The Uncertain Fate of Sea Silk – Part 2

Pinna Nobilis washed ashore

In our previous Blog we shared with you one of the most awe inspiring examples of submerged heritage in the Mediterranean Sea; the story of sea silk. Sea silk is made from the long silky filaments, also known as byssus, secreted by a gland at the base of pen shells, especially the noble pen shell (Pinna nobilis). The noble pen shell is endemic to the Mediterranean Sea and can be found along a large part of the Greek coastline, often in shallow waters. You will find more information on the physiology and habitat of the noble pen shell in our previous Blog. This Blog will shed some light on what in the meantime is starting to look as the doomed fate of this beautiful example of submerged heritage.

Despite their status as an endangered species the number of noble pen shells has continued to decline over the years due to over-fishing, destruction by trawling and anchoring, the decline of sea grass fields and pollution. The last two years the rate of their decline has jumped to unprecedented levels. A new enemy has emerged that is causing mass mortality events throughout the Mediterranean. These evidence gathered by the science community these past few months shows beyond a doubt that an environmental tragedy is emerging.

Noble pen shell washed ashore

Last September Greece was struck by Medicane (Mediterranean Hurricane) Zorba. After a number of days with gale force winds battering the Greek coastline the calm returned. Walking on the beach I was shocked by the large number of noble pen shells that had washed ashore. That shock turned to surprise when I took a closer look at the uprooted shells. All the shells were completely empty ... devoid of the fleshy tissue that normally fills them. Three months later Greek newspaper Kathemerini confirmed my fear that ‘the’ parasite had reached Greek waters.

The first reports of mass mortality events started appearing late 2016 with large numbers of dead noble pen shells being found along the coastline of Spain. Spanish scientists established that the deaths were caused by a new species of parasite (Haplosporidium pinnae) that was discovered in the digestive system of the dead shells. The main symptoms of infection include mantle retraction, no reaction to stimuli and open valves, as they are no longer able to close their shells. The chance of survival for individual members in infected communities is incredibly limited.

In May 2018 the International Union for Conservation of Nature (IUCN) sounded alarm calling central and western Mediterranean countries to action and releasing a number of recommendations aimed at monitoring the outbreak and establishing a rescue strategy.

In the meantime the parasite has reached Greek waters. Scientists from the Hellenic Center for Marine Research (HCMR) and the University of the Aegean have confirmed the presence and rapid spread of the parasite both around mainland Greece and the islands. An inventory late 2018 of 13 sites around the Aegean island of Lesvos showed that the noble pen shell population had been depleted by 93 percent. Similar observations continue to be reported. A cure has yet to be found. The scientific community now fears for the very survival of the species.

“Scientists are now racing to understand how the parasite spreads and its life cycle – essential information for a successful rescue program. One theory is that it could be spreading through phytoplankton, the bivalve’s food source, but nobody knows for certain” (Ekathimerini, 2018).

A number of initiatives have been taken to identify healthy shells and to place them in aquaria, hoping to save them and possibly breed them. Early 2019 the Greek scientific community called upon the scuba diving community to start reporting on the whereabouts of surviving specimens so that they may be relocated to research facilities.

To be continued.