MARINE AND FISHERY SCIENCES 36 (2): 137-147 (2023)
ABSTRACT. This paper aims to provide information about the behavior and diet composition of
fiddler crabs. The large percentage of sediments present in the stomach of fiddler crabs proves that
fiddler crabs play an important role in aerating the soil, which would help in the growth of mangrove
and wetland plants. Observations were done in sandy, muddy, and coralline substrates for four
months. Thirty fiddler crabs were collected for laboratory test of their diet composition. Sediments
had the highest percentage in the stomach content of the fiddler crabs (60%), followed by chum
(25%), and leaf particles (15%). The analysis of the fullness of their stomach showed that it was
highly significant (df =2, MS =2.09, F =34.34, p =0.001). While the fiddler crabs ate all three col-
ors of mangrove leaves, it preferred to forage on yellow leaves (n =104) followed by the brown
leaves (n =78) and the green leaves (n =77), proving that nutrient recycling occurs in the mangrove
area. The existence of the fiddler crabs contributes to a more stable mangrove ecosystem. In addi-
tion, this study is the first assessment of fiddler crabs documented in Mindanao, Philippines. Results
of the study can be used as a baseline for the protection of mangrove ecosystem species.
Key words: Antagonistic behavior, bioturbation, mangrove, sediments, soft-bottom ecosystem.
Comportamiento y composición de la dieta del cangrejo violinista en Guang-guang, Dahican,
Mati City, Davao Oriental, Filipinas
RESUMEN. Este trabajo tiene como objetivo proporcionar información sobre el comportamiento
y la composición de la dieta de los cangrejos violinistas. El gran porcentaje de sedimentos presentes
en el estómago de los cangrejos violinistas demuestra que los cangrejos violinistas juegan un papel
importante en la aireación del suelo, lo que ayudaría al crecimiento de las plantas de manglares y
humedales. Las observaciones se realizaron en sustratos arenosos, fangosos y coralinos durante cua-
tro meses. Treinta cangrejos violinistas fueron recolectados para estudiar en de laboratorio la com-
posición de su dieta. Los sedimentos tuvieron el porcentaje más alto en el contenido estomacal de
los cangrejos violinistas (60%), seguidos de la carnada (25%) y las partículas de hojas (15%). El
nivel de llenado del estómago mostró que era altamente significativo (df =2, MS =2,09, F =34,34,
p =0,001). Si bien los cangrejos violinistas comieron los tres colores de las hojas de mangle, prefi-
rieron alimentarse de las hojas amarillas (n =104), seguidas de las hojas marrones (n =78) y las
hojas verdes (n = 77), lo que demuestra que el reciclaje de nutrientes ocurre en la zona de manglares.
Los cangrejos violinistas contribuyen a mantener un ecosistema de manglar más estable. Además,
este estudio representa la primera evaluación de cangrejos violinistas documentada en Mindanao,
Filipinas. Los resultados se pueden utilizar como referencia para la protección de las especies del
ecosistema de manglares.
Palabras clave: Comportamiento antagonista, bioturbación, manglar, sedimentos, ecosistema de
fondos blandos.
Received: 16 January 2023
Accepted: 16 March 2023
ISSN 2683-7595 (print)
ISSN 2683-7951 (online)
Journal of the Instituto Nacional de
Investigación y Desarrollo Pesquero
This work is licensed under a Creative
Commons Attribution-
NonCommercial-ShareAlike 4.0
International License
Marine and
Fishery Sciences
Behavior and diet composition of fiddler crabs in Guang-guang, Dahican,
Mati City, Davao Oriental, Philippines
1Fisheries Catch Assessment Project, Davao Oriental State University, Mati, Philippines. 2Faculty of Agriculture and Life Sciences (FALS),
Davao Oriental State University, Mati, Philippines. ORCID Ivy M. Nallos, Edison D. Macusi
The Genus Uca contains about 100 species of
semi-terrestrial marine crabs which includes fid-
dler crabs, sometimes called ‘calling crabs’
(Rosenberg 2019). Fiddler crabs are members of
the Family Ocypodidae of brachyuran crabs,
marine animals that recently invaded the land.
They are active on the surface at low tide, feeding
on soil debris, bacteria, and algae (Zeil et al.
2006). Sandy beaches, mudflats, mangrove areas,
and salt marshes are all locations where fiddler
crabs can be found. Fiddler crabs rely on the sed-
iment which they use for food, burrowing, and for
collecting bacteria, debris, and benthic macroal-
gae (Ribeiro and Iribarne 2011). The intertidal
zone and the nearby marine and terrestrial habi-
tats are connected by fiddler crabs, which are rec-
ognized as ecosystem engineers and significant
connectors of energy flow. According to a recent
study, fiddler crabs are the primary food source
for some fish and may be more important than
was previously thought as food for predators
(Grande et al. 2018).
Fiddler crabs are recognized for having
extraordinary claws. Male claws are much larger
compared to those of females, who have claws of
the same size. They stay close to their burrows to
quickly escape from predators, as well as find
shelter from the heat and water loss (Macintosh et
al. 2002).Male fiddler crabs use their minor claw
for feeding and the major claw for displaying and
fighting. Major claws are typically brightly col-
ored and four to five times longer than minor
claws, making up around one-third of the total
body mass of the crab. Female fiddler crabs have
two tiny claws almost always cryptic (Rosenberg
2001). Fiddler crabs can tolerate a wide range of
salinities, high temperatures, and low levels of
oxygen (Nagelkerken et al. 2008). Fiddler crabs
attract a female for mating by waving their
enlarged claw. Courtship activity of male fiddler
crabs peaks semi-monthly and coincides with the
peak in the temporal distribution of receptive
female fiddler crabs. A female fiddler crab mate
once a month, 4-5 days prior to one of the semi-
monthly spring tides. The relationship between
the timing of reproduction and tide cycles may
represent an adaptation to maximize the likeli-
hood that the last stage of planktonic larvae will
be carried by tidal currents to substrates suitable
for adults (Swanson et al. 2013). They protected
themselves against other fiddler crabs or preda-
tors using their enlarged claws (Bergey and Weis
2006). Burrows are the most crucial resource for
the reproduction and survival of fiddler crabs, and
males must defend them for females to be attract-
ed to them. Each fiddler crab concentrates its ter-
ritorial defenses on a single burrow (Mautz et al.
2011). Research on the behavior of fiddler crabs
is critical to understanding when and how much
sediments impact and how they affect the overall
functioning of ecosystems. As with other inter-
tidal invertebrates, their activity is significantly
influenced by tides. According to several studies,
fiddler crabs only engage in surface behaviors
including feeding, burrowing, and mating during
low tide and stay in their burrows during high tide
(Reinsel 2004; Sanford et al. 2006; Zeil and
Hemmi 2006; Dugaw et al. 2009). Fiddler crabs
can significantly influence the ecology of man-
grove communities, acting as ecological engi-
neers by adjusting resources accessible to marsh
plants and by changing the physical, chemical,
and biological characteristics of these communi-
ties of soft sediments (Smith et al. 2009). Fiddler
crab bioturbation would improve the oxygenation
of the sediments and promote the growth of man-
grove saplings (Macusi and Tipudan 2021). Vari-
ous species of fiddler crabs, each have different
behaviors like feeding, mating, walking, etc., can
be found in the same habitat in many tropical
environments (Nordhaus et al. 2009; Shih 2012).
Because there are few studies of fiddler crabs
in the Philippines, this paper provides a new
understanding of the species. The objective of
138 MARINE AND FISHERY SCIENCES 36 (2): 137-147 (2023)
this study was to provide information about the
behavior and diet composition of fiddler crabs in
Guang-guang, Dahican, Mati City, Davao Orien-
tal. Findings of this study will be used as a refer-
ence for anyone interested in studying fiddler
crabs in the Philippines, particularly in Min-
danao, and will provide additional information to
assist in the development of a conservation strat-
egy for various marine species.
Study area
The study area was located in the mangroves of
Guang-guang, Barangay Dahican under the
municipality of Mati City, Province of Davao
Oriental. The study area is situated at 60° 55'N
and 126° 15'E. The area is characterized by
sandy, sandy-muddy, sandy coralline and muddy
substrate with different species of seagrasses
thriving in it. The Guang-guang mangrove area is
part of the National Integrated Protected Areas
System (NIPAS) as Protected Landscape/
Seascape under Proclamation No. 451 dated July
31st, 1994 of the Philippine government with an
approximate area of 168 km2(Abreo et al. 2020)
(Figure 1).
Data collection
The study focused on observing the behavior
and the diet composition of fiddler crabs in rela-
tion to low tide in Guang-guang, Dahican, Davao
Oriental. Three sampling stations of 10 ´10 m
quadrats each were established on the shoreline
Figure 1. Study area in Guang-guang Dahican, Mati City, indicating different substrates (blue colors).
of Guang-guang: sandy, muddy, and sandy
coralline. Each station had four quadrats for easi-
er observation on their behaviors with a minimum
distance of 100 m from each other to maintain
independence and prevent one station from being
influenced by the others. The behavior of fiddler
crabs was assessed according to their sex,
whether they perform courting, defending bur-
rows, fencing or predation, waving, enhancing
their burrows, walking/grasping or foraging. For-
aging activities of fiddler crabs were also catego-
rized as either collecting leaves (grasping the
item and retreating to the burrow) or foraging
(slow walking, associated with tapping or tasting
sediment or litter). In each station, fiddler crabs
and their burrows were counted and burrows
were examined for leaf taking. The duration of
the observation and counting of fiddler crabs took
one hour during low tide and these activities were
photographed for documentation. The regularity
of the observation in the three stations were thrice
a week for four months. Three different hues of
leaves (green, yellow, and brown) were tied with
a thread, anchored by a bamboo stick, and placed
close to the burrows for determining the preferred
color. Scoring and observation were carried out
by evaluating the leaves that were eaten or miss-
ing and scored positive when fiddler crabs had
bite marks on that particular leaf color or if leaves
were missing. Ten randomly selected burrows
from each of the three stations were used in the
experiment. Each station was observed for one
hour during low tide. The experiment was repeat-
ed four times and then after the fourth test, crab
burrows were sampled for leaf coloration in each
of the three stations.
Laboratory work
Species identification and diet composition of fid-
dler crabs
Fiddler crab species were identified by mor-
phological characteristics from Rosenberg (2019)
and by using the following taxonomic references:
the Austruca annulipes (Milne Edwards, 1837),
Gelasimus vocans (Linnaeus, 1758), Tubuca
capricornis (Crane, 1975), Tubuca urvillei (Milne
Edwards, 1852), Paraleptuca crassipes (White,
1847) and Tubuca alcocki (Shih, Chan and Ng
2018). Thirty male and female fiddler crabs from
each of the three stations were sampled for diet
composition. These samples were collected in the
field and placed immediately in 70% ethanol and
brought to the laboratory. Fiddler crabs were
injected with 10% formalin solution to stop the
digestion process and then they were pho-
tographed. Next, fiddler crabs were dissected and
stomach contents were washed with distilled
water, transferred to a solution of 10% formalin,
and stained with safranin red (this stain was used
because it was the only one that could be found in
the laboratory during the study). Contents were
classified into distinguishable food categories,
e.g. leaf, algae, and sediments. Stomach fullness,
percentage of the total volume visible contributed
by each of the food categories, and frequency of
occurrence of different food categories were
determined. To get the percentage of the stomach
fullness the following values were D0=0%, D1=
25%, D2=50%, D3=75% and D4=100%.
Categorizing food Items from crab stomachs
By using a dissecting microscope, food items
in the stomach of each crab were classified as
sediment, leaves, or algae. There were also stom-
ach samples in which no leaf fragments were
found. Foraging behaviors of fiddler crabs were
also classified according to whether they forage
(slow walking, associated with tapping or tasting
sediment and litter) or collect leaves (grasping the
item and retreating into the burrow).
Data analysis
All count data were first checked for normal
distribution before comparisons were made. If
data were not normally distributed, they were
log10 transformed and checked again for normal
140 MARINE AND FISHERY SCIENCES 36 (2): 137-147 (2023)