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

Abstract

Two methodological approaches are compared with regard to their usellness for providing explanations of observed faunal differences along Atlantic-Mediterranean coasts. A set of distribution data of ascidians containing 5 19 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 p 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-Atlantic 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 Atlantic-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 mostly found in the European Atlantic 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.Q 1998 The Linnean Society of London ADDITIONAL KEY WORDS:-marine frontiersecotonesp diversitydetrended correspondence analysisshallow-water tunicates.

Is the north side of the Strait of Gibraltar more diverse than the south side? A case study using the intertidal peracarids (Crustacea: Malacostraca) associated to the seaweedCorallina elongata

“…Hydrozoans and sponges have great affinity with the Mauritanian regions (Carballo et al, 1997;Medel & López-González, 1998), probably due to the entrance of tropical and subtropical species into the Mediterranean through the Strait of Gibraltar during the Pliocene. The groups which present more biogeographical affinities with the Mediterranean region, in a similar manner to peracaridean crustaceans, are the Ascidians and Anthozoans (López-González, 1993;Naranjo, 1995;Naranjo et al, 1998). This trend has been also observed for opisthobranch molluscs (Cervera et al, 2004), which are characterized by planctotrophic-lecitotrophic larvae, a lack of Mediterranean endemisms and scarce number of amphiatlantic species (García-Gómez, 2002).…”

“…Data taken fromLópez-González, 1993;López de la Cuadra & García-Gómez, 1994;Carballo et al, 1997;Medel & López-González, 1998;Naranjo et al, 1998. gateways between the Atlantic Ocean and the Mediterranean Sea. Data taken fromLópez-González, 1993;López de la Cuadra & García-Gómez, 1994;Carballo et al, 1997;Medel & López-González, 1998;Naranjo et al, 1998. gateways between the Atlantic Ocean and the Mediterranean Sea.…”

A realignment of marine biogeographic provinces with particular reference to fish distributions

“…species of reef fishes with some 30% endemism (Floeter et al, 2008); opisthobranch gastropod endemism is about 36% (García & Bertsch, 2009) and tunicate endemism about 31% (Naranjo et al, 1998). To the south, the warm-temperate Southwest Africa Province is now called the Benguela Province (BP).…”

Environmental factors affecting reproduction and development in ascidians and other protochordates

“…The southern hemisphere Corella eumyota Traustedt, 1882 recently appeared in two harbors in Brittany, France (Lambert 2004), and is likely to spread farther in the northern hemisphere. Naranjo et al (1998) compiled distribution data for 519 Atlantic and Mediterranean ascidian species; they found very few common to both sides of the Atlantic other than those known to be associated with anthropogenic transport. Half the species have very limited distribution, and correlated with that, the highest diversity was found for the Caribbean and Mediterranean.…”