Cent. Eur. J. Biol. • 8(9) • 2013 • 843-847
DOI: 10.2478/s11535-013-0208-7
Central European Journal of Biology
Number of ghost crab burrows does not
correspond to population size
Communication
Willian T.A.F. Silva1,2,*, Tereza C.S. Calado1
Integrated Laboratories of Marine and Natural Sciences,
Federal University of Alagoas, 57051-090 Maceió-AL, Brazil
1
Institute of Evolutionary Sciences of Montpellier,
Group of Developmental and Evolutionary Biology,
University of Montpellier 2, 34095 Montpellier, France
2
Received 12 December 2012; Accepted 08 May 2013
Abstract: Ghost crabs are distributed worldwide on sandy beaches, and several studies have associated the number of ghost crab
burrows with the levels of anthropogenic impacts on the beaches under study. However, our results show that the use of
ghost crab Ocypode quadrata burrows to assess levels of anthropogenic impacts on sand beaches may not be accurate,
as previously thought, because the number of burrows does not represent an estimate of the population size. In addition,
we propose three hypotheses to explain the extremely low number of individuals/number of burrows ratio: the “secret
chamber”, the “multiple openings”, and the “one crab, several burrows” hypotheses. We also observed an unusual sex ratio.
Keywords: Ocypode quadrata • Ocypodidae • Sandy beaches • Anthropogenic impacts
© Versita Sp. z o.o.
1. Introduction
The ghost crab Ocypode quadrata (Fabricius, 1787),
also known as maria-farinha in Brazil, is the only
species of the genus Ocypode that can be found on
the Brazilian coast [1]. These semi-terrestrial crabs
are the largest permanent residents of the upper
beach and play an important role as predators and
scavengers [2].
Ghost crab burrows increase in diameter as distance
from the water line increases [3], and they show a variety
of shapes, varying in orientation and depth [4,5]. The
abundance of ghost crab burrows has been reported in
several ecological and population studies and seems to
be influenced by several ecological aspects. Abundance
increases with distance from the water line [6]; sources of
food [6,7]; sand moisture [8]; and smaller sediment grain
size [9]. These burrows have long been used worldwide
as an estimation of population size [10,11] and a tool
for assessment of human impacts on sandy beaches
[12-15]. In a field survey, we tested the hypothesis that
the number of ghost crab O. quadrata burrows on sandy
beaches corresponds to the estimated number of crabs
in the population.
2. Experimental Procedures
Four surveys were conducted on two beaches: Avenida
beach (9º40’16.8”S 35º44’36.4”W) and Pontal sand
bar (9º42’08.4”S 35º46’56.9”W), in northeast Brazil
(Figure 1) between October 2006 and March 2007.
Avenida beach is considered urban and shows several
types of human activities, including recreational activities
(e.g. beach soccer games, picnicking, sunbathing and
fishing) and the presence of cleaning tractors that clean
the beach. Pontal sand bar is considered non-urban
with regard to levels of anthropogenic impacts and is
also a protected area, thus limiting human activities. We
excavated a total of 370 burrows throughout the entire
* E-mail: [email protected]
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Number of ghost crab burrows does not
correspond to population size
Figure 1.
Localization of the sites under study.
study period and analyzed the relationship between the
number of burrows excavated and the number of crabs
found in the burrows. An area of 600 square meters (6 m
in width and 100 m in length) was delimited on the upper
beach parallel to the water line and all burrows within
the delimited area were excavated. Since crabs have
nocturnal behavior, the excavations were conducted
during daylight hours so that the crabs could be found
in their burrows, and were performed using a branch
of morning-glory (commonly found in the study sites),
a trenching shovel and hand trowels. The branch of
morning-glory was used to keep track of the burrow tube
orientation and curves in the event of cave-ins during
the excavations. Burrows were back-filled after each
survey was completed.
3. Results and Discussion
Our surveys showed that the number of burrows and
number of crabs in the urban site differed significantly
(P<0.01) from the non-urban site (Figure 2). This finding
supports the use of number of burrows to assess levels
of anthropogenic impacts on sandy beaches. However,
the number of burrows was also significantly different
from the number of crabs in both sites (P<0.01),
meaning that it does not represent an accurate estimate
of crab population size. The lowest crab abundance
was found in our second survey in Avenida beach and
corresponded to only 14.9% of the number of burrows.
The highest crab abundance was found in the first
survey in Pontal sand bar and corresponded to only
27.0% of the number of burrows.
Our results are of great importance for the
interpretation of previous studies on the ecology of ghost
crabs and conservation of sandy beaches. Several
studies on sandy beaches have been carried out
worldwide using the abundance of ghost crab burrows
to assess the levels of anthropogenic disturbance.
Significant differences between the number of ghost
crab burrows in urban and non-urban beaches have
been found in many others [12-21]. In all these studies,
the number of burrows had a negative association
with the levels of human disturbance in the observed
beaches. These results, and many others, must now be
considered with care.
Three hypotheses are proposed to explain the
low number of individuals/number of burrows ratio.
The first possible explanation is the “secret chamber”
hypothesis (Figure 3a). This supposes that crabs isolate
themselves from the outside by constructing a wall of
sand at some point in their burrow extension, creating a
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W.T.A.F. Silva, T.C.S. Calado
Figure 2.
Abundance of ghost crab burrows and individuals throughout the study period. Black bars represent number of burrows. Grey bars
represent number of individuals collected. Letters represent the results of the statistical analyses (chi-square tests for goodness-of-fit),
different letters representing statistically significant (P<0.01) differences.
Figure 3.
Hypotheses proposed to explain the low number of individuals/number of burrows ratio. a) “secret chamber” hypothesis; b) “multiple
openings” hypothesis; c) “one crab, several burrows” hypothesis.
chamber that could not be found during the excavations.
During the surveys, some crabs were found in burrows
that had already been excavated and where no crab
had previously been found. The second explanation is
the “multiple openings” hypothesis (Figure 3b), which
supposes that burrows have more than one or two
openings that are considered different burrows during
excavations due to the lack of apparent connection
between them. The third explanation is the “one
crab, several burrows” hypothesis (Figure 3c), which
postulates that each crab constructs several burrows
that may be used in different situations. Burrows have
been reported to have two main functions for ocypodid
crabs: they are used as a shelter against predation
by non-breeding crabs, and as refuge and copulation
burrows by breeding crabs [5,22,23]. Other possible
uses would include feeding and territoriality. However,
it is also possible that the behavior of constructing
several burrows may merely be a consequence of poor
memory; that is, crabs do not remember where their
burrows are and start constructing a new burrow in a
different place. Regardless of the general biology of the
ghost crab, it has been proposed that the difference
in number of burrows between urban and non-urban
beaches may represent behavioral differences between
these populations [12].
Our results contradict those found for Heloecius
cordiformis [24], another semi-terrestrial ocypodid
crab; according to Warren, the apparent abundance of
crabs and the number of open burrows are significantly
correlated. However, H. cordiformis lives in mangrove
swamps and mudflats. The difference between our
results could be explained by the difference in habitat of
O. quadrata and H. cordiformis, but other studies suggest
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Number of ghost crab burrows does not
correspond to population size
otherwise. A comparison between three methods for
estimating the population density of Uca annulipes,
another ocypodid crab that inhabits mangrove mudflats,
showed that the two indirect methods, binocular count
and burrow count, underestimated and overestimated,
respectively, the abundance of crabs when compared
to the excavation method [25,26]. This suggests that
habitat type is not the cause of the discrepancy between
number of burrows and population size. In a previous
study on ghost crabs, burrow count and a method similar
to binocular count were used to elucidate different
aspects of the biology of ghost crabs, but a comparative
methodological approach was not used [27]. This is
the first time that the significance of the relationship
between number of burrows and crab abundance of O.
quadrata has been tested.
Another intriguing find in this study was the sex ratio
in the sample. Both at Avenida beach and at Pontal
sand bar, the total number of males in the sample was
significantly different from the total number of females
(χ2=4.50, df=1, P<0.05, for Avenida beach; χ2=48.60,
df=1, P<0.01, for Pontal sand bar). At Avenida beach,
the sex ratio was 7:1, while at Pontal sand bar the sex
ratio was 19:1. Although the population size at Pontal
was greater, it showed a higher sex ratio disequilibrium
than Avenida.
In a different study, researchers collected emergent
crabs during their nocturnal activities throughout the
year at a different beach in Brazil [28]. Their sample did
not display any sex ratio disequilibrium (χ2=1.758, df=1,
P>0.18; statistics not published in the original article).
Since ghost crabs are primarily nocturnal [29,30] and
our study was conducted during daylight hours, our
results raise questions about the possible explanations
for the observed ratio. Crabs are believed to be in their
burrows during daylight hours, but we could not find
them, suggesting that there are alternative daytime
locations. Further studies on this species need to
be conducted in order to understand its population
dynamics and behavior. The effectiveness of using the
number of ghost crab burrows to assess anthropogenic
impacts on sandy beaches seems to hold true, but this
method cannot be used to estimate population size. If
not used with careful interpretation, this method can
yield doubtful and inaccurate estimates.
Acknowledgements
The authors are grateful to Manoel José da Silva and
all LabMar interns for assistance with the fieldwork.
The authors would like to thank the anonymous
reviewers for their comments and suggestions to
improve the quality of the paper. The author was
supported by the Institutional Programme of Scientific
Initiation (PIBIC) with a scholarship granted by the
Research Support Foundation of the State of Alagoas
(FAPEAL), Brazil.
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