Journal of Threatened
Taxa | www.threatenedtaxa.org | 26 October 2025 | 17(10): 27754–27760
ISSN 0974-7907 (Online) | ISSN 0974-7893 (Print)
https://doi.org/10.11609/jott.9161.17.10.27754-27760
#9161 | Received 27 May 2024 | Final received 03 September 2025 | Finally
accepted 07 October 2025
Exploring carapace phenotypic
variation in female Fiddler Crab Austruca annulipes (H. Milne Edwards, 1837): insights into
adaptive strategies and ecological significance
Vaishnavi Bharti 1 , Sagar Naik 2 &
Nitin Sawant 3
1–3 School of Biological Sciences and
Biotechnology, Zoology Discipline, Goa University, Goa 403206, India.
1 vb.vaishnavibharti@gmail.com, 2
sagarnaik1018@gmail.com, 3 nitin.sawant@unigoa.ac.in (corresponding
author)
Editor: Kareen Schnabel,
National Institute of Water & Atmospheric Research Ltd, Wellington New
Zealand. Date of publication:
26 October 2025 (online & print)
Citation:
Bharti, V., S. Naik & N. Sawant (2025). Exploring carapace
phenotypic variation in female Fiddler Crab Austruca
annulipes (H. Milne Edwards, 1837): insights into
adaptive strategies and ecological significance. Journal of Threatened Taxa 17(10): 27754–27760. https://doi.org/10.11609/jott.9161.17.10.27754-27760
Copyright: © Bharti et al. 2025. Creative Commons Attribution 4.0 International License.
JoTT allows unrestricted use, reproduction, and
distribution of this article in any medium by providing adequate credit to the
author(s) and the source of publication.
Funding: None.
Competing interests: The authors declare no competing interests.
Acknowledgments: We are extremely grateful to Dr. Preeti Antonetta Pereira for her guidance and knowledge. Special thanks to Ms. Anita Bharti for her support during field visits and collection of specimens.
Abstract: Fiddler crabs (Ocypodidae: Crustacea: Arthropoda) are globally documented
but relatively understudied along the Indian coastline. Austruca
annulipes, common across Indian mangrove
habitats, remains insufficiently explored in terms of female morphology. While
male fiddler crabs are recognized for their prominent chelae, females of A. annulipes exhibit notable polymorphism. In this
short-term study conducted in the mangroves of Sauta Vaddo (Baga), Goa, we documented
14 distinct female morphs, alongside observations of male moulting.
The study highlights the scope for future research into the adaptive
significance of female phenotypic variation in this species.
Keywords: Alternative reproductive tactics,
behaviour, carapace pattern, colour
variation, mangrove, morphology, moulting, Ocypodidae, polymorphism, sexual dimorphism.
Fiddler crabs are semi aquatic
crustaceans (Arthropoda: Crustacea: Ocypodidae) found
in the intertidal zones. They are found in dense numbers in mangroves,
mudflats, salt marshes throughout the tropics, and subtropics. They form an
important part of mangrove ecosystems and are considered flagship species, and
ecosystem engineers (Peer et al. 2015). The fiddler crabs widely known as
‘calling crabs’ display characteristics of sexual dimorphism. The females have
symmetrical, small-sized chelipeds while the males have asymmetrical chelipeds
one exceedingly large and another much smaller, minor cheliped (Bouchard et al.
2013). The enlarged cheliped is used in nuptial display by males to impress the
females before mating and the enlargement occurs at random, resulting in a
right-handed or left-handed claw in about equal proportions (Yamaguchi 1977).
More than 100 species of fiddler
crabs are recognized at present, many of which are studied for their behaviour, ecology, and systematics (Silva et al. 2024).
One overlooked species is Austruca annulipes (H. Milne Edwards, 1837), commonly called the
Porcelain Fiddler Crab (Fulmali et al. 2021). The
species has been included in global studies of taxonomy, population biology,
feeding, and burrow characteristics, however, these studies have nearly
exclusively considered the male fiddler crab only, and a very little data is
available on females. An exception is a study of South African mangroves where
14 variations in female patterns are documented showing female polymorphism in A.
annulipes (Peer et al. 2015). Fiddler crabs, like
many other animals, have dynamic carapace colour and
pattern changes, especially during moulting, which
can be timed to tidal or lunar cycles (Brown & Webb 1949). They are known
to undergo rapid colour changes in response to
stress, during courtship, and as a result of thermo-regulation (Hemmi et al. 2006). The colour
changes that result are more noticeable in females than in males of the same
size (Detto et al. 2008). Hence, in this study, we
examine and document the phenotypic variation in female carapace morphology of Austruca annulipes
and whether it represents ecologically adaptive polymorphism, potentially
driven by selective pressures such as predation, habitat complexity, or mate
recognition.
Materials and Methods
The study was conducted in
mangroves off Sauta Vaddo, Baga-Calangute, Goa (15.5661º N, 73.7581º E) (Image 1)
during December 2023. The mangroves have expanded into the abandoned fields
nearby creating mangrove edge ecosystems which are affected by the tidal cycle.
These provide suitable habitat for fiddler crabs that have populated these
areas over the past 20 years. The river Baga runs
through the mangroves and the area has seen little human disturbance since the
fields were abandoned (Image 2).
The variation in female carapace colour and pattern was observed in the field and
photographed using Sony HX400V, and Nikon Coolpix P1000. Microscopic characters
such as minor cheliped, abdomen, and walking legs were photographed using
stereomicroscope Zeiss Stemi 508 with Axiocam camera. The study area map was created using Qgis 3.34. The species was identified using taxonomic keys
given by White A (1847) and WORMS (World Register of Marine Species) online
database (marinespecies.org). The crabs were handpicked and scooped by random
sampling during low tide and collected & tagged in containers,
and brought to the lab as prescribed by Darnell et al. (2019). Each
specimen was assigned a unique identification code comprising the site
abbreviation, date of collection (day–month–year), and an individual serial
number (e.g., SV17122301–SV17122350). In total, 50 specimens were catalogued,
including 25 males and 25 females. The specimens were then freeze-killed and
immersed in 80% absolute ethanol (Hampton et al. 2013). This study was conducted with the
Ethical permission no. GUZ/IAEC/23-24/N42. Basic morphometric analyses (Crane
2015) were performed for all specimens and the carapace length (CL), carapace
breath (CB), & breadth of front (BF) was recorded for both sexes; major propodus length (MPL) & major dactyl length (MDL) for
males; and length of propodus (LP) & length of
abdomen (LA) for females were recorded separately.
Results
The morphometric data collected
(Table 1) from these crabs support that all observed
morphs, belong to the same species, Austruca
annulipes. A total of 14 female variations (Table
2), adult males and four male moults (Image 3) were
recorded during the present field study. The females were slightly smaller than
the males in size and illustrated more colour, and pattern
variations compared to males, while the males were similar in size but showed
varied patterns on carapace as they underwent periodic moulting.
Discussion
The study confirms pronounced
sexual dimorphism in A. annulipes, with
significant polymorphism observed in females. Male carapace patterns appear
linked to periodic moulting, while female variation
is more persistent, and diverse. The documentation of 14 female morphs in this
Indian population parallels findings from South Africa (Peer et al. 2015),
although with differing pattern types. Five morphs resemble male-like androchromic traits, while the remaining exhibit heterochromic features. The ecological and genetic
underpinnings of such polymorphism remain unclear, warranting further
investigation. Colour variation in fiddler crabs has
been linked to light, stress, and social signalling
(Crane 1944, 1958; Taborsky 2008; Tate & Amar
2017). While in females it is linked mostly with sexual selection and female
ornamentation, indicating towards alternative reproductive tactics (Oliveira et al. 2008; Diamant et al. 2021). This study adds valuable
observational data, supporting the case for more detailed ecological, and
genetic research into female-specific traits in A. annulipes.
Conclusion
This study provides the first
documentation of carapace polymorphism among female Austruca
annulipes in the Baga
mangroves of Goa, India. Observations indicate size-related variation and
distinct colour morphs across individuals. Though moulting patterns might be influenced by lunar cycles, this
was not a primary focus of the current study. Given their role as bioindicators
and ecosystem engineers, deeper insight into the species’ phenotypic diversity
can contribute to broader mangrove conservation strategies. Future research
should explore the genetic basis and ecological drivers of polymorphism in
female A. annulipes.
Table 1. Morphometric
data (mm) for males and females
of Austruca annulipes.
|
|
Morphometric character |
Male (n = 25): Mean ± SD |
Female (n = 25): Mean ± SD |
|
1 |
Carapace length (CL) |
16.42 ± 1.41 |
13.57 ± 1.57 |
|
2 |
Carapace breath (CB) |
9.84 ± 0.61 |
8.16 ± 0.62 |
|
3 |
Breath of front (BF) |
2.5 ± 0.19 |
2.21 ± 0.16 |
|
4 |
Major propodus
length (MPL) |
26.74 ± 2.30 |
- |
|
5 |
Major dactyl length (MDL) |
15.14 ± 1.62 |
- |
|
6 |
Length of propodus
(LP) |
- |
4.72 ± 0.515 |
|
7 |
Length of abdomen (LA) |
- |
6.13 ± 0.68 |
For
images - - click here for full PDF
References
Bouchard, J.,
J. Poupin, R. Cleva, J.
Dumas & V. Dinhut (2013). Land, mangrove and freshwater
decapod crustaceans of Mayotte region (Crustacea Decapoda).
Atoll Research Bulletin 592: 1–60. https://doi.org/10.5479/si.00775630.592
Brown, F.A.
& H.M. Webb (1949). Studies of the Daily Rhythmicity of the Fiddler Crab, Uca. Modifications by Light. Physiological Zoology 22(2):
136–148. https://doi.org/10.1086/physzool.22.2.30152036
Crane, J.
(1944). On the colour changes of Fiddler crabs (Genus Uca)
in the field. Zoologica Scientific
Contributions of the New York Zoological Society 29(15): 161–168. https://doi.org/10.5962/p.203572
Crane, J.
(1958). Aspects of
social behavior in fiddler crabs, with special reference to Uca
maracoani (Latreille). Zoologica Scientific Contributions of the New
York Zoological Society 43(10): 113–130. https://doi.org/10.5962/p.203380
Crane, J.
(2015). Fiddler Crabs
of the World: Ocypodidae: Genus UCA. Princeton University Press, 766
pp.
Darnell,
M.Z., P.R. Backwell & P. Munguia (2019). Frequency and latency of
autotomy of a sexually selected fiddler crab appendage. Journal of
Experimental Marine Biology and Ecology 523: 151255. https://doi.org/10.1016/j.jembe.2019.151255
Detto, T., J.M. Hemmi
& P.R.Y. Backwell (2008). Colouration
and colour changes of the fiddler crab, Uca capricornis: a
descriptive study. PLoS ONE 3(2):
e1629. https://doi.org/10.1371/journal.pone.0001629
Diamant, E.S.,
J.J. Falk & D.R. Rubenstein (2021). Male-like female
morphs in hummingbirds: the evolution of
a widespread sex-limited plumage
polymorphism. Proceedings
of the Royal Society B
Biological Sciences 288(1945): 1–10. https://doi.org/10.1098/rspb.2020.3004
Fulmali, M., S. Kumaresan,
A. Saravanakumar & R. Samuthirapandian
(2021). Customisation of bioturbatory structure and spatio-temporal burrow density variations and allometric
growth pattern by porcelain fiddler crab (Austruca
annulipes H. Milne Edwards, 1837) in polluted and
unpolluted habitats of Vellar Estuary, India. Thesis
for Master of Science in Marine Biology and Oceanography. Annamalai University.
Hampton, K.,
M. Hopkins, J. McNamara & C. Thurman (2013). Intraspecific variation in
carapace morphology among fiddler crabs (Genus Uca)
from the Atlantic coast of Brazil. Aquatic Biology 20(1): 53–67. https://doi.org/10.3354/ab00545
Hemmi, J.M., J. Marshall, W. Pix, M.
Vorobyev & J. Zeil
(2006). The variable colours of the
fiddler crabUca vomerisand their relation to background and
predation. Journal of Experimental Biology 209(20):
4140–4153. https://doi.org/10.1242/jeb.02483
Oliveira,
R.F., M. Taborsky & H.J. Brockmann
(2008). Alternative
Reproductive Tactics: An Integrative Approach. Cambridge University Press, 518
pp.
Peer, N.,
N.A. Miranda & R. Perissinotto (2015). A review of fiddler crabs (genusUcaLeach, 1814) in South Africa. African
Zoology 50(3): 187–204. https://doi.org/10.1080/15627020.2015.1055700
Silva, D.J.A.,
M.F. Erickson & D.M.A. Pessoa (2024). The role of carapace colour in species recognition in a fiddler crab. Animal Behaviour 216: 37–44. https://doi.org/10.1016/j.anbehav.2024.07.016
Tate, G.J.
& A. Amar (2017). Morph specific foraging behavior by a polymorphic raptor under variable
light conditions. Scientific Reports 7(1): 1–12. https://doi.org/10.1038/s41598-017-07829-x
WoRMS World Register of Marine Species
- Austruca annulipes
(H. Milne Edwards, 1837). Accessed on 2.vi.2023. https://www.marinespecies.org/aphia.php?p=taxdetails&id=955178#:~:text=DecaNet%20eds.%20(2024).%20DecaNet.%20Austruca%20annulipes%20(H.%20Milne%20Edwards%2C%201837).%20Accessed%20through%3A%20World%20Register%20of%20Marine%20Species%20at%3A%20https%3A//www.marinespecies.org/aphia.php%3Fp%3Dtaxdetails%26id%3D955178%20on%202024%2D02%2D16
Yamaguchi, T. (1977). Studies on the handedness of the
fiddler crab, Uca lactea.
Biological Bulletin 152(3): 424–436. https://doi.org/10.2307/1540430