Distribution, abundance and population structure of Pinctada radiata (Mollusca: Bivalvia) in southern Tunisian waters (Central Mediterranean)

Abstract

The pearl oyster Pinctada radiata is an alien species introduced to the Mediterranean Sea and recorded in Tunisia several years ago. However, since its record in the Gulf of Gabes, no studies have been carried out about the spread of this invasive mollusc. Thus, the status of this species is still poorly known and there is a knowledge gap about its distribution and abundance. The present work is a contribution to the knowledge of the pearl oyster distribution and densities in southern Tunisian waters, at depths between 0 and 100 metres. The results showed a scattered distribution pattern of the species according to location and depth ranging between 0 and 145 ind.m-2. Pinctada radiata was encountered from the intertidal zone to 40 m depth, with a highest population densities recorded at depth range of 2-20 m. The total stock was estimated to be 27584.9 ± 11504.5 million individuals. Oyster distribution seems influenced by the substrate type. This high population of pearl oysters was associated with large cover of seagrass Posidonia oceanica which provides an excellent substratum for attachment. The littoral zone seems not to be the preferred habitat for the proliferation of this immigrant species. Oysters' size increased steadily with depth, ranging from a mean of 37.98 ± 0.40 mm SH at 0-1 m to 60.98 ± 0.68 mm SH at 20-100 m depth range. Size structure analysis showed that deep water population was dominated by large individuals reaching 96 mm SH. Findings of this study suggested that P. radiata is well proliferate in the Gulf of Gabes.