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Marine Biology Research
ISSN: 1745-1000 (Print) 1745-1019 (Online) Journal homepage: http://www.tandfonline.com/loi/smar20
Biology and interspecific interactions of the alien
crab Percnon gibbesi in the Maltese Islands
Marija Sciberras & Patrick J. Schembri
To cite this article: Marija Sciberras & Patrick J. Schembri (2008) Biology and interspecific
interactions of the alien crab Percnon gibbesi in the Maltese Islands, Marine Biology Research,
4:5, 321-332, DOI: 10.1080/17451000801964923
To link to this article: http://dx.doi.org/10.1080/17451000801964923
Published online: 30 Sep 2008.
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Biology and interspecific interactions of the alien crab Percnon gibbesi
in the Maltese Islands
MARIJA SCIBERRAS & PATRICK J. SCHEMBRI
Department of Biology, University of Malta, Msida, Malta
Spatial and bathymetric distribution, population density, habitat preferences, fecundity, breeding season and interspecific
interactions of the alien grapsoid crab Percnon gibbesi (H. Milne-Edwards, 1853) from the Maltese Islands (Malta and Gozo)
are compared among localities in the Mediterranean where established populations have been reported since 1999. In the
Maltese Islands, habitat preferences and bathymetric distribution were similar to those in other Mediterranean localities.
Spatial distribution was found to be limited by the availability of the boulder habitat in which this crab nearly always occurs.
Fecundity was higher in the Maltese Islands than in Linosa and Lampedusa, the breeding season lasting from the end of
May until September. On Maltese shores the habitat of the alien overlapped with that of the native grapsid Pachygrapsus
marmoratus (Fabricius, 1787) (Crustacea: Brachyura: Grapsoidea) and, to a lesser extent, that of the native xanthid Eriphia
verrucosa (Forska˚l, 1775). Laboratory studies to assess the possible interactions of the alien species with P. marmoratus
suggest that the latter shows a competitive advantage over P. gibbesi, since 80.8% of encounters between the two species were
initiated by P. marmoratus, and in 80% of the encounters it prevailed. This suggests that P. marmoratus is unlikely to be
excluded from its natural habitat by the alien species, and that significant spatial resource partitioning on the part of P.
marmoratus is unlikely to occur.
Key words: Interspeciﬁc interactions, Maltese Islands, Pachygrapsus marmoratus, Percnon gibbesi, population density
Increased trade and tourism associated with globali-
zation have facilitated one of the least reversible
human-induced global changes: the homogenization
of the Earth’s biota through the establishment and
spread of alien species (Kolar & Lodge 2002). The
crab Percnon gibbesi (H. Milne Edwards, 1853), a
grapsoid (superfamily Grapsoidea) of uncertain fa-
milial assignation (Schubart et al. 2000), has a wide
natural latitudinal and temperature range that ex-
tends from Baja California to Chile on the eastern
Pacific coast (Hendrickx 1995). On the western
Atlantic coast its range extends from Florida to
Brazil. Along the eastern Atlantic coast its range
includes Madeira, the Azores, the Cape Verde Islands
and the coast of Africa from Morocco to Ghana and
offshore islands in the Gulf of Guinea (Manning &
Holthius 1981; Relini et al. 2000). This species has
recently been reported from the Mediterranean Sea,
where in 1999 it appeared concurrently in Ibiza,
Spain (Muller 2001) and two locations in Italy:
Linosa Island in the Straits of Sicily (Relini et al.
2000) and Capo Passero in southern Sicily (Mori &
Vacchi 2002). Since that time, P. gibbesi has con-
tinued to spread rapidly throughout the Mediterra-
nean basin. Other reports from Italy include
Pantelleria and western Sicily (San Vito, Capo Gallo
and Ustica) (Pipitone et al. 2001), the northern and
Ionian coasts of Sicily, the Tyrrhenian coast of
Calabria, the islands of Ischia and Ponza, southern
Sardinia, Isola delle Femmine (northwest Sicily)
(Cannicci et al. 2004), the central Tyrrhenian
extending from the coasts of Cilento to the Pontine
Islands, and the Gulf of Naples (Russo & Villani
2005). Reports from Spain include Dragonera and
Es Pantaleu islands southwest of Mallorca (Deudero
et al. 2005). Reports from Greece include: Xero-
campos and the Gulf of Messiniakos (Crete), Anti-
kythira Island and Rhodes Island (Cannicci et al.
Correspondence: M. Sciberras, Department of Biology, University of Malta, Msida MSD2080, Malta. E-mail:
Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory,
University of Copenhagen, Denmark
Marine Biology Research, 2008; 4: 321332
(Accepted 4 February 2008; Printed 17 October 2008)
ISSN 1745-1000 print/ISSN 1745-1019 online # 2008 Taylor & Francis
Marine Biology Research 2008.4:321-332.
2006; Thessalou-Legaki et al. 2006). Reports from
Turkey include Uc Adalar and Kas-Antalya (Yokes &
Galil 2006). Percnon gibbesi was first recorded from
Malta in 2001 (Borg & Attard-Montalto 2002).
Numerous mechanisms for the introduction and
spread of P. gibbesi in the Mediterranean have been
proposed. These include natural range expansion
by adult migration or larval drift with Atlantic
surface currents that enter the Mediterranean via
the Strait of Gibraltar, favoured by a climatic
change towards warmer waters (Pipitone et al.
2001); transport in the ballast tanks of vessels
(Cannicci et al. 2006; Yokes & Galil 2006), or
with fouling and boring communities on ship hulls
(Mori & Vacchi 2002); and intentional release or
accidental escape from aquaria (Borg & Attard-
Montalto 2002). Whatever the mechanism of
introduction, this species has established breeding
populations in all Mediterranean localities where it
has been reported. The remarkable success of this
alien to colonize and establish itself in the Medi-
terranean has been linked to several attributes of
P. gibbesi itself and the recipient communities,
including favourable local environmental factors
such as seawater temperature and absence of
competitors (Pipitone et al. 2001); availability of
unoccupied niches (Cannicci et al. 2004; Fanelli &
Azzurro 2004); the crab’s ability to rapidly adapt to
different types of habitats (Pipitone et al. 2001); its
flexible feeding habits (Cannicci et al. 2004); its
long reproductive period (Fanelli & Azzurro 2004);
its ability to produce several clutches of eggs each
year (Mori & Vacchi 2002); and its long plankto-
trophic larval stage (Pipitone et al. 2001).
Even if the biology of a species in its native range is
known, the consequences of an invasion are often not
easily predicted (Ruiz et al. 1997) since various
checks and balances that would normally limit
population growth of the invader might be different
or non-existent in the new environment. For example,
the absence of predators and competitors for the alien
ctenophore Mnemiopsis in the Black Sea, contrary to
in its native habitat, had severe impacts on the Black
Sea ecosystem in the late 1980s and early 1990s. The
reduction in zooplanktivorous fish populations in the
Black Sea due to overfishing prior to the ctenophore
outbreak, combined with predation by Mnemiopsis on
fish eggs, contributed to the success of Mnemiopsis in
the Black Sea and subsequent reductions in fish
populations (Purcell et al. 2001). The appearance of
yet another invasive ctenophore (Beroe) that preyed
on Mnemiopsis promoted recovery of the Black Sea
ecosystem from effects of the Mnemiopsis invasion
(Shiganova et al. 2001). Invasions may thus be
regarded as ecological ‘experiments’ that could pro-
vide information on the structure (Crooks & Khim
1999) and resistance (Stachowicz et al. 1999) of
natural communities, or on perturbations caused by
the sudden arrival of new species into the system
(Lambert et al. 1992; McDonald et al. 2001; Walton
et al. 2002; Occhipinti-Ambrogi & Savini 2003;
Branch & Steffani 2004).
Information on the biology and ecology of P.
gibbesi in the Mediterranean is steadily accumulating
and indicates that this crab species interacts with its
environment differently in different localities where
it has been established. In this study we report field
observations on the distribution, density and biology
of P. gibbesi in the Maltese Islands. We also report
results of a laboratory-based study to investigate the
competitive advantage in one-on-one interspecific
interactions between the alien species and the native
syntopic grapsid, Pachygrapsus marmoratus (Fabri-
cius, 1787), with which the alien was observed to
share habitat and interact with during field observa-
Material and methods
Species studied in the Maltese Islands were identi-
fied by comparing crabs collected in the field with
characteristics of Percnon gibbesi detailed in Schmitt
(1939) and Williams (1984). All specimens exam-
ined were typical P. gibbesi.
Field surveys to assess the bathymetric distribution,
habitat preferences and interaction with other spe-
cies were carried out between July and October 2004
at 23 sites around the Maltese Islands (Figure 1).
Observations were made and samples collected from
Malta and Gozo two of the islands that make up
the Maltese archipelago. At 20 of the sites investi-
gated, the population density between the 2 m depth
contour and the shore was determined as the
number of crabs in replicate virtual quadrats, in
habitats where P. gibbesi was observed. The number
of replicates varied from 5 to 49 (see Table II) based
on size of the area investigated. Bottom areas of
approximately 1 m
were estimated visually using
landmarks. At Cirkewwa (Site 1 in Figure 1), St.
Julians (Site 8), Marsascala (Site 10) and Hondoq ir-
Rummien (Site 19), changes in crab population
density with depth were studied for two depth strata:
within 2 m and from 2 to 4 m, during the morning
(10.0012.00 h). At Pembroke (Site 7 in Figure 1),
changes in crab population density with time of day
were studied on six consecutive days by conducting
quadrat counts between 11.00 and 14.00 h and then
again between 17.00 and 19.30 h.
322 M. Sciberras & P. J. Schembri
Marine Biology Research 2008.4:321-332.
Reproductive condition of female crabs was ex-
amined at weekly intervals between July and October
2004, and at monthly intervals between November
2004 and May 2005, with the exception of April
2005 when no samples were collected. Sexual
maturity of the crabs was assessed based on abdom-
inal morphology. Females with domed abdomens
were considered mature; those with suboval abdo-
mens were considered immature. Males with abdo-
mens closely apposed to the thoracic sternites that
could not be easily pried open with a dissecting
needle, were considered immature. Maturity status
(whether immature or mature) and fecundity status
of females (whether females carry eggs or not) were
determined using a sample of 270 crabs, of which
158 were male and 112 were female crabs. To
investigate the relationship between number of eggs
and size (measured as the maximum carapace
length, CL), egg masses from 30 ovigerous females
collected from the field were fixed in 70% alcohol
and weighed; subsamples of eggs were then re-
moved, weighed and the number of eggs in each
was counted using a stereomicroscope. Prior to
weighing, all egg samples were cleared of setules
and other adhering debris. The total number of eggs
in the egg mass was then calculated by simple
Specimens of Pachygrapsus marmoratus and Percnon
gibbesi were collected by hand between December
2004 and March 2005. Pachygrapsus marmoratus
were collected from rock pools, beneath boulders
and within rock crevices at Pembroke (Site 7 in
Figure 1). Percnon gibbesi were collected from
Pembroke (Site 7 in Figure 1) and Marsascala
(Site 10 in Figure 1) depending on weather and
sea conditions. In the laboratory, each crab was
sexed and measures taken of its carapace length
(CL), carapace width (CW), chela length (total
propodus length, TPL) and chela width (total
propodus width, TPW) using vernier callipers (ac-
curate to 90.1 mm). The crabs were held in plastic
aquaria (31.5 cm19.5 cm 19.5 cm deep) sup-
plied with aerated seawater at 208C. They were left
to acclimate to laboratory conditions for a period of
7 days prior to behavioural observations. During
these experiments the crabs were not fed to motivate
aggressive behaviour between the two crab species.
To ensure competitiveness in behavioural observa-
tions, only crabs were used that had no missing
chelipeds or recently regenerated chelipeds and hard
exoskeletons (Sneddon et al. 1997).
Individuals of both species were grouped into
three size classes; large (L) with CL greater than
30 mm, medium (M) with CL between 20 and 30
mm and small (S) with CL smaller than 20 mm. In
total, nine size combinations were investigated
(Table I). A minimum of 10 one-on-one interspecific
encounters were staged for each size combination by
placing the two crabs in a plastic aquarium (length
29.5 cmwidth 20 cmdepth 19 cm). The bottom
of the test aquarium was layered with gravel and
filled with aerated seawater at 208C. The crabs were
allowed a settling time of 30 min in isolation;
keeping the two individuals separate with an opaque
vertical partition across the test aquarium, after
Figure 1. Map of the Maltese Islands showing the location of the 23 sites investigated.
The alien crab Percnon gibbesi from Malta 323
Marine Biology Research 2008.4:321-332.
which, the partition was raised gently to initiate the
encounter. To reduce the possibility of chemical
communication, the air pump was switched off
during the settling and observation period to mini-
mize mixing. Each encounter lasted for a maximum
of 1 h or until the contest was ended by one crab
injuring its opponent. The actions of both crabs were
recorded for subsequent analysis, using a video
Percnon gibbesi was found at all 23 sites investigated
during this study. This species showed a preference
for boulder fields (boulder size: 30 cm up to 1 m or
more in maximum diameter) with surfaces either
bare of sessile macrobenthic organisms but covered
with a microalgal film; covered with encrusting algae
or algal turf; or with a moderate cover of erect
macroalgae (see Table II). Occasionally, crabs were
also present on wide bare rock ledges or in crevices
on vertical rock walls with minimal vegetation cover
(see Table II). However, this species was never
observed on sandy bottoms, homogeneous rocky
sea beds, bottoms with sparse and widely scattered
boulders, or in seagrass meadows.
In the Maltese Islands, P. gibbesi is strictly subtidal
and limited to the uppermost reaches of the infra-
littoral zone. The bathymetry at the sites studied
ranged from 0.05 to 4 m (Table II), 4 m being the
lower depth limit with significant accumulation of
boulders. However, the possibility that individual
crabs occurred in slightly deeper water cannot be
excluded for four of these sites, namely Cirkewwa;
Hondoq ir-Rummien; Wied il-Ghasri; and Dwejra
(Ghar Zerqa) where the boulder habitat favoured
by this species extended to depths greater than 4 m,
making observations of these well-camouflaged
crabs in dark spaces amongst boulders at these
depths difficult. Observation became more proble-
matic due to the agility with which the crabs
retreated at the slightest disturbance or movement.
However, in most localities, the boulder habitat
gradually merged into a homogeneous rocky or
sandy bottom at depths less than 4 m; here no crabs
were observed beyond the boulder zone.
For the 20 localities where population counts were
made, the mean population density of P. gibbesi
ranged from 1.690.5 to 11.997.1 crabs m
(Table II). Population density was significantly
different between sites (KruskalWallis H-test; PB
0.05); the highest records were for Marsascala,
Hondoq ir-Rummien and Dahlet Qorrot, possibly
because at these localities, the favoured boulder
bottoms extended as a band for relatively long
distances along the shore rather than being restricted
to small patches. Conversely, a lower population
density was obtained at sites such as Ghajn Tuffieha,
Birzebbugia, Fra Ben and Mgarr ix-Xini, where
availability of adequate habitat was limited. At
Ghajn Tuffieha, the crabs were restricted to a small
patch of boulders surrounded by a sandy bottom. At
Birzebbugia, boulders and crevices favoured by P.
gibbesi were absent within the first 23 m from shore,
while at Fra Ben and Mgarr ix-Xini, the majority of
boulders were heavily covered by macroalgae.
The mean population density changed signifi-
cantly with depth at sites where P. gibbesi occurred,
both above and below 2 m depth (MannWhitney
U-test; PB0.05) (Figure 2). Higher mean popula-
tion densities were obtained at depths smaller than
2 m compared to those obtained at 2 to 4 m
depths. A two-fold increase in the mean population
density between morning (before 13.30 h; 5.293.4
) and late afternoon (after 16.00 h;
10.195.7 crabs m
) suggests that P. gibbesi be-
comes most active towards dusk (MannWhitney
U-test; P B0.05). The difference in population
density between morning and evening counts is
not related to tides since the average tidal range for
Table I. The size combinations investigated in interspeciﬁc interactions between Pachygrapsus marmoratus and Percnon gibbesi (the number
of replicate trials for each size combination tested is also given).
Size combination Pachygrapsus marmoratus Percnon gibbesi Number of replicate trials
LL Large Large 10
LM Large Medium 12
LS Large Small 10
ML Medium Large 12
MM Medium Medium 13
MS Medium Small 12
SL Small Large 11
SM Small Medium 12
SS Small Small 11
Large (L) indicates crabs with CL larger than 30 mm, medium (M) indicates crabs with CL between 20 and 30 mm, and small (S) indicates
crabs with CL smaller than 20 mm.
324 M. Sciberras & P. J. Schembri
Marine Biology Research 2008.4:321-332.
the Maltese Islands is 6 cm, with a spring tide
maximum of 20 cm.
Carapace length (CL) of crabs collected from the
field ranged from 6.3 to 37.2 mm (the largest male
collected). The largest female had a carapace length
of 35.6 mm. During July to October, crabs with CL
between 21 and 35 mm constituted 79% of the
sample, whereas crabs with CL less than 21 mm and
more than 35 mm made up 19 and 2% of the
sample, respectively (Figure 3).
Ovigerous females occurred between the end of
May and September; 73.2% of mature females
collected from July to October carried eggs, whereas
98% of the females collected between November
and March were mature but not ovigerous, suggest-
ing that the crab breeds during the summer months.
Brood size produced by mature ovigerous females
ranged between 254 eggs (total number of eggs in
the smallest egg mass) and 32,0409281 eggs
(estimated from three sub-samples taken from the
largest egg mass) per brood. There was a significant
positive correlation between log number of eggs per
brood and the log carapace length of ovigerous
Table II. The mean population density, depth range, general habitat and microhabitat of Percnon gibbesi at 23 sites in the Maltese Islands.
density9SD (crabs m
Cirkewwa 19 3.492.2 0.64.0 1; 2; 4 a; b; c; d
L-Ahrax tal-Mellieha 21 5.593.5 1.43.0 2 a; b; c
Mellieha Bay 14 4.893.0 0.30.6 3 a; b
Imgiebah 16 4.592.2 3.0 2 a; b
St. Paul’s Bay 10 4.392.0 1.52.5 2 b; c
Fra Ben 7 3.992.4 0.51.5 2 a; c; d
Pembroke 43 5.493.9 0.62.6 2 a; b; c
St. Julians 32 2.491.7 1.23.0 2 a; b; c
Ghar id-Dud 43 5.292.4 1.54.0 2 a; b; c; e
Marsascala 21 11.997.1 0.34.0 2 a; b; c
Xrobb l-Ghagin 29 5.592.5 0.52.0 2 a; e
Birzebbugia 6 4.593.3 0.3-0.6 1; 2 b; c; d
Ghar Lapsi 17 5.795.2 0.13.0 2 a; b; c; d
Golden Bay 8 4.693.1 1.52.0 3 a; b; c
Ghajn Tuffieha 5 1.690.6 2.0 3 a; b
Paradise Bay 25 5.892.5 0.32.0 3 a; b; c
Mgarr ix-Xini 9 3.792.1 0.6 1; 2 a; b; c; d
Mgarr Harbour 12 3.392.6 1.01.5 2 a; b
Hondoq ir-Rummien 20 10.096.0 0.63.6 2 a; b; c; d
Dahlet Qorrot 49 8.995.2 0.052.0 2 a; b; c
Ramla l-Hamra Bay 3a;b
Wied il-Ghasri 4a;b;c
Dwejra (Ghar Zerqa) 4b;c
N is the number of replicate quadrats used for estimation of population density. No population counts were made at Ramla l-Hamra Bay,
Wied il-Ghasri and Dwejra (Ghar Zerqa) due to high water turbidity and poor visibility at the time of observation.
Key to general habitat types: 1vertical rock faces with crevices that merge into a rocky bottom with dense cover of macroalgae at depths
less than 4 m; 2boulder field which merges into a bare rocky bottom covered by dense macroalgae or seagrass meadows at depths less
than 4 m; 3boulder field which merges into a sandy bottom at depths less than 4 m; 4boulder field which extends beyond a depth
of 4 m.
Key to microhabitats in which Percnon gibbesi were observed: aboulders with surface bare of sessile macrobenthos, but with a cover of
microalgae; bboulders covered by encrusting algae or algal turf; cboulders with a moderate cover of erect macroalgae; drock ledges
or crevices in rock faces which are either bare of vegetation or have a cover of encrusting algae or algal turf; evertical rock faces bare of
sessile macrobenthos but with a cover of microalgae.
Figure 2. Mean population density (individuals per m
Percnon gibbesi at depths above 2 m and between 2 and 4 m, at
St Julians (m), Cirkewwa (j), Hondoq ir-Rummien () and
Marsascala ('). The population density was estimated between
10.30 and 13.30 h. Error bars represent the 95% conﬁdence
interval above and below the mean.
The alien crab Percnon gibbesi from Malta 325
Marine Biology Research 2008.4:321-332.
females (Pearson product-moment correlation; r
0.82, P B0.001). Juveniles (crabs with CL less than
15 mm) were first observed at the end of September
and continued to appear throughout the winter
months until at least March; this suggests that
recruitment into the population occurred during
this period, however, given the low number of crabs
observed (very small Percnon seek shelter deep in
crevices during rough weather) it could also indicate
Analysis of abdomen morphology suggested that
female P. gibbesi reach sexual maturity at a CL of
15.016.0 mm; this was supported by the fact that
the smallest ovigerous female sampled had a CL of
16.1 mm. Assessment of size at sexual maturity
using abdomen morphology gave identical results for
males and females. All crabs studied were homo-
chelate at all sizes (apart from individuals with
injuries or regenerating chelae). A morphometric
change indicated by an upward inflection in male
chelae length at CLs of 1213 mm was not seen in
females (Figure 4).
Field observations of P. gibbesi suggested that this
species feeds using a combination of microphagy and
macrophagy. They used their chelipeds to scoop the
biofilm or detritus from the surfaces of boulders;
they also used their chelae to either grasp pieces of
macroalgae that floated past in the water current
when these came within striking distance or to tear
off pieces of macroalgae attached to boulders. In the
field, P. gibbesi were observed to feed on the
following algae: Stypocaulon (Halopteris) scoparium,
Hypnea musciformis, Liagora viscida, Gelidium crinale,
Sphacelaria sp., Caulerpa racemosa, Jania rubens,
Padina pavonica and Dictyopteris polypodioides.On
one occasion, a large male was observed to emerge
from its shelter and grasp a jellyfish that was floating
just above the bottom. The crab dragged it back into
its shelter where it presumably ate it.
In the field, P. gibbesi was observed to overlap in
habitat with the native grapsid, Pachygrapsus mar-
moratus, and to a lesser degree with the native
xanthid, Eriphia verrucosa (Forska˚l, 1775); on sev-
eral occasions during the population density counts,
either or both of these decapods were observed
within the same 1 m
virtual quadrat as P. gibbesi
(Table III). Although interactions between P. gibbesi
and P. marmoratus were rarely observed in the field,
when P. gibbesi was surrounded by P. marmoratus
some of which were larger than the alien and were
within 10 cm of its hiding place P. gibbesi remained
confined to its shelter. When the alien was sur-
rounded by P. marmoratus that were smaller than it,
both species foraged independently of each other.
When P. gibbesi touched one of the native crabs with
its pereiopods, however, the latter was observed to
advance quickly towards the intruder, which would
then retreat to a distance of ca. 20 cm away from the
aggressor. No interactions between P. gibbesi and E.
verrucosa were observed in the field.
Interspecific interactions were recorded as a series of
discrete action patterns (acts), described in Table IV.
The initiator of an encounter was defined as the
first crab to move towards its opponent or to make
physical contact with its opponent. Pachygrapsus
marmoratus initiated 80.8% of the interactions for
all size combinations investigated; the remaining
interactions were initiated by Percnon gibbesi
The winner of an encounter was the individual
that elicited repeated retreats from the other, or
successfully caught its opponent inflicting damage,
or even consuming it. Encounters with no winner
Figure 4. Biplot of mean chelae length with carapace length for
male (k,n157) and female ( ,n113) Percnon gibbesi. Only
crabs with both chelae intact and no sign of regeneration were
used in this analysis.
Figure 3. The sizefrequency distribution of Percnon gibbesi
collected from sites around the Maltese Islands during the period
July to October 2004. The total sample consisted of 251 crabs.
326 M. Sciberras & P. J. Schembri
Marine Biology Research 2008.4:321-332.
occurred when one crab moved towards its oppo-
nent but no reaction was elicited from the opponent,
or when the two crabs showed an equal number of
submissive (retreat or low merus display) and
aggressive (medium or high merus display, attack,
or chelae wave display) reactions. Pachygraspus
marmoratus won 80% of the trials within most size
combinations investigated (Figure 6). Percnon gibbesi
won encounters in only 3% of the trials, and only
when matched with a smaller P. marmoratus (Figure
6). In 13 out of 22 encounters between a large or
medium P. marmoratus and a small P. gibbesi, P.
marmoratus deliberately attacked P. gibbesi and killed
it. A large or medium-sized P. gibbesi, on the other
hand, was never observed to inflict injury or kill a
smaller-sized P. marmoratus.
Percnon gibbesi, the ‘loser’ in most encounters,
performed more acts of retreat than P. marmoratus.
Acts of retreat performed by P. gibbesi constituted
92% of all observed acts of retreat (Figure 7).
Conversely, acts of aggression performed by P.
marmoratus constituted 95% of all observed acts of
aggression (Figure 7).
Cheliped displays were good predictors of which
species was likely to win an encounter; such displays
were used as indicators of the relative aggressive
‘drive state’ of the animal to claim a limited resource
(Hazlett & Bossert 1965). Pachygraspus marmoratus
made extensive use of cheliped displays compared to
P. gibbesi (Figure 8). When P. marmoratus was equal
in size to P. gibbesi (LL, MM, SS), P. marmoratus
performed more cheliped displays than P. gibbesi.
Table III. Co-occurrence density (individuals of Percnon gibbesi, Pachygrapsus marmoratus and Eriphia verrucosa in the same m
the 11 locations where spatial overlap was observed.
Density (individuals m
Site Percnon gibbesi Pachygrapsus marmoratus Eriphia verrucosa
Cirkewwa 4 1
L-Ahrax tal-Mellieha 9 1
Mellieha Bay 7 1
Pembroke 26 1
Ghar id-Dud 8 1
Marsascala 1 5
Xrobb l-Ghagin 3 2
Birzebbugia 1 2
Ghar Lapsi 3 4
Mgarr ix-Xini 6 1
Dahlet Qorrot 11 1
St. Paul’s Bay 1 1
Birzebbugia 3 1
Ghar Lapsi 22 1
Paradise Bay 3 1
Dahlet Qorrot 22 1
Mellieha Bay 7 1 1
Xrobb l-Ghagin 3 5 1
Ghar Lapsi 3 4 1
Table IV. Action patterns used during interspeciﬁc encounters
between Percnon gibbesi and Pachygrapsus marmoratus in the
Advance The crab moves towards the other
Retreat The crab retreats (moves away) from the
Touch Any physical contact by means of the
pereiopods or chelae between the two
The ‘neutral’ posture of a crab where
the body is held close to the substratum
with the chelae folded inwards
The body is raised up a little off the
substratum by the ambulatory legs and
the chelae are partly spread laterally
The crab stands on the tips of the
dactyls of the walking legs with the body
well off the substratum and chelae fully
Attack in high-intensity
The crab moves towards its opponent
on the tips of the dactyls and snaps the
chelipeds from an open to a closed
position in an attempt to trap and grasp
Chelae wave display The crab raises the chelipeds up and
down simultaneously or in an alternative
fashion in front of its opponent
The alien crab Percnon gibbesi from Malta 327
Marine Biology Research 2008.4:321-332.
When P. gibbesi was smaller than P. marmoratus (LS,
LM, MS) it did not perform any cheliped displays.
When a small P. marmoratus was matched with a
large P. gibbesi (SL) the two performed almost the
same number of cheliped displays (Figure 8).
The basin-wide distribution of breeding populations
of the alien grapsoid Percnon gibbesi, since its
introduction in the Mediterranean Sea, suggests
that abiotic and biotic conditions favour establish-
ment and expansion of this alien. One critical aspect
of predicting the impact of introduced species is to
determine the range of habitats that are likely to be
occupied. In Mediterranean sites studied to date,
including the Maltese Islands, P. gibbesi indicates a
preference for the infralittoral zone amongst
boulders with surfaces almost devoid of sessile
macrobenthic organisms (Pipitone et al. 2001; pre-
sent study) but covered by thin algal felts, or
amongst mostly bare boulder surfaces with limited
macroalgal cover (Cannicci et al. 2004; Deudero
et al. 2005; present study). The high affinity of
P. gibbesi for boulder bottoms may be due to
Figure 6. The percentage of interactions won by each crab species
in each of the nine size combinations (refer to Table I for
abbreviations). Black bars represent the percentage interactions
won by Pachygrapsus marmoratus, white bars represent the
percentage won by Percnon gibbesi and hatched bars represent
the percentage of interactions with ‘no winner’.
Figure 7. The number of retreat and advance acts by Pachygrapsus
marmoratus (dotted bars) and Percnon gibbesi (black bars) in each
of the nine size combinations (refer to Table I for abbreviations).
Figure 8. The number of cheliped displays, including medium
and high-intensity merus displays, attack in high merus display,
and chelae wave display, by each crab species (black bars represent
Pachygrapsus marmoratus, white bars represent Percnon gibbesi)in
each of the nine size combinations (refer to Table I for abbrevia-
Figure 5. The percentage of interactions initiated by each crab
species in each of the nine size combinations (refer to Table I for
abbreviations). Black bars represent the percentage interactions
initiated by Pachygrapsus marmoratus, white bars represent the
percentage initiated by Percnon gibbesi.
328 M. Sciberras & P. J. Schembri
Marine Biology Research 2008.4:321-332.
increased protection from predators, and a readily
available food supply which includes small algae and
epibenthic organisms (Pipitone et al. 2001), as well
as the algal turf and sparse macroalgae growing on
the boulder surfaces (as observed in the present
In the Maltese Islands, it appears that P. gibbesi’s
spatial and bathymetric distribution are controlled
by factors such as habitat type and availability; the
absence of this species from habitats other than
boulder fields, and its high abundance in areas where
boulder fields cover large areas along the shore,
supports this theory. Sites where P. gibbesi was
observed were typically characterized by boulder
screes, such as Ghajn Tuffieha, Golden Bay, Im-
giebah, Paradise Bay, Dahlet Qorrot and Hondoq ir-
Rummien, or characterized by shores at the bottom
of cliffs such as L-Ahrax tal-Mellieha, or at valley
mouths such as Mgarr ix-Xini and Wied il-Ghasri.
As suggested by Deudero et al. (2005), variability in
abundance of P. gibbesi between replicate quadrats
within the same site may be explained in terms of a
patchy distribution within a heterogeneous habitat.
During this study, no partitioning of crab size with
depth as reported in Deudero et al. (2005) was
observed, since crabs with a CL less than 10 mm
were recorded at depths shallower than 2 m as well
as between 2 and 4 m depth, 4 m being the lower
depth limit for the crab at the sites studied here. The
maximum depth range of P. gibbesi in the Maltese
Islands (7 m) reported by Borg & Attard-Montalto
(2002) falls within the limits reported in other
studies in the Mediterranean (Mu
ller 2001; Deu-
dero et al. 2005), but is far less than that reported in
its native range: 29 m (Mori & Vacchi 2002). At sites
investigated in Spain, Deudero et al. (2005) contend
that bathymetric distribution is restricted to the
upper metres to avoid predation by macrocarnivor-
ous feeders such as groupers Epinepheleus spp. or
bass Serranus spp. However, within its native range P.
gibbesi has been reported from dense algal canopies
(Mori & Vacchi 2002), which would presumably
furnish shelter and protection from predators, thus
enabling it to inhabit deeper waters. The mean
population densities of P. gibbesi in the Maltese
localities studied (Table II) are larger than those
reported by Deudero et al. (2005) for Balearic
waters (0.227.66 individuals per 200 m
nera Island and Es Pantaleu; 3 individuals per m
along the southwest of Mallorca). However, these
two estimates of population density are not compar-
able, since our samples are taken in patches of the
preferred habitat (boulder bottoms) whereas sam-
ples taken by Deudero et al. (2005) were from the
entire seabed including habitats not preferred. No
data on the proportion of bottoms consisting of
fields of boulders are provided by Deudero et al.
(2005), so we cannot estimate whether the popula-
tion densities of P. gibbesi in its preferred habitat are
comparable between the Balearics, Spain and the
Maltese Islands. Apart from the present study, that
by Deudero et al. (2005) is the only one that has
reported population densities of P. gibbesi in the
Mediterranean. Population density during evening
hours was on average twice that for morning counts,
suggesting that P. gibbesi becomes more active
towards dusk, presumably to avoid diurnal preda-
tors. Similarly, observations by Cannicci et al.
(2004) at Capo Gallo, Isola delle Femmine, show
that the crab is most active during dusk and at night
when light intensity is low.
In the Mediterranean, P. gibbesi was described as
strictly herbivorous by Puccio et al. (2006); others
have reported that it feeds on both plant and animal
matter (Cannicci et al. 2004; Deudero et al. 2005).
Deudero et al. (2005) reported opportunistic feed-
ing on algae, pagurids and polychaetes. Stomach
analyses of crabs from northwest Sicily showed that
animal matter, primarily gastropod and crustaceans,
constituted 43.2% of the stomach contents (Can-
nicci et al. 2004). This flexibility in feeding is
probably a key factor that has facilitated the spread
of P. gibbesi in the Mediterranean. Field observations
from the present study, and those by Muller (2001),
suggest that plant material constitutes the bulk of the
diet; however, P. gibbesi was observed to feed on a
jellyfish. Stomach content analysis of P. gibbesi and
laboratory feeding experiments are currently under
way to determine the food preferences of P. gibbesi.
Puccio et al. (2003) hypothesized that the breed-
ing season for P. gibbesi in the Mediterranean is likely
to begin in April as it does in Madeira, since the
surface temperature of seawater in the Mediterra-
nean is similar, if not higher than that surrounding
the archipelago at Madeira. Results from the present
study do not support this hypothesis, since ovigerous
females in the Maltese Islands were recorded be-
tween the end of May and September. The smallest
ovigerous female recorded by Fanelli & Azzurro
(2004) had a smaller carapace length (14 mm)
than the smallest one collected in the Maltese
Islands. This suggests that female crabs from the
southern coast of Italy attain physiological maturity
before those in the Maltese Islands. However,
fecundity in the Maltese Islands was on average
twice as high as that reported by Puccio et al. (2003)
for specimens with the same carapace length col-
lected in Linosa and Lampedusa during the same
period of the year as for the Maltese Islands.
Increased fecundity of P. gibbesi in the Maltese
Islands may thus counterbalance the shorter breed-
ing season, relative to Madeira, such that the species
The alien crab Percnon gibbesi from Malta 329
Marine Biology Research 2008.4:321-332.
maintains a high reproductive output; this is yet
another factor that may have facilitated its rapid and
successful spread. In the present study, juveniles
(CLB15 mm) were observed at the end of Septem-
ber/beginning of October, possibly suggesting that in
the Maltese Islands, recruitment does not occur in
early summer, as assumed by Deudero et al. (2005)
for the western Mediterranean. This may explain
why individuals with carapace length less than 16
mm made up only a small percentage of those
collected between July and October (Figure 3).
Growth in the chelae of P. gibbesi shows a pattern
similar to that of Corystes cassivelaunus (Pennant,
1777), Eurynome aspera (Pennant, 1777) and Macro-
podia rostrata (Linnaeus, 1761) reported by Hartnoll
(1974), where male chela length has a much larger
positive allometry than that of females, which show
low positive allometry. In the Maltese population,
sexual dimorphism seems to manifest at an early
stage during the crab’s life cycle; at CLs as small as
1213 mm, male chela length showed an upward
inflection relative to that of the female. The sexual
dimorphism of the chelae may be a consequence of
their use by the male in combat, display and court-
ship (cf. Hartnoll 1974).
Introductions of alien species have been identified
as one of the leading causes of endangerment and
extinction of native species (Czech & Krausman
1997; Mooney & Cleland 2001). Competition in
particular is increasingly recognized as a major
means through which non-indigenous species have
an impact on native species (Nichols et al. 1990;
Byers 2000). From this study and that by Muller
(2001), it appears that P. gibbesi and P. marmoratus
are potential competitors for space and according
to Muller (2001), also for food since the two
species have been observed to occur in close
proximity. Whether or not the two species actually
compete for food in the Maltese Islands remains a
question at present; detailed studies on diet are
currently under way.
What has been described on some Mediterranean
shores as a vacant niche for P. gibbesi, that is, free of
competition from other brachyuran species (Pipi-
tone et al. 2001; Deudero et al. 2005), might
actually be ‘competition ground’ for resources es-
sential to both the alien and native species in the
Maltese Islands. On Maltese shores, P. marmoratus
normally occupies supralittoral rock crevices and
rock pools. It also occupies rock crevices and
boulders in the mediolittoral and uppermost infra-
littoral zones on most rocky shores (Paul Galea,
unpublished dissertation, 1995), where it has been
observed to overlap with P. gibbesi (present study,
Table III). Spatial overlap with Eriphia verrucosa was
also observed, but to a lesser extent since on Maltese
shores E. verrucosa typically inhabits deeper water in
the upper infralittoral, only occasionally appearing in
the uppermost regions of the infralittoral in calm
weather (Schembri & Lanfranco 1984).
Laboratory experiments from the present study
indicate that when competing for space, P. marmor-
atus dominates the interactions with P. gibbesi,
irrespective of the size combination being investi-
gated. The behaviour of the ‘winner’ and ‘loser’
differed considerably; the ‘winner’ (P. marmoratus in
the majority of the agonistic interactions) performed
more advance acts and cheliped displays, whereas
the ‘loser’ (P. gibbesi on most occasions) showed
more submissive behaviour by retreating from, and
avoiding contact with, its aggressor. According to
game theory, contests should begin with energeti-
cally inexpensive movements, such as display acts,
followed by more energetically demanding acts
involving physical contact, and then on to injurious
behaviour (Sneddon et al. 1997). In interactions
between P. marmoratus and P. gibbesi, encounters
consisted mainly of visual displays such as medium
and high merus displays, attack in high merus
display, and chelae wave displays; they seldom
escalated into injurious fights. This suggests that
the crabs assessed each other visually to decide the
outcome of contests. The use of the chelipeds in
visual displays indicates that chelae size may be the
‘resource holding potential’ (RHP) (Sneddon et al.
2000) by which the two crabs assessed each other’s
strength and likelihood of winning.
The laboratory results presented here suggest that
P. marmoratus shows competitive advantage over P.
gibbesi. Hence, P. marmoratus has a greater chance of
winning a territorial dispute in the field. The native
species is thus unlikely to be excluded from its
natural habitat by the alien. It is also unlikely that
significant spatial resource partitioning on the part
of P. marmoratus will occur.
The authors are most grateful to Edwin Lanfranco
for identifying the algal specimens, Prassede Vella
and Dr Joseph A. Borg for providing valuable
literature, and Dr Katrin Fenech for translating
literature from German to English. Special thanks
to Edmond Sciberras, Darren Cordina and Claude
Cassar for their valuable help in specimen collection.
Earlier drafts of this paper beneﬁtted from the
comments of three anonymous referees for which
we are thankful. This research was supported by
funds from the University of Malta Research Com-
mittee, for which we are grateful.
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