Good news for the Fan Mussel ?
The Fan Mussel, the bivalve mollusc of the Mediterranean waters
The Fan Mussel or noble pen shell (Pinna nobilis) is a bivalve mollusc which is seen only in the Mediterranean. Despite its great size (50 to 70 cm), this shellfish very often goes unnoticed in the midst of the aquatic grass-beds. Found at depths of between 2 and 10 metres, embedded vertically in the sand, it only allows the upper half of its shell to be seen.
Although it was fairly common until the middle of the 20th century, it suffered a sharp decline due to coastal development and the pollution and use of boats that followed on from that. The species has been protected since 1992 and may not be gathered or intentionally destroyed.
The Fan Mussel’s mortality rate
Since the autumn of 2016, many naturalist observations and scientific studies have brought to light a new mass mortality event affecting this species in the western Mediterranean (one outbreak coming up from Spain and another from Corsica). A priori this mortality has been caused by a unicellular parasite of unknown origin (Haplosporidium pinnae), but the researchers are also talking about a possible micro-bacterial disease. The populations of this protected species have dropped dramatically on our coasts, with survival rates near zero.
Hopes of recovery for the species in the Thau basin
Much more surprising is the very recent colonisation of the Thau basin by the species. Until now, the Fan Mussel was only very occasionally found on the shores, except for the Sète and Marseillan inlets. Recently, volunteer counting carried out by Biotope employees has revealed the presence of a high number of young individuals, of about fifteen centimetres, on the edge of eelgrass beds on the beaches of Mèze, Balaruc and Marseillan Plage (on the lagoon side) at least.
The densities are very high in places (several dozen specimens in 100 linear metres, all alive) and will require in-depth studies (details of distribution in the Thau basin). An awareness and protection campaign with the town halls and with the bathers, who will inevitably notice them, will also be necessary (they appear from a depth of 1m).
It is important to monitor the evolution of this new population which could form a temporary means of escape from the epidemic which is sweeping the sea. The CRIOBE, a scientific body in Perpignan, is also working on the subject.