DISCUSSION
Our study presents for the first time a detailed and an accurate picture
of Chthamalus stellatus population distribution in the
Mediterranean Sea. Previous studies (Crisp, Southward, & Southward,
1982; Pannacciulli et al.,1997; Shemesh et al., 2009) used only a single
or a very few individuals to represent each local population. Using 63
SNPs in 14 populations, each consisting of 15–28 specimens, enabled us
to get a more reliable picture of this barnacle’s populations in the
Mediterranean as well as the Eastern Atlantic.
The various analyses performed in our study clearly demonstrate four
distinct clusters (see fig. 2-5). The Eastern Atlantic (EA) cluster,
consists of the western peripheral populations of C. stellatus ,
namely, Biarritz, the Canaries and Madeira. This cluster was
long-established in previous studies that show the separation between
the Atlantic and the Mediterranean populations of C. stellatus(Crisp et al., 1982; Pannacciulli et al.,1997; Shemesh et al., 2009).
The Western Mediterranean (WM) cluster, which consists of Bizerte,
Málaga and Melilla, is a sister cluster to the EA cluster. Two of the
cluster’s locations, Málaga and Melilla, are geographically adjacent to
the Strait of Gibraltar, and therefore naturally influenced by the
surface current entering from the Atlantic and flowing along the
Northern Africa coast (Millot & Taupier-Letage, 2005). The third
location (Bizerte) is geographically quite distant, closer to the
Mid-Mediterranean locations studied here, but probably still influenced
by the flow along the coast. The resemblance between the Atlantic
populations and a distant Mediterranean population of barnacles is
reported here for the first time. The Eastern Mediterranean (EM)
cluster, that consists of Bodrum, Dubrovnik, Fažana, Larnaca and
Rethymno, is well-defined in all our different analysis approaches. The
Mid-Mediterranean (MM) cluster consists of Bastia, Birżebbuġa and
Pantelleria. Bastia, at the northwestern coast of Corsica, belongs
geographically to the Western Mediterranean basin. The other two MM
locations are also close to the Western Mediterranean. Nevertheless, the
MM cluster emerged as a sister group to the EM cluster in all our
analysis approaches. While this is not surprising for Birżebbuġa and
Pantelleria, the Bastia population is an integral part of this cluster.
This finding is supported by a previous C. stellatus study,
clustering Bastia, Genoa and nearby locations with MM populations and
not with the Atlantic cluster (Pannacciulli et al., 1997). The only
exception to this cluster analysis is presented in two of the 10 trees
(Fig. 6) in which the population of Pantelleria is clustered with the
WM.
Establishment of new barnacle populations, as well as other sessile
animals, mainly occurs by current-assisted larval distribution
(Johannesson, 1988). The pelagic stage of C. stellatus is about
22 days, allowing for extensive connectivity between populations
(Pannacciulli, Manetti, & Maltagliati, 2009). The well-documented
surface current, coming from the Atlantic Ocean and entering the
Mediterranean through the Strait of Gibraltar can easily bring barnacle
larvae to the shores of Málaga and Melilla. Previous studies have
included these locations as part of the Atlantic region population,
separated from the rest of the Mediterranean by the Almeria-Oran Front
(Pannacciulli et al., 1997). But, according to our results, it appears
that the Almeria–Oran Front is not impermeable to the propagules ofC. stellatus which is reflected in the resemblance of the Western
Mediterranean populations of both sides of the front. Patarentello et
al. (2007) reviewed over 20 population studies of 58 different marine
species across the Atlantic-Mediterranean range, aiming to comprehend
phylogeographical patterns, including potential barriers in the
Mediterranean Sea. The patterns obtained from their data were very
diversified, even between closely related species. The three major
patterns were (i) full congruence between Atlantic and Mediterranean
clades; (ii) distinct Atlantic and Mediterranean clades, where the
Almeria–Oran front serves as the Atlantic–Mediterranean
phylogeographical break; (iii) an Eastern Mediterranean clade that is
distinct from the Western Mediterranean and Atlantic Ocean clade where
the Sicily Strait and the Messinian Strait serve as a phylogeographical
boundary (see also Villamor, Costantini, & Abbiti, 2014). Our results
further expand the second and third models of Patarentello et al. (2007)
and divide the populations to four clusters. In contrast to the previous
above-mentioned analyses, that are based on a limited number of markers
and small samples of specimens representing each population, our results
that are based on a larger set of informative markers and a large
population sample, look more reliable.
The Atlantic current, entering through the Strait of Gibraltar, extends
east along the shores of Northern Africa (Hamad, Millot, &
Taupier-Letage, 2006; Millot & Taupier-Letage, 2005; Poulain et al.,
2013) (Fig. 8) elucidates the resemblance between Bizerte and the other
WM cluster populations and the influence by the sister cluster of the EA
populations. This Atlantic current system is likely to assist larval
distribution entering the Mediterranean. It appears that the influence
of this current does not spread further east and we can only find traces
of the connection between the Bizerte and the Pantelleria populations
(Fig. 6), which can also be explained by local gyres. The EM cluster is
well defined and spans over a large part of the Mediterranean, from the
eastern shores through the Aegean Sea and north into the Adriatic Sea.
This part of the Mediterranean is influenced by the Asia Minor current
as well as some cyclonic and anti-cyclonic gyres (Pinardi, Arneri,
Crise, Ravaioli, & Zavatarelli, 2006) and we therefore witness
increased connectivity between the different populations. The northern
part of the Western Mediterranean is influenced by currents from the
area of Sicily (Pinardi et al., 2006), and therefore it is not clear why
the population resembles the MM populations and not a WM one like
Bizerte. The mixing of water by the mesoscale gyres in the Tyrrhenian
sea (Fig. 8) may contribute to the resemblance between the Bastia
population and that of the other MM populations. Pantelleria is very
close to the border between the well-defined branches of the EA/WM and
the MM/EM, and indeed found to be genetically close to both. Although
belonging to the WM cluster in most analysis, it is part of the WM
cluster in two of the population trees, right next to its geographically
neighboring population of Bizerte.
We may also speculate that the separation between the Western and
Eastern Mediterranean populations of C. stellatus is a result of
the geological history of the Mediterranean. One of the most conspicuous
events that shaped the composition of flora and fauna of the
Mediterranean is the Messinian Salinity Crisis (MSC) that started at the
end of the Miocene, about 6 million years ago
(Krijgsman,
Hilgen, Raffi, Sierro, & Wilson, 1999) when the connection between the
Mediterranean and the Atlantic was cut. The water balance of the
Mediterranean was, and still is, deficient and is compensated by the
Atlantic inflow through the Strait of Gibraltar. The Messinian Salinity
Crisis ended in the Zanclean flood, occurred 5.33 million years ago when
the Strait of Gibraltar opened and have refilled the
Mediterranean
Sea. During the MSC, the water level and salinity of the Mediterranean
fluctuated and it was dried up and refilled repeatedly during the few
million years of the Messinian stage, leaving behind lakes of different
salinities (Hsu, Ryan, & Cita, 1973). Some of these lakes might be a
refuge, inhabited by resistant organism like intertidal barnacles that
naturally withstand fluctuating temperatures and salinities. We may
propose that eastern and middle populations of C.stellatus are a relic of the MSC lakes while the WM populations
resemble the Atlantic “invaders” arriving like many other organisms in
the Zanclean flood.
In order to better understand how these populations were shaped, the
selective pressures of the habitats, like salinity and temperature, must
be considered. When compared to other species of barnacles (Bhatnagar &
Crisp, 1965), C. stellatus was found to be better adapted to
higher temperatures while less inhabiting lower salinity niches. Low
salinity has been correlated with reduced number of eggs per brood inC. stellatus (Barnes and Barnes, 1965). These adaptive advantages
can contribute to a bigger, stable and more polymorphic population in
the higher salinity and temperature conditions of the EM. Indeed, the EM
populations have more alleles per position and their expected
heterozygosity is significantly higher than all other populations (Figs.
7a–b). The EA populations, on the other hand, have significantly
smaller number of alleles per position, lower expected heterozygosity
and lower percent of polymorphic positions (Figs. 7a–c), presumably
reflecting the adaptive difficulties of C. stellatus in the lower
salinity and temperature of the East Atlantic Ocean. The Atlantic
current influence on the Mediterranean is also evident in the low
salinity region stretching from the Strait of Gibraltar along the
Northern Africa shores to Bizerte (Fig. 8). As expected from this, the
WM populations do have less alleles per position and their expected
heterozygosity is significantly lower than the EM populations. The
environmental conditions may also explain the establishment of EM
populations of C. stellatus in the western part of the
Mediterranean. Mean surface salinity and temperature of the Northern
Tyrrhenian and the Ligurian Seas are the highest in the Western
Mediterranean Basin (UNEP/MAP 2012, p 23). These favorable conditions
could allow for the establishment of a polymorphic population like the
one in Bastia.
Patarentello et al. (2007) have concluded that genetic diversity does
not necessarily decrease in a direction either from the Atlantic Ocean
to the Mediterranean or even to the Adriatic Sea. The higher genetic
variability of the EM population can be partially explained by the
central-marginal hypothesis, also termed the Carson hypothesis (Carson,
1959; Eckert, Samis, & Lougheed, 2008; Sagarin & Gaines, 2002). The
hypothesis claims that range margins exhibit less genetic diversity and
greater inter‐population genetic differentiation compared to range
cores. Since the Eastern Mediterranean shores are not a naturally
occurring edge of the population this effect can only be seen in the
Atlantic where C. stellatus does not spread further south to the
Western Africa shore and even the Cape Verde Chthamalus was shown
to be a different species (Tikochinski et al., 2020).
Yet another explanation may be the advantage of variability within the
eastern basin populations in overcoming the higher temperature changes
as well as other rapid condition changes and processes typical to this
part of the Mediterranean, especially since the opening of the Suez
Canal.
Our study presents a unique opportunity to study processes of population
settlements in sessile animals, the influence of oceanographic
conditions and processes including selection and genetic variation. In
order to better understand and solidify some of our speculations, more
studies of this range of geographic distribution, population size and
genetic polymorphism are needed.