Seasonal and diel changes in the structure of a crustacean decapod assemblage associated to a shallow Cymodocea nodosa meadow in northern Tunisia (Mediterranean Sea). An overview of Mediterranean decapod taxocoenoses

A study on the decapod assemblage inhabiting a shallow meadow of Cymodocea nodosa in the bay of La Goulette (Tunisia) was carried out from February 2009 to February 2010. Monthly samples (with replicates) were taken in the morning and at night, with a small Agassiz trawl. In total, 11699 specimens belonging to 41 species were caught. Significant day-night and seasonal changes are mainly related to movements (feeding) and recruitments. Higher abundance and richness were reported at night. Analyses did not reveal significant relationships between plant phenology, abundance and richness or between total abundance and temperature, but significant correlations with a time lag of 3 - 4 months between these factors exist, which could be related to hatching and larval development period. The nocturnal and diurnal values in the diversity and equitability indexes are quite similar throughout the year (peaks in spring - early summer, minimum in autumn - early winter). This pattern is due mainly to the strong dominance of a few species, with maximum abundances during their recruitment events, whose seasonal and day-night changes have been determined. Besides, an overview of the decapod assemblage associated to this seagrass has been carried out for the Mediterranean Sea. The assemblage can be divided into two groups (linked to the leaves and to the sediment). The differences in species composition among different Mediterranean areas must be related to different sampling methodologies, feeding and reproductive strategies, but also to the layout and influence of the surrounding habitats.