Hi folks and welcome back to another edition of Marine life Mediterranean style.
This time I am going move away from sponges and write about other species endemic to the Mediterranean, namely Pinna Nobilis, a fan mussel.
What is the Pinna Nobilis
Pinna Nobilis is a long-lived Mediterranean endemic species, considered one of the biggest bivalve molluscs in the Mediterranean Sea. It lives at depths ranging between 0.5 and 60mtr. It has a wide distribution across coastal areas, occurring mainly in seagrass meadows, but also present in other habitats such as rocky bottoms.
The bivalve shell is usually 30–50 cm long, but can reach 120 cm. Its shape differs depending on the region it inhabits.
It attaches itself to rocks using strong byssus (a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface) composed of many silk-like threads which used to be made into cloth. The animal secretes these fibres from its byssus gland. They consist of keratin and other proteins and may be as long as 6 cm. The inside of the shell is lined with brilliant mother-of-pearl.
What is its role in the ecosystem
It plays a key ecological role by filtering water and retaining large amounts of organic matter from suspended detritus contributing to water clarity. Like all pen shells, it is relatively fragile to pollution and shell damage.
As with other members of its genus, Pinna Nobilis hosts symbiotic shrimp which live inside its shell. It is believed that when it sees a threat, the shrimp warns the host, perhaps by retracting its claws or even by pinching. The clam then closes shut. It has been demonstrated that the shrimp has a similar filter-feeding diet to its host and the relationship is likely mutualistic.
A mass mortality event
A mass mortality event affecting Pinna Nobilis populations was first detected in 2016 along the Spanish coast. The still ongoing mortality outbreak has been found to be caused by a pathogen (Haplosporidium pinnae), which spreads rapidly giving a mortality rate of 80-100% across many regions. Only a few populations are known to remain pathogen-free. However, these are geographically isolated and located in sites with very specific environmental conditions. Alas Malta is not one of these regions. In the past, Pinna Nobilis has been threatened with extinction, due in part to fishing, incidental killing by trawling, boats anchoring, and the decline in seagrass fields. It doesn’t help when pollution kills eggs, larvae, and in some cases the adult mussels.
Populations of this fan mussel are strongly dependent on the survival of adults. Successive hermaphrodism (e.g. changing sex at different stages in life) make the recovery of populations more difficult and less likely to happen. Whereas other fan mussels mainly reproduce during the summer months, Pinna Nobilis reproduces between July and October.
Are they endangered
Its uniqueness attracts many scuba-divers across the Mediterranean. Besides, its surfaces are colonized by other benthic species, including algae and macroinvertebrates, thus increasing the local biodiversity.
The European Council Habitats Directive 92/43/EEC, on conservation of natural habitats and the wild fauna and flora, proclaims that P. nobilis is strictly protected (by the Annex IV of EEC, 1992) – all forms of deliberate capture or killing of fan mussel specimens are prohibited by law.
Thanks for reading the blog. Catch you soon.
Written By: Sarah Shaw