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)