Page 23 - Bioconstructions_2018

Mediterranean Bioconstructions Along the Italian Coast 83

2.2.8 Serpulid Reefs, Including Biostalactites
Serpulidae are a large family of polychaetes that produce a calcareous tube in
which they live (Bianchi, 1981). Tubes typically encrust any hard substrate
and may be considered “secondary framework builders”, filling crevices and
cementing rubble in coralligenous reefs and other marine bioconstructions
(Bianchi et al., 1995). The scissiparous species Salmacina dysteri (Huxley,
1855) may form large “pseudocolonies”, which are nevertheless very fragile
and do not exhibit any significant constructional capacity (Bianchi and
Morri, 2000). In contrast, some gregarious species can grow erect attached
to each other, thus acting as “primary frame builders” and forming huge
aggregates (Aliani et al., 1996). The individual tubes are small (rarely longer
than 10cm or wider than 1cm), but their aggregations can form reef-like
structures more than 1m thick that may cover tens of square metres
(Bianchi et al., 1995). Serpulid reefs are well known in the geological past
(Fagerstrom, 1987), and mass occurrences of recent serpulid species have
been reported from many sites (Ten Hove, 1979). In Italy, major serpulid
reefs, formed by Ficopomatus enigmaticus (Fauvel, 1923) or Hydroides dianthus
(Verrill, 1873), have been described from brackish lagoons (Bianchi and
Morri, 1996, 2001), but these will not be further discussed here.
In fully marine waters, serpulids may develop conspicuous assemblages
in submarine caves, even encrusting preexisting speleothemes, as first
described for the Mediterranean by Ten Hove and Van Den Hurk (1993)
and in Italy by Bianchi and Morri (1994) and Antonioli et al. (2001).
Particular serpulid structures in submarine caves are the so called bio-
stalactites. They develop in completely dark parts based on single (e.g. Cape
of Otranto) or few (e.g. Plemmirio Islands) (Fig. 7C) serpulid species,
whose aggregations become substrate for smaller invertebrates and bacteria
(Belmonte et al., 2009; Guido et al., 2012). Biostalactites grow far from the
entrance on rocky walls where generally Serpulidae dominate (Bussotti
et al., 2006) and where other benthic species are less competitive. Bio-
stalactites can protrude a few cm from the basal substrate (Plemmirio) but
also up to 2m (Cape of Otranto, Fig. 7A) and they can be also oblique or
horizontal. Freshwater inflows (even intermittent) seem to favour their
formation. The engineering species always have been referred to the genus
Protula, but recent genetic analyses of the Otranto species showed the
existence of a still unnamed species. The growth of Otranto biostalactites
started about 6000 years ago, corresponding with the sea level during its
postglacial rise (Belmonte et al., 2009, 2016). The outer surfaces of the
biostalactites are encrusted by living and dead fauna of several invertebrate