This month, we’re learning about the noble pen shell, ‘Pinna nobilis’ – a large species of mediterranean clam.
Most people will think of a small black shell and tasty orange flesh when confronted with the word ‘mussel’. Commonly consumed throughout the Mediterranean, these molluscs have irregularly shaped shells and can measure up to 12cm long. The mussel you are thinking of is the common mussel Mytilus edulis. Now imagine a mussel reaching 120cm in length, and you might consider that the fleshy orange morsel inside is large enough to feed a family! The stuff of fantasy surely? Actually no, and indeed found on a coast near you; enter the fan mussels.
The first thing to mention about these gargantuan molluscs is that they are very strictly protected by Annex IV of the EEC Habitats directive and all forms of capture or harvesting are prohibited by law. This is exceptionally important to respect as they are incredibly sensitive to pollution and are listed as endangered within the Mediterranean.
Typically found in soft sediments and in seagrass meadows up to a depth of 60 metres, fan mussels are solitary, which is atypical compared to the common variety which form beds. They do share the mussel beard known as the Byssus filament, but use it to attach themselves to hard surface within the sediments. The filaments are made of keratin like our fingernails and are found beneath the substrate boundary towards the anterior of the mussel and in total about one third of the shell is buried. Historically, these fibres were used in weaving cloth such as sea silk which can then be used to create all sorts of clothing. The Romans valued sea silk so much that only the wealthiest individuals could afford it.
This anchor is very important in keeping the animal upright and in an ideal feeding position. Interestingly, the seagrass is also very important for them as seagrass blades help to slow down the flow of water and protect the mussels from being pushed over. The combination of the mussel and the seagrasses forms the foundation of a diverse and healthy ecosystem where shrimps and many small crustaceans hide away. These crustaceans then form an important food source for many species of fish along the coastline.
Pinna nobilis is also affected by a number of threats which contribute to its poor abundance. Mass mortality events were experienced across the Western Mediterranean in early autumn 2016. No one was able to determine exactly why they were dying until various researchers stumbled upon haplosporidia – a eukaryotic parasitic animal which most closely resembles terrestrial fungus in its lifestyle – in the digestive system of the mussel. The rate of spread of the parasite was so fast that it killed whole populations of mussels over hundreds of kilometres of coastline and consequently pushed it into the critically endangered zone.
The destruction of seagrass meadows through pollution and commercial fishing have also had a significant impact on standing stocks. Net dragging is highly destructive of the benthic habitats as it decimates everything in its path. Sessile Pinna nobilis is often a victim in these kinds of activities. Typically, the fishermen are back before the sea bed has had a chance to recover and this keeps it in a degraded state, permanently. It is not just commercial fishermen either as free divers have targeted them for the easy pickings since Egyptian times. Fishing activities of all kinds affect them negatively.
Although infrequent, shipping disasters also take their toll. In January 2012, the cruise ship Costa Concordia ran aground off the coast of Italy and tragically 32 people lost their lives. A lesser-known fact is that the location where Concordia partially sank was also the site of an established Pinna nobilis population and the ships presence on the sea bed threatened to destroy them all. As part of a monumental effort to rescue the surviving mussels, scientific divers carefully removed them manually and relocated them to a suitable location. Fortunately, the survival rates were pretty good and hence the damage was limited to some degree.
As ever, climate change is a serious threat to these organisms. Studies have shown that as water temperature rises, the juvenile survival rate decreases, which could dramatically affect larval dispersal rates and population growth. Increased oceanic acidification due to increased carbon dioxide levels are also expected to increase stress pressures on standing stocks.
Ecologically they are very important as proficient filter feeders. Filter feeding is an essential natural component for the removal of detritus which is in suspension throughout the water column. These mussels retain the detritus within their digestive system which helps clean up the water. Typically, the higher the biomass of filter feeders the lower the turbidity; this is particularly good for divers. Not just that, but the loss of such a monster mussel would be a big blow to conservational efforts in the region.
Gibraltar’s coastline has already lost its seagrass beds and looking to the future we must carefully ponder the fate of the monster mussel. With an appropriate monitoring programme and careful coastline management we can build a sanctuary for them on our coastline for generations to come.