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.