Towards a knowledge of marine boundaries using ascidians as indicators: characterising transition zones for species distribution along Atlantic-Mediterranean shores

Two methodological approaches are compared with regard to their usefulness for providing explanations of observed faunal differences along Atlantic-Mediterranean coasts. A set of distribution data of ascidians containing 519 species and 80 genera was used for this purpose. Similarity methods were applied to establish the faunal affinities between distribution areas, and the measurement of β diversity and ecological distances along unidimensional gradients by non-lineal rescaling (using Detrended Correspondence Analysis) was undertaken for determining the steepness of ecotones existing along spatial gradients; these were analysed to detect the principal faunal changes, as well as their concordance with the commonly accepted limits for the classical biogeographical areas. There are fewer widely distributed species of shallow-water ascidians than in the Indo-Pacific region. Marked environmental changes within the Atlantic Ocean translate into significant differences in the composition of ascidian assemblages both in a latitudinal range and between the western and eastern sides. Few species show an amphi-Adantic range and most of these correspond with typically cosmopolitan species which are in general associated with shipping traffic or other forms of man-made transport. Three main endemism areas can be distinguished for Adantic-Mediterranean ascidian fauna involving genera and species: Caribbean, Mediterranean and south-west African. The rest of the shallow-water regions act as transitional or buffer zones where the genetic flows are generally maintained, although an increase in the number of certain species is found in some areas. This transitional role is mosdy found in the European Adantic coasts from the British Isles towards the Senegalian subregion, where gradual changes in ascidian populations occur: cold-water and tropical species at their southern and northern boundaries are substituted by temperate and subtropical species at the entrance of the Mediterranean.