MARINE AND FISHERY SCIENCES 36 (2): 137-147 (2023)

https://doi.org/10.47193/mafis.3622023010506

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.

137

*Correspondence:

edison.macusi@dorsu.edu.ph

Received: 16 January 2023

Accepted: 16 March 2023

ISSN 2683-7595 (print)

ISSN 2683-7951 (online)

https://ojs.inidep.edu.ar

Journal of the Instituto Nacional de

Investigación y Desarrollo Pesquero

(INIDEP)

This work is licensed under a Creative

Commons Attribution-

NonCommercial-ShareAlike 4.0

International License

Marine and

Fishery Sciences

MAFIS

ORIGINAL RESEARCH

Behavior and diet composition of fiddler crabs in Guang-guang, Dahican,

Mati City, Davao Oriental, Philippines

IVY M. NALLOS1, 2 and EDISON D. MACUSI1, 2, *

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 https://orcid.org/0000-0003-3752-4847, Edison D. Macusi

https://orcid.org/0000-0002-9714-1074

INTRODUCTION

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.

MATERIALS AND METHODS

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

139

NALLOS AND MACUSI: BEHAVIOR AND DIET COMPOSITION OF FIDDLER CRABS

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)

data distribution and homogeneity of variance

using Kolmogorov-Smirnov test. Once the

requirement of ANOVA was satisfied, then all

tests were considered statistically significant at p

£0.05. Post hoc analyses using Tukey’s HSD test

and the modified Tukey’s HSD test for unequal

sample N were performed. The Kruskal-Wallis

test was used to analyze the frequency of various

behaviors and the diet composition of fiddler

crabs in order to compare them when data trans-

formation did not work out for normal distribu-

tion and homogeneity of variance.

RESULTS

Species composition

Family Ocypodidae is the family of fiddler

crabs found in different stations: the muddy,

sandy coralline, and sandy muddy substrate in the

study area. Six species were identified from sam-

ples: Austruca annulipes (Edwards, 1837),

Gelasimus vocans (Linnaeus, 1758), Tubuca

capricornis (Crane, 1975), Tubuca urvillei

(Edwards, 1852), Paraleptuca crassipes (Adams

and White, 1848), and Tubuca alcocki (Shih,

Chan and Ng 2018). In species identification, the

genus level was used due to a lack of exact infor-

mation about their species composition. Pictures

taken during the sampling period were compared

to descriptions from Rosenberg (2014) (Figure 2).

Behavior of fiddler crabs

There were different behaviors of fiddler crabs

observed in the study area. They consisted of

antagonistic, walking, foraging, waving, and bur-

row enhancement. The frequency of these activi-

ties during the observation days were analyzed

using Kruskal-Wallis test and no significant dif-

ferences in terms of the walking (df =2, H =2.65,

p=0.266), foraging (df =2, H =0.36, p =0.834),

141

NALLOS AND MACUSI: BEHAVIOR AND DIET COMPOSITION OF FIDDLER CRABS

Figure 2. Side by side, comparison of species found in the

study area together with published photos from

Rosenberg (2014) and their corresponding names.

Austruca annulipes (Edwards, 1837) (A and B),

Gelasimus vocans (Linnaeus, 1758) (C and D),

Tubuca capricornis (Crane, 1975) (E and F), T.

urvillei (Edwards, 1852) (G and H), Paraleptuca

crassipes (Adams and White, 1848) (I and J), and T.

alcocki (Shih, Chan and Ng 2018) (K and L).

antagonistic (df =2, H =0.36, p =0.834) and bur-

row enhancement activities of the various fiddler

crabs (df =2, H =5.71, p =0.058) were detected.

Observations were done in three different sub-

strates (muddy, coralline, and sandy substrates)

for three days to assess their behaviors. Waving

and antagonistic activity were two patterns of

activity that only male fiddler crabs were able to

perform more actively than female fiddler crabs

(Figure 3).

The various behaviors of fiddler crabs

observed included courting (waving), defending

burrows (antagonistic), and burrow enhancement

mostly performed by Paraleptuca crassipes (Fig-

ure 4 A, D, and E). A waving behavior was usual-

ly performed by Paraleptuca boninensis (Figure

4 B), while fencing/predation behavior was per-

formed by Tubuca dussumieri (Figure 4 C), and

foraging behavior was performed by Tubuca

capricornis (Figure 4 F). In addition, walking

was performed by Gelasimus tangeri (Figure 4

G), and foraging was also performed by Paralep-

tuca chlorophthalmus (Figure 4 H).

Diet composition

Fiddler crabs were collected to identify their

stomach content. Most of stomach samples con-

tained sediment, leaf fragments, and algae. The

post hoc comparison showed that sediments com-

prised 34% of stomach contents followed by 12%

of chum and 10% of leaf particles. Comparison of

sediment contents of stomachs from the various

substrates showed no significant differences (df =

2, H =1.19, p =0.551), and the same was

observed for chum (df =2, H =2.17, p =0.339)

and leaf contents (df =2, H =1.38, p =0.501)

when compared to those in the sandy, muddy, and

coralline area.

Mangrove leaf preference

During the three-day experiment regarding leaf

preference of fiddler crabs, the most eaten leaves

were the yellow ones with a total count of n =

104. In the muddy substrate, yellow leaves were

the most eaten among the three colors (n =27),

while green leaves were the least consumed (n =

23). The highest count of total leaves eaten in the

coralline substrate was yellow leaves (n =37),

while brown leaves were the least eaten (n =21).

For the sandy muddy, the highest count of eaten

leaves were also yellow leaves (n =40), while

green leaves were also less consumed (n = 29)

(Figure 5 A). On the third day of the experiment,

some leaves were missing in each station, both

green and yellow leaves. In the muddy station,

the number of leaves missing for the three colors

were the same (2). In coralline areas, yellow

leaves had two missing leaves (2) compared to

the others, one for the green (1) and none for the

brown (0). For sandy areas, green and yellow

leaves had the same number of missing leaves (6)

while brown leaves had the lowest number (5)

(Figure 5 B).

DISCUSSION

Females invested more time feeding and fed

50% faster than males. For example, Uca vocans

(Rumphius, 1705) was the most dominant fiddler

crab species on sandy beaches and was particular-

ly active, feeding at approximately twice the rate

of other species (Weis and Weis 2004).Composi-

tion of the substrate is also significantly altered

by foraging and burrowing activities (Posey

1987) and could affect biological processes like

meiofauna reproduction (Ólafsson and Ndaro

1997) and growth of young mangrove plants

(Macusi and Tipudan 2021). Foraging activity

was the most frequently performed activity by

female fiddler crabs compared to males, followed

by walking. To increase the size of their arms out

of proportion to the growth of their bodies, large

male fiddler crabs engage in grabbing and pinch-

ing structures. Even crab mating can be utilized

142 MARINE AND FISHERY SCIENCES 36 (2): 137-147 (2023)

143

NALLOS AND MACUSI: BEHAVIOR AND DIET COMPOSITION OF FIDDLER CRABS

Figure 3. Total count of various behaviors of fiddler crabs observed in the three different substrates (A). Male fiddler crabs per-

formed both antagonistic and waving activities more actively. Percentage of stomach contents of fiddler crabs (B).

Figure 4. The various behaviors of fiddler crabs displayed in the three stations (muddy, sandy, and coralline). A, D, and E)

Paraleptuca crassipes. B) P. boninensis. C) Tubuca dussumieri. F) T. capricornis. G) Gelasimus tangeri. H) P. chloroph-

thalmus.

Antagonistic Walking Foraging Waving Burrow

enhancemnet

Behaviors

Stations

Frequency

Average

0.66

0.64

0.62

0.60

0.58

0.56

0.54

800

700

600

500

400

300

200

100

Muddy Sandy coralline Sandy muddy

ABCD

EFGH

as a predation technique. Males of ‘directing’ or

‘herding’ fiddler crab species trap stressed

females between their body and their major claw

before pushing and pulling them into burrows

(Zucker 1986). The bioturbation carried on by

fiddler crabs through burrowing, feeding and

ventilation is crucial for coastal wetlands world-

wide (Stieglitz et al. 2013; Xiao et al. 2017). Crab

burrows become waterlogged occasionally as a

result of tidal flushing, which results in material

exchange between the burrow water and overly-

ing water across the sediment-water interface

(Xie et al. 2019). Burrow flushing creates asym-

metric distributions of dissolved oxygen along

burrow walls and surrounding sediments, and

bioturbation can significantly increase dissolved

oxygen uptake (Liu et al. 2019).

Stomach contents of fiddler crabs consisted of

sediments, chum, and leaf particles. When

observed under the microscope, sediments and

chum were consistently more prevalent than leaf

particles in the stomach of fiddler crabs. At each

station, some leaves were tied near their burrows

and after some time they disappeared, while some

leaves bore a scratch mark and others were eaten,

indicating that leaves were also part of the diet of

the species. Since energy is much more readily

available in plants than in animals, the low nitro-

gen concentration of plants can prove to be a sig-

nificant limiting nutrient for herbivores (Boyd

and Goodyear 1971). Although having a greater

expected mass and availability than animal

meals, plants are less nutrient-dense than those

used for food by herbivores (Wolcott and O’Con-

nor 1992).

Mangrove forest sediments contain large num-

bers of active bacteria (Alongi et al. 2005). These

bacterial communities break down organic

144 MARINE AND FISHERY SCIENCES 36 (2): 137-147 (2023)

Figure 5. Total count of eaten (A) and missing (B) mangrove leaves.

Frequency (N)

Stations

Muddy Sandy coralline Sandy muddy

Stations

Muddy Sandy coralline Sandy muddy

Green Yellow Brown

debris, including leaf litter, and incorporate rem-

ineralized nutrients into mangrove sediments,

making them available to plants and detritivores

(Koch and Wolff 2002). Fiddler crabs, together

with other detritivores, consume between 20%

and 80% of the carbon from mangrove leaf litter.

They are the most active and noticeable detriti-

vores in the mangrove forest (France 1998;

Bouillon et al. 2008; Kristensen et al. 2008). By

utilizing these resources, detritivores contribute

more to the cycling of nutrients in mangrove sys-

tems than any other trophic group by mass per

unit of time (Koch and Wolff 2002). This con-

cept, however, had been questioned, and even

pointed out that partially degraded mangrove

leaves are likely insufficient to support crab

growth (Bouillon et al. 2004). When gut samples

of fiddler crabs were viewed under the micro-

scope and analyzed, 60% of their content was

sediment. Fiddler crabs are specialized sediment

consumers. They use their mouthparts to extract

organic material from the sand or mud sediments.

Excess inorganic sediments are later released as

tiny pellets, which typically cover the ground

near burrows. Because male crabs only have one

feeding claw, it is helpful to identify crab species

by its gender. Therefore, they feed longer and

scoop more quickly than female crabs. After the

primary claw reaches a particular size, it

becomes useless as a feeding instrument (Moruf

and Ojetayo 2017).

CONCLUSIONS

This study was conducted due to the lack of

information about the behavior and diet composi-

tion of fiddler crabs. Subsequent studies could

benefit from additional data collection to better

understand their roles in the mangrove ecosys-

tem. Firstly, a similar study involving the record-

ing of digital video cameras for their various

behaviors ought to be carried out. Secondly, Rose

Bengal staining should have been used to deter-

mine the gut contents of the fiddler crabs and

additional burrow sampling should have been

used to assess the diets of fiddler crabs. Lastly,

identifying the species of fiddler crabs could be

done at the molecular level.

ACKNOWLEDGEMENT

The authors would like to thank PENRO/

CENRO staffs who assisted and allowed the

study to be conducted in Guang-guang, Dahican,

Mati City. We thank Mr Mike Bersaldo for mak-

ing the map for this study.

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NALLOS AND MACUSI: BEHAVIOR AND DIET COMPOSITION OF FIDDLER CRABS