INTRODUCTION
Barnacles of the genus Chthamalus are a major worldwide component of the rocky intertidal zone of tropical and sub-tropical shores, with few species penetrating into temperate latitudes. The star barnacle,Chthamalus stellatus Poli, is a species with wide geographical range, covering the Mediterranean Sea, the North-Eastern Atlantic coasts, and the offshore Eastern Atlantic islands: Madeira, Azores, and Cape Verde (Southward, 1976; Stubbings, 1967). Recently it was demonstrated that the Cape Verde Islands population is an independent Evolutionary Significant Unit (ESU), a sister clade of C. stellatus (Tikochinski, Motro, Simon-Blecher, & Achituv, 2020).C. stellatus is absent from the North-Western Africa Atlantic coast, where it is replaced by another species of Chthamalus ,C. montagui Southward, which is widely distributed in the Mediterranean. Burrows, Hawkins, & Southward (1992; 1999) suggested that differences in the distribution of these two species and especially the absence of C. montagui from the Atlantic islands are related to the lifespan of their pelagic stage. The larvae of C. stellatus are larger than those of C. montagui, they live longer and can disperse further offshore. This strategy appears to allowC. stellatus to maintain populations on offshore islands. However, C. stellatus could not spread further south to the North-West African sores and the neighboring islands, presumably because of incompatible salinity and temperature conditions (Bhatnagar & Crisp, 1965). Several molecular studies have aimed to resolve the population structure of Chthamalids in the Mediterranean Sea and the oceanographic forces leading to this structure. Pannacciulli, Bishop, & Hawkins (1997) showed that in both species there is a separation between the Atlantic and Mediterranean populations, with C. montaguiexhibiting a greater separation than that of C. stellatus . Since the Mediterranean Chthamalids that are located close to the Strait of Gibraltar resembled the Atlantic ones, they hypothesized that the Almeria–Oran Front in the Alboran Sea is a major barrier to larval dispersion and therefore restricts gene flow between the Atlantic and the Mediterranean of the two Chthamalus species. The Almeria–Oran Front is regarded as a major transition zone between the Atlantic and the Western Mediterranean for 58 marine species, includingC. montagui (Patarnello, Volckaert, & Castilho, 2007). Using three molecular markers, Shemesh, Hochon, Simon-Blecher, & Achituv (2009) examined the distribution of three Chthamalid species, C. stellatus , C. montagui and Euraphia depressa , in the Eastern Atlantic, the Mediterranean Sea and the Black Sea. Their wide selection of sampling sites revealed a significant genetic structure among the populations of these Chthamalids. However, for C. stellatus , the structure was based only on one marker, EF1.
In our presented study we used two molecular markers to examine the distribution of C. stellatus in the Mediterranean. While the number of collecting sites (14) is slightly smaller than in the abovementioned studies, they adequately represent both the Eastern and the Western basins of the Mediterranean, as well as a few locations in the Eastern Atlantic. Sample sizes from each location, nevertheless, are quite large, enabling us to perform population genetic analyses and to construct dendrograms which are based on population samples, and not on just a single or a very few individuals from each location. Thus, we get a more accurate and more reliable picture of the population structure than that depicted by previous studies, which used only a single or a very few individuals to represent an entire local population.
Due to its life cycle and possible mode of settlement and distribution, we suggest that Chthamalus stellatus is a suitable marine animal for studying how geological events and hydrographic conditions shape the fauna in the Mediterranean Sea.