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(Ostrea, Crassostrea) and arks (Arca, Scapharca) are representatives of the
monophyletic species-rich subclass Pteriomorphia (Matsumoto, 2003).

The taxonomic resolution of this subclass is puzzling (Purchon, 1987; Waller,
1990; Morton, 1996; Sa1vini-Plawen and Steiner, 1996; Cope, 1996, 1997), and led
some authors to hypothesize a polyphyly (Carter, 1990; Starobogatov, 1992; Giribet and
Distel, 2003), probably due to a rapid radiation event in its early evolution (Campbell,
2000; Steiner and Hammer, 2000; Matsumoto, 2003).

One of the best known species included within this subclass is the Mediterranean
endemie fan mussel, Pinna nobilis Linnaeus, 1758. It belongs to the superfamily
Pinnoidea which shows its closer genetic relationship (on mitochondrial bases) with the
Pteriomorphia superfamilies Pterioidea and Osteoidea (Matsumoto, 2003). It occurs at
depths between 0.5 and 60 m, in soft and sandy bottoms mostly characterized by
seagrass meadows (Zavodnik, 1967; Zavodnik et al. , 1991; Katsanevakis, 2005);
individuals attain lengths whose maximum values of l m (Zavodnik et al., 1991) led to
consider the species as one of the largest among bivalves.

In the western Mediterranean, P. nobilis reproduces in the summer period, from
June to August (De Gaulejac, 1995). Embryos diffuse in the water colurnn as veliger
larvae stadium (Butler et al., 1993) before settling in the sediment and anchoring by
means ofbyssus produced by juveniles. A very little is known about the length oflarval
life cycle of P. nobilis which can be considered equivalent to a period of 5- l Odays (De
Gaulejac and Vicente, 1990). Anyway, according to what found for bivalves' larvae by
Gosling (2003), it is supposed that P. nobilis veliger exhibits daily migration across the
water colurnn tending to pass daylight hours in less illuminated and deeper waters and
night time on superficial ones (Peharda and Vilibié, 2008). Taking into account such
trend P. nobilis larvae could diffuse by means oftidal currents especially during the

50

Gìan Luca Dedola: ANALISI DELLA VARIABILITÀ GENETICA DI PATELLA FéRRUGINEA, PATEL-LA UL l'SSIPONENSIS (MOLLUSCA: GASTROPODA) E PINNA
NOBILIS (MOLLUSCA: BIVALVIA): lLCONTRJBUTO DEl DATI MOLECOLARl ALLA CONSERVAZIONE DI SPECIE MINACCIATE- Tesi di douorato in Scienze
della natura e delle sue risorse • indiri7.zo BK>logi.a Ambientale, Università degli Studi di Sassarl