02-22). "The Complete Fiddler Crab Care Guide: Tank, Food, Facts and More…". Fishkeeping World. Retrieved 2021-04-21.
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