Depth and temporal distribution of vagile fauna associated with Posidonia oceanica meadows in Cap Zebib, north-eastern Tunisian coastline

Abstract

A study on plant features and motile fauna was carried out within a Posidonia oceanica (L.) Delile bed in Cap Zebib (north-eastern Tunisian coastline). Fauna was sampled in two stations located at 3 and 12 m depth. A total of 154 species represented by 5592 individuals were collected. Taxa with the highest number of species included Crustacea (22), Mollusca (47) and Polychaeta (46). Among Crustacea, Leptocheirus pilosus, Elasmopus rapax and Lysianassa longicornis were recorded for the first time in Tunisian P. oceanica meadows. The species richness was found to be generally higher at the shallow station. Multivariate analyses showed a pronounced temporal variation in collected populations’ structure. Moreover, the distribution of some groups, in particular molluscs and crustaceans, appeared to be mainly influenced by ‘depth’ factor showing a clear zonation pattern for some species. It was also showed with Spearman test that the faunal features of the two latter taxa (molluscs and crustaceans) are related with the shoot density and Leaf Area Index. However, no correlation was found between the plant parameters and polychaetes community characteristics. In addition, environmental factors, i.e. temperature and dissolved oxygen were found to be correlated with species richness and Shannon–Wiener diversity for each zoological group. Resume Une etude fut realisee sur les caracteristiques de la vegetation et sur la faune mobile dans un herbier de Posidonia oceanica (L.) Delile, a Cap Zebib, sur la cote nord-est de la Tunisie. La faune a ete prelevee dans deux stations a 3 et 12 m de profondeur. Au total 5592 individus appartenant a 154 especesont ete collectes. Les taxons qui comptent le plus grand nombre d’especes incluent les Crustaces (22), les mollusques (47) et les polychetes (46). Parmi les crustaces, Leptocheirus pilosus, Elasmopus rapax et Lysianassa longicornis sont enregistres pour la premiere fois dans les prairies tunisiennes de P. oceanica. La richesse specifique etait generalement plus elevee dans la station la moins profonde. Des analyses multivariees ont montre une variation temporelle prononcee de la structure des populations collectees. De plus, la distribution de certains groupes, en particulier des mollusques et des crustaces, semble etre influencee par le facteur ‘profondeur’ et montre un schema de zonage clair chez certaines especes. Le test de Spearman a montre que les caracteristiques faunistiques des deux derniers taxons (mollusques et crustaces) sont correlees avec la densite de l‘herbier et l’Indice foliaire. Cependant, aucune correlation entre les parametres de l‘herbier et les caracteristiques de la communaute des polychetes n’a ete mise en evidence. La richesse specifique et l’indice de Shannon-Wiener, pour chacun des groupes zoologiques, sont correles a la temperature et a l‘oxygene dissous du milieu.