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For this study 100 specimens per population were collected and examined morphologically using standard biometric techniques. Measurements were carried out using precise instruments : electronic digital calliper at 0.01mm precision and electronic scale at 0.01g precision. Five morphological parameters were considered with the object to compare the five sites :
-Total weight (except for site SA1)
-Dorsal side (DS)
The analysis concerns:
1-Comparison of morphological parameters with the use of statistical tests (Kruskal-Wallis test ).
2- Comparison of C and R coefficients according to Hynd (1955)
3-Study of the relative growth parameters (Dagnélie, 1982; Box and Cox,1982) in each population using allometric tests (regression lines)
4-Analysis of morphological parameters’ variation based on the
use of a standard animal (Aloui-Bejaoui &. al., 2003).
9th International Conference on Shellfish RestorationNovember 15–19, 2006 - Charleston, South Carolina, USA
MORPHOLOGICAL COMPARISON BETWEEN GEOGRAPHICALLY DISTANT POPULATIONS OF THE PEARL OYSTER PINCTADA RADIATA INVADING THE EASTERN MEDITERRANEAN SEA
Comparison of morphological parameters with the use of statistical tests and Box and Whisker Plot :
The Kruskal-Wallis Test of the Medians results in significant difference amongst the populations for the Height (Test statistic=162,78, P-value <0,001), the width (Test statistic=164,14, P-value <0,001 ), the thickness (Test statistic=203,98, P-value <0,001), the dorsal side (Test statistic=132,77, P-value <0,001) and the total weight (Test statistic=138,35, P-value <0,001) at the 95% confidence interval .
The Student T-test as well as the Box and Whisker Plot indicates that animals from Saronikos Gulf (SA1) present the:
- higher values for height followed by the ROD , SA2 and LAT populations. KER present the lowest values.
- greater values for width, as it was observed for height, and animals from Tunisia (KER) the lowest.
- higher values for thickness followed by the LAT, ROD and SA2 populations which seem not to differ significantly and at the end again the KER population presents the lowest values.
P. radiata from Tunisia (KER) present significant differences (p<0,001) from all the other sites and have the lowest values for total weight. At the other end, animals from Saronikos Gulf (SA2) (weight measurements from SA1 were not available) have the greater values. SA2 population is significantly different from all other sites except Rodos (ROD).
MATERIAL AND STUDY AREAS
Individuals were collected from 5 different sites. Three prominent established populations of Pinctada radiata have been selected from Greece: the populations of Rodos (ROD) and Saronikos Bay (SA1, SA2), one population from Tunisia Kerkennah islands (KER) and one from Syria, Lattakia area (LAT). Sampling was carried out by diving.
- Convexity in SA1, SA2 and LAT present the higher values. Pinctada radiata in ROD and KER is the less convex.
-The variations of Dorsal Side are not statistically linked to the other morphologic parameters. DS is a parameter highly influenced by the ecological conditions. The variations of this parameter are significant for only a couple of sites: SA1 and KER. The first site presents the more important value of R whereas KER, as for the other parameters, presents the lowest one.This indicates an almost equal increase for the two parameters in the Tunisian population while at the other sites height has a faster growth rate than DS.
The more important total weight of a 50 mm height standard animal is observed in SA2 and LAT and the lowest one in KER. All the results confirm a better relative growth of the populations of Saronikos Gulf (height and width). The total weight in SA2 is more important than in the other populations. The population of KER presents the lowest values. The population of LAT presents a good development in total weight, comparatively to its metric growth. Among the three populations of Greece, ROD is the one that presents the less good growth in terms of height and total weight. This population shows a shape which develops more in width.
The Kruskal-Wallis test tests the null hypothesis that the medians of one parameter within each of the 5 levels of Station are the same. The data from all the levels is first combined and ranked from smallest to largest. The average rank is then computed for the data at each level. Since the P-value is less than 0,05, there is a statistically significant difference amongst the medians at the 95,0% confidence level. The Box-and-Whisker Plot determine which medians are significantly different from which others.
These differences are also obvious on the following graphs:
The equations of the regression lines have permitted to calculate the theoric values of weight after having fixed the value of the metric parameters of the individuals belonging to the different populations (standard animal). This value has been fixed in a way it could be the nearest possible of the means calculated refering to the different populations : H=50 mm
To summarize the results and visualize better the differences between sites we performed a Euclidean clustering method for all measured parameters except Total Wet Weight. The produced dendrogram suggests that the SA1 population is completely different from all the others which form two subgroups, the Greek sites together (SA2 and ROD) and the Syrian and Tunisian (LAT and KER) on the other subgroup.
There is no differences observed according to an east-west transect. Pinctada radiata is adapting to its new environmental conditions in spite of the geographic distance from its native area. Saronikos Gulf (Greece) population have the greater values and animals from Kerkennah Islands (Tunisia) present significant differences from all the other sites and have the lowest values in all parameters measured. It is assumed that environmental parameters are responsible for the observed morphometrical differences and shell morphology of Pinctada radiata has been shown to vary especially with salinity (Al Sayed & al., 1997). In Tunisia, the thermic conditions are very favorable and enable high fecundities of Pinctada radiata (Zouari and Zaouali,1994 ), just like for the other bivalves. This situation leads to different modalities of growth.
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