Spatial and seasonal distribution of ascidians in a semi-enclosed basin of the Mediterranean Sea

A total of 25 species of ascidians were collected in the Mar Piccolo of Taranto, a semi-enclosed Mediterranean basin. Three are non-indigenous for the Mediterranean Sea: Microcosmus squamiger, Polyandrocarpa zorritensis and Distaplia bermudensis. The substrate features, season and depth affect the distribution of ascidians in the study area. Some species, such as Pyura dura and Pyura microcosmus, were found only on artificial substrates, while Ascidiella aspersa was almost exclusively recovered on natural bottoms. Seasonal variation in the ascidian distribution and abundance seems to be due mainly to their biological cycles, larval recruitment and adaptation. During the autumn and winter the most abundant species were Clavelina phlegraea and Ciona intestinalis, while A. aspersa was particularly abundant during spring. Depth and more directly light intensity play an important role in ascidian distribution. In the upper few metres the shallow-water species Polyandrocarpa zorritensis was abundant due to its photopositive larvae. Even though the distribution and abundance changed significantly between substrates, seasons and depths, the most abundant species in the Mar Piccolo of Taranto were Clavelina phlegraea, Ciona intestinalis and Styela plicata all of which are able to tolerate the variations in environmental conditions, low rate of water renewal and continuous silting of this semi-enclosed sea. Assuming the role that the above mentioned species have as marine pollution indicators and the abundance recorded for some of them, a high degree of environmental stress can be confirmed for the Mar Piccolo of Taranto. A comparative list of the ascidians recorded in this and previous studies is also reported