Melilla and Chafarinas Islands (Templado et al. 2004, 2006), where it is common but
with relatively low densities. Astroides calycularis is also widely distributed along
the coasts of Morocco with spectacular occurrences around Cape Tres Forcas
(Bazairi et al. 2013) and in the Al Hoceima National Park (Franzosini and Limam
2004) and Algeria. Its southeastern limit seems to be located in Cap Bon and Zembra
Island (Tunisia), where Boudouresque et al. (1986) pointed out that this coral covers
extensive surfaces of shallow rocky bottoms down to 15 m depth.
Further north, Musco et al. (2016) recorded well-developed populations in the
Sorrento Peninsula and the islands of Ischia and Capri along the coast of Campania;
the Cilento Coast; the area between Calabria and Sicily, including the Straits of
Messina and the Aeolian Islands; the northwestern coast of Sicily, including Ustica
and the Egadi Islands; and the islands in the Strait of Sicily. Some colonies were
found along the coast of Croatia (Grubelić et al. 2004) which suggests that the
geographical range of A. calycularis is expanding northwards.
The strictly coastal habitat of A. calycularis and its preference for shaded enclaves
in the upper infralittoral belt expose this coral to the impact of human activities. This,
coupled with its narrow range of tolerance for temperature, seems to be causing its
decline. On the other hand, its planula larvae show negative buoyancy and demersal
behaviour, crawling along the rocky wall to find a suitable place on which to settle
(Goffredo et al. 2010). Hence, the species’ larvae have low dispersal capabilities,
which leads to a restricted gene flow and connectivity, increasing differentiation
among populations because of isolation by distance (Casado-Amezúa et al. 2012).
All the above has determined that the orange coral is nowadays protected under
Appendix II of the Bern Convention, Annex II of Barcelona Convention and
Appendix II of CITES (with all scleractinian corals). It is also catalogued as
“vulnerable” in the Spanish and Andalusian Catalogues of Endangered Species
(see Sect. 10.5) but considered as of “least concern” in the IUCN Red List.
10.2 Rare Species
The vast majority of species are rare, having a low number of individuals or a
restricted spatial occurrence. A particular species may be rare for several reasons,
namely: small absolute population size, small geographic range, highly specialized
mode of life in conditions that are themselves uncommon, low population density,
incidental occurrence in environments not normally occupied by the species, successful breeding at only a few sites (Vermeij and Grosberg 2018) or simply be an
artefact due to scarce knowledge linked to the availability of the relevant taxonomic
specialists (Bianchi et al. 2014). Species which are represented anywhere by few,
scattered individuals implies good dispersal abilities, while species which are
restricted to a very peculiar habitat and are abundant there, but that habitat in itself
is rare and/or inaccessible, are rather characterized by direct development and low
dispersal potential, and their rarity may be an artefact due to our access limitation
(Gaston 1994). It has long been recognized that populations below a critical density
10 Invertebrates: The Realm of Diversity
365
with relatively low densities. Astroides calycularis is also widely distributed along
the coasts of Morocco with spectacular occurrences around Cape Tres Forcas
(Bazairi et al. 2013) and in the Al Hoceima National Park (Franzosini and Limam
2004) and Algeria. Its southeastern limit seems to be located in Cap Bon and Zembra
Island (Tunisia), where Boudouresque et al. (1986) pointed out that this coral covers
extensive surfaces of shallow rocky bottoms down to 15 m depth.
Further north, Musco et al. (2016) recorded well-developed populations in the
Sorrento Peninsula and the islands of Ischia and Capri along the coast of Campania;
the Cilento Coast; the area between Calabria and Sicily, including the Straits of
Messina and the Aeolian Islands; the northwestern coast of Sicily, including Ustica
and the Egadi Islands; and the islands in the Strait of Sicily. Some colonies were
found along the coast of Croatia (Grubelić et al. 2004) which suggests that the
geographical range of A. calycularis is expanding northwards.
The strictly coastal habitat of A. calycularis and its preference for shaded enclaves
in the upper infralittoral belt expose this coral to the impact of human activities. This,
coupled with its narrow range of tolerance for temperature, seems to be causing its
decline. On the other hand, its planula larvae show negative buoyancy and demersal
behaviour, crawling along the rocky wall to find a suitable place on which to settle
(Goffredo et al. 2010). Hence, the species’ larvae have low dispersal capabilities,
which leads to a restricted gene flow and connectivity, increasing differentiation
among populations because of isolation by distance (Casado-Amezúa et al. 2012).
All the above has determined that the orange coral is nowadays protected under
Appendix II of the Bern Convention, Annex II of Barcelona Convention and
Appendix II of CITES (with all scleractinian corals). It is also catalogued as
“vulnerable” in the Spanish and Andalusian Catalogues of Endangered Species
(see Sect. 10.5) but considered as of “least concern” in the IUCN Red List.
10.2 Rare Species
The vast majority of species are rare, having a low number of individuals or a
restricted spatial occurrence. A particular species may be rare for several reasons,
namely: small absolute population size, small geographic range, highly specialized
mode of life in conditions that are themselves uncommon, low population density,
incidental occurrence in environments not normally occupied by the species, successful breeding at only a few sites (Vermeij and Grosberg 2018) or simply be an
artefact due to scarce knowledge linked to the availability of the relevant taxonomic
specialists (Bianchi et al. 2014). Species which are represented anywhere by few,
scattered individuals implies good dispersal abilities, while species which are
restricted to a very peculiar habitat and are abundant there, but that habitat in itself
is rare and/or inaccessible, are rather characterized by direct development and low
dispersal potential, and their rarity may be an artefact due to our access limitation
(Gaston 1994). It has long been recognized that populations below a critical density
10 Invertebrates: The Realm of Diversity
365