Biologia 65/2: 294—300, 2010
Comparative morphometric study of the invasive pearl oyster
Pinctada radiata along the Tunisian coastline
Sabiha Tlig-Zouari, Lotfi Rabaoui, Ikram Irathni, Moctar Diawara
& Oum Kalthoum Ben Hassine
Unité de Recherche de Biologie, Ecologie et Parasitologie des Organismes Aquatiques, Campus Universitaire, Université Tu-
nis El Manar, Faculté des Sciences de Tunis, Département de Biologie, 2092 Tunis, Tunisie; e-mail: firstname.lastname@example.org
Abstract: In order to study the relative growth of the pearl oyster Pinctada radiata in Tunisia, a total of 330 individuals of
this species were collected from six sites along the Tunisian coastline. Quantitative measurements of collected oysters were
conducted for shell height, shell length, shell width, hinge length, height and width of the nacreous part and wet weight.
The size structure of the sampled populations was described and the relative growth between different morphometric
characteristics was estimated as allometric growth lines for the six P. radiata samples. It appeared that the majority of
examined samples were dominated by large individuals that exceed a shell height of 42 mm. The maximum size (100.5 mm),
recorded in Bizerta lagoon, is bigger than that recorded elsewhere in particular in the Red Sea. Size distribution analysis
also showed that the majority of P. radiata samples were dominated by two or more size groups. Differences of allometric
regression were found between the examined samples for the tested relationships. Moreover, the Factorial Discriminant
Analysis, coupled with Ascending Hierarchic Classification, classified the sub-populations according to geographic locations.
Key words: morphometry; pearl oyster; Pinctada radiata; Tunisian coastline
The pearl oyster Pinctada radiata (Leach, 1814) is
one of the first exotic immigrants that arrived to the
Mediterranean Sea through the Suez Channel (Mon-
terosato 1878). This indo-pacific bivalve has success-
fully passed through this Canal and spread through-
out the Mediterranean basin colonizing continually new
habitats. Indeed, this species has been recorded sev-
eral times in both eastern and western basins (Galil
& Zenetos 2002; Gofas & Zenetos 2003; Zenetos et al.
2005). It was also mentioned in the Adriatic (Vio &
De Min 1996). In Tunisia, P. radiata was reported,
for the first time, by Bouchon-Brandely & Berthoule
(1891) and Vassel (1897) in the Gulf of Gabes (Southern
coastline) where it had confined and proliferated form-
ing very dense populations (Tlig-Zouari 1993). In fact,
outside the gulf of Gabes, the presence of this bivalve
is very sporadic along the eastern Tunisian coastline
(Ktari-Chakroun & Azzouz 1971; Tlig-Zouari 1993).
P. radiata is now established along nearly the entire
Tunisian coastline, from the Libyan frontier to Bizerta
lagoon which represents the northern limit of its cur-
rent distribution. P. radiata abounds in many areas of
the Tunisian coastline: the Gulf of Gabes, Monastir Bay
and the Gulf of Tunis (Tlig-Zouari et al. 2009).
Pinctada radiata is an epifaunal suspension feeder
of the subtidal zone and a fouling species. It is a protan-
dric hermaphrodite species and the size of first sexual
maturity happens at a size of 17 mm. It was reported in
Tunisia that sex inversion occurs in shells of 32–57 mm
length and gonad maturity is controlled by temperature
(Tlig-Zouari 1993; Tlig-Zouari & Zaouali 1994). Stud-
ies carried out on P. radiata outside its original area
are scarce (Tlig-Zouari 1993; Yassien et al. 2000; Mo-
hammed & Yassien 2003) and ecological and biological
characteristics of this species within its new habitats
are spatially and temporally limited.
Pinctada radiata is consumed and appreciated in
some Tunisian coastal regions such as Kerkennah is-
lands, the island of Djerba and in the region of Bizerta.
Thus, it can be an interesting and complementary food
source. However, biological and ecological characteris-
tics of the species, in Tunisia, remain poorly known.
The only studies about Tunisian P. radiata popula-
tion were carried out in the Archipelago of Kerkennah
and interested the species reproduction, its growth and
its demographic and biometric features (Tlig-Zouari
1993; Tlig-Zouari & Zaouali 1994, 1995, 1998). The
knowledge-gap about relative growth and stocks of P.
radiata along the Tunisian coastline, in particular the
northern and eastern sectors, led us to make a research
about this bivalve. The present study aims to character-
ize the morphometry and compare the relative growth
of 6 Tunisian P. radiata populations from the northern
and eastern coasts of the country.
c ?2010 Institute of Zoology, Slovak Academy of Sciences
Morphometry of Pinctada radiata along the Tunisian coasts
Fig. 1. The sampling sites of the six studied Pinctada radiata
samples: N1 – Bizerta lagoon (in Bizerta coast), N2 – Tunis north
lagoon, N3 – La Marsa (in Tunis gulf), E1 – Hammamet harbour
(in Hammamet coast), E2 – Monastir and E3 – Stah Jaber (in
Material and methods
Six samples (N1: Bizerta lagoon, N2: Tunis north lagoon,
N3: La Marsa, E1: Hammamet harbour, E2: Monastir and
E3: Stah Jaber) of wild P. radiata specimens were collected
by divers at 3–6 m depths in 6 sites from the northern and
eastern coasts of Tunisia (Fig. 1). At each site, 55 specimens
were randomly collected within an area of 200 m2which was
randomly chosen (Tlig-Zouari & Zaouali 1995, 1998). A to-
tal of 330 specimens, ranging 18–100.51 mm shell height
(SH), were used in this study. Morphological measurements
of live oysters were conducted after shell cleaning. Speci-
mens were measured for shell height (SH), shell length (SL),
shell width (SW), height and width of the nacreous part
(Hpn and Lpn) and hinge length (HL) on the left shell to
the nearest 0.01 mm by a digital vernier calliper (Fig. 2).
Parts of the measurement were done similarly to previous
studies about Pinctada genus (Hynd 1955; Hwang & Oku-
tani 2003). Wet weight (WW) including shell and soft body
was weighted to the nearest 0.01 g by an electronic scale.
The samples were grouped by size classes and the demo-
Fig. 2. Morphological measurements of the shell in Pinctada ra-
graphic structures of the considered sub-populations were
represented by histograms. Shell heights of samples were
compared using the non-parametric test of Kruskal-Wallis.
Relative growth between different morphometric char-
acteristics was investigated using the shell height (SH) as
reference variable. The relative growth was then represented
as allometric growth regressions (lines) for the six P. radi-
ata samples by applying the linear least squares method
for log-transformed data of each characteristic (Schwartz
1963; Sokal & Rholf 1994). A single allometric relationship
between wet weight (WW) and shell height (SH) was also
performed for each P. radiata sample. The parameters a, b
of allometric equations were estimated and analysis of co-
variance (ANCOVA) was used to compare the difference in
tested variables among populations. Factorial Discriminant
Analysis (FDA) and the Ascending Hierarchic Classification
(AHC) were also applied using the log-transformed morpho-
metric data to check differences among the studied samples.
Statistical analyses of the data were performed using Excel,
SPSS 11.5 and Rgui package softwares.
S. Tlig-Zouari et al.
Fig. 3. Distribution of Pinctada radiata individuals according to the total shell height (SH) in six examined populations.
Table 1. Estimated parameters of allometric growth (for the relationships ln (SL)–ln (SH), ln (SW)–ln (SH), ln (Hpn)–ln (SH), ln
(Lpn)–ln (SH), ln (HL)–ln (SH) and ln (WW)–ln (SH)) of Pinctada radiata in the six examined populations at 95% confidence
N1N2 N3 E1 E2E3
ln(Hpn)–ln(SH) 0.988 0.192 0.980 0.986 0.238 0.893 0.896 0.176 0.918 0.943 0.023 0.079 1.003 0.264 0.981 0.996 0.233 0.943
ln(Lpn)–ln(SH)0.851 0.275 0.927 0.823 0.305 0.860 0.871 0.220 0.773 0.849 0.261 0.914 1.068 0.620 0.968 1.008 0.347 0.839
ln(HL)–ln(SH)0.497 1.864 0.577 0.747 0.833 0.825 0.720 0.979 0.483 0.692 0.981 0.901 0.778 0.630 0.829 0.638 1.212 0.715
ln(WW)–ln(SH) 2.673 7.764 0.822 2.554 7.485 0.898 1.784 4.526 0.522 3.613 11.60 0.943 3.086 9.640 0.974 2.663 8.148 0.889
0.718 1.095 0.860 0.748 0.889 0.848 0.967 0.103 0.809 0.927 0.235 0.949 1.027 0.149 0.968 0.972 0.093 0.884
1.095 1.476 0.850 0.892 0.715 0.753 0.766 0.106 0.579 1.387 2.624 0.936 1.162 1.709 0.908 1.111 1.518 0.836
Explanations: SH – shell height (mm); SL – shell length (mm); SW – shell width (mm); HL – hinge length (mm); Hpn – height of
nacreous part (mm); Lpn – width of nacreous part (mm); WW – wet weight (g).
Total SH average values of the examined samples
ranged between 33.57 ± 10.13 mm and 63.50 ± 16.09
mm. The majority of the analyzed P. radiata sub-
populations were dominated by large individuals whose
size were equal or exceeded 42 mm (Fig. 3). In con-
trast, the samples of Tunis north lagoon (N2) and La
Marsa (N3) showed lower average values of total height.
They were distinguished by specimens whose size did
not exceed 42 mm (Fig. 3). It is worth noting that the
maximum size (100.51 mm) recorded during the present
study was recorded in N1 (Bizerta lagoon). Although
normality assumption of shell height (SH) data of the
whole sample was satisfied (Shapiro-Wilk test = 0.956;
P < 0.001), some samples did not show a normal distri-
Morphometry of Pinctada radiata along the Tunisian coasts
Fig. 4. Comparison of relative growth of the different measurements taken on Pinctada radiata individuals from six populations
considering the shell height (SH) as a variable reference. For explanations see Table 1.
bution, e.g., N1 and E1. In addition, a non-parametric
procedure using Kruskal-Wallis test showed a signifi-
cant difference between the examined samples (H =
151.364; P < 0.001).
The parameters of allometric lines among ln(SL),
ln(SW), ln(Hpn), ln(Lpn), ln(HL), ln(WW), and
ln(SH), estimated at 95% confidence intervals, are
shown in Table 1. The relationships between these vari-
ables, except for ln(WW) and ln(SH), exhibited close
linear regressions, with exponent (b) 0.093–1.095 for
ln(SL) vs ln(SH), 0.106–2.624 for ln(SW) vs ln(SH),
0.023–0.264 for ln(Hpn) vs ln(SH), 0.220–0.347 for
ln(Lpn) vs ln(SH) and 0.630–1.864for ln(HL) vs ln(SH).
Exponents in allometric relationships for ln(WW) vs
ln(SH) were comprised between 4.526 (N3) and 11.60
(E1) (Table 1). Morphological comparisons of relation-
ships between P. radiata samples are presented in Ta-
ble 1 and Figure 4.
With the exception of the morphometric relation-
ship ln(HL)–ln(SH) within N3 (La Marsa), all tested
variables were highly correlated (high R2values) for
all examined samples (Table 1). Significant differences
in the tested relationships between samples were found
with analysis of covariance (ANCOVA, P < 0.0001),
although some overlaps in the plot area were observed
(Table 2). It is worth noting that for the majority of
tested relationships, the regression line of the sample
N1 was obtained separated from those of other sam-
ples. These latter showed always overlaps in the plot
area. The larger overlaps in the plot area was especially
observed, between N2, N3, E1, E2 and E3, with the
relationships ln(SL)–ln(SH), ln(SW)–ln(SH), ln(Hpn)–
ln(SH), ln(Lpn)–ln(SH), and ln(WW)–ln(SH) (Fig. 4).
Factorial Discriminant Analysis (FDA) done based
on the same data gave a better discrimination (Fig. 5).
The first two axes explained 64% of the variability in
the data (38% for axis 1 and 26% for axis 2). In spite
of the overlap observed between the samples, it is pos-
sible to see that the sample E1 tend to be separated
from the rest of samples. High overlap was observed be-
S. Tlig-Zouari et al.
Table 2. Results of statistical analysis on log linear relationships of morphological characters between Pinctada radiata populations by
Metric relationships dfSum of squaresAverage squares
Pr > F
For explanations see Table 1.
Fig. 5. Scatterplot of the first two FDA (Factorial Discriminant Analysis) axes of the log-transformed data of the six Pinctada radiata
tween the samples E2 and E3. N2 and N3 individuals
were grouped separately in both sides of axis 1. In ad-
dition, FDA separated the six samples and a Student–
Neuman–Keuls multiple-range test separated four ho-
mogenous groups: E1, N2, N3 and E2+E3+N1. A bet-
ter separation was obtained with the Ascending Hierar-
chic Classification which showed two clusters. The first
was represented by the Northern samples (N1, N2 and
N3) with a separation of the population N1. As for the
second cluster, it was formed by the Eastern samples
(E1, E2 and E3) and it consisted of two sub-clusters,
one represented by E1 and another by E2 + E3 (Fig. 6).
The size structure of the six P. radiata samples showed
Fig. 6. Ascending Hierarchic Classification of the six Pinctada
Morphometry of Pinctada radiata along the Tunisian coasts
that with the exception of the two sub-populations of
Tunis Gulf (N2 & N3) characterized by the dominance
of small specimens (Ht ≤ 42 mm), the other samples
were dominated by large pearl oysters (Ht ≥ 42 mm)
(Fig. 3). Not similarly to the latter result, the study
carried out about P. radiata sub-populations in the
Archipelago of Kerkennah (Gulf of Gabes, Southern
coast of Tunisia) showed that the majority of individu-
als had a size lower than 42 mm with individuals rang-
ing between 1 and 74 mm (Tlig-Zouari 1993). The lat-
ter author explained this fact by a continuous recruit-
ment of the species in Kerkennah islands which is due
to a continuous reproduction activity along the year
(Tlig-Zouari & Zaouali 1994). Size distribution analy-
sis also showed that the majority of P. radiata samples
were dominated by 2 or more size groups. Differences
in size distribution might be due to various environ-
mental factors, such as temporally and spatially vari-
able predation or exploitation or adverse environmental
episodes. Otherwise, similar observations were found in
the Archipelago of Kerkennah where P. radiata popu-
lations were reported to be dominated by four cohorts
during most of the year; such an observation was ex-
plained by an irregular seasonal growth and continuous
reproduction activity and recrutement of the species
(Tlig-Zouari & Zaouali 1994, 1995).
It is worth noting that the maximum size (100.5
mm), recorded in Bizerta lagoon (N1) is higher than
that observed by Seurat (1929) (85 mm) in Gabes Gulf
(south of Tunisia) and Tlig-Zouari (1993) in Kerkennah
islands (74 mm). This size is larger than that recorded
by Yassien (1998) and Yassien et al. (2000), respec-
tively, in the Red Sea (93.2 mm) and Eastern Mediter-
ranean (64 mm).
The comparison of relative growth features of P.
radiata showed differences between the studied samples.
Larger differences were observed in the relationships
ln(SW)–ln(SH) (Fig. 4). This result indicated that SW,
among the six characters examined (SH, HL, SL, SW,
Hpn, Lpn), greatly differs between samples, because
larger overlaps in the plot area of the log-linear relation-
ships were observed by other combinations of characters
(ln(SL)–ln(SH), ln(Hpn)–ln(SH) and ln(Lpn)–ln(SH)).
High correlation was also found between SW and SH
within the sub-populations of Kerkennah Islands (Tlig-
Zouari & Zaouali 1998). The latter authors also men-
tioned significant differences in this relationship be-
tween the examined samples. Other large differences
were observed with the relationship ln(SH)–ln(WW).
These differences could be caused by WW, which dif-
fered from one sample to another due to the variabil-
ity of environmental conditions and availability of food
resources. This study also revealed a variable relative
growth of SW between the different prospected locali-
ties and indicated that the pearl oysters of Hammamet
harbour (E1) and Stah Jaber (E3) have the widest SW
than those recorded in the other sites. Similar variabil-
ity in length-weight relationships was previously ob-
served in the south Tunisian coastline by Tlig-Zouari
(1993) and Tlig-Zouari & Zaouali (1994) who explained
that such variability could be in relation with individu-
als’ size (SH) and the availability of food resources. The
latter authors also explained that within large-sized in-
dividuals, the growth of SH is highly reduced but their
shells became wider.
The comparison of the relationship SW-SH inves-
tigated during this study with that of other species be-
longing to the same genus from different areas showed
that P. radiata in Monastir (E2) has a higher SW in-
crease against SH than those of P. fucata (Gould, 1850),
P. margaritifera (L., 1758) and P. sugillata (Reeve,
1857) in Taiwan (Hwang et al. 2007), Korea (Shimizu
1999) and Japan (Wada 1984). Furthermore, it is worth
noting that the variable SW was also considered as the
most important morphological character in the aqua-
culture of other pearl oyster species; e.g., P. fucata, P.
margaritifera and P. sugillata since it greatly affects the
number and size of the inserted pearl nuclei (Hwang et
al. 2007). These differences could be in relation to en-
vironmental and therefore nutritional factors or to ge-
netic ones as was reported for P. fucata populations in
Japan (Wada 1984). However, because it has a thicker
SW, the species P. radiata has great potential for use
as a supplementary edible resource and also as a nacre
Morphometric differences in relation to genetic fac-
tors have been reported with P. radiata (mentioned as
P. imbricata and P. albina) in Australia (Colgan & Pon-
der 2002). Otherwise, the study of P. radiata (men-
tioned as P. imbricata) populations of Japan (Wada
1975, 1982) and Bahrain (Beaumont & Khamdan 1991)
showed the existence of morphometric variations. The
latter authors have not excluded the hypothesis that
these morphometric differences may be also related to
environmental conditions, in particular the exposure to
wave action, salinity and temperature. Moreover, the
temperature also affects the size of pearl oysters. In
the Gulf of Mexico, for example, the size of oysters de-
creases at low latitudes because the elevated tempera-
ture reduces the allocation of energy to growth somatic
(Klinck et al. 1992). Similar relative growth differences
have been mentioned among the stocks of P. margari-
tifera in French Polynesia (Pouvreau & Prasil 2001).
According to these authors, such relative growth dif-
ferences are probably due to temperature, the renewal
rate of water and food availability around the bivalves.
The four sub-population assemblages, obtained
with FDA and CHA, almost matched with geographi-
cal separation of the six samples. This could be in re-
lation with environmental conditions which differ from
one sector to another. In fact, N1 is localized in Bizerta
lagoon, N2 + N3 in Tunis Gulf, E1 in Hammamet Gulf
and E2 + E3 in Monastir bay (Fig. 1).
Summarising, the present study described the rel-
ative growth of the alien species P. radiata in North-
ern and Eastern Tunisian coastline. Except for La
Marsa sample (N3), all populations showed high cor-
relations between the various combined variables, with
some variability from one station to another. The
most significant differences between samples were es-
S. Tlig-Zouari et al.
pecially obtained with the relationships ln(SW)–ln(SH)
and ln(WW)–ln(SH). Multivariate analyses (FDA and
CHA) enabled to classify the samples in four sub-
groups. Such differences might be due to the difference
in geographic locations. Further genetic investigations
are necessary to prove wether morphometric differences
of Tunisian P. radiata sub-populations are related to
The authors are grateful to the referees who helped to im-
prove the quality of the manuscript.
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Received April 28, 2009
Accepted August 28, 2009