The Noble Pen Shell (Pinna nobilis) is an endemic species of the Mediterranean, the largest bivalve in the Mediterranean Sea and among the largest bivalves in the world. It usually grows between 30 and 50 cm but can reach lengths of up to 120 cm and live to be around fifty years old. It lives in sandy bottoms and seagrass meadows, from the surface up to about sixty meters in depth. It uses the rear pointed end of the shell to bury itself partially into the sediment and attach to it using numerous so-called byssal threads. The other end is fan-shaped, ranging from light brown to reddish color. The two shells are identical and the pen shell opens them only about a centimeter to filter the sea water and collect plankton and organic particles from it. It can filter hundreds of liters of sea on a daily basis. If it feels threatened, it will close its shells tightly. The little crab (Nepinnotheres pinnotheres), which often lives inside it, can also warn it of danger.
The noble pen shell has always been a prized bivalve. Its durable byssal threads were used to make royal clothing, ornaments and gloves that shone like gold in the sun. Due to their grand size and smooth iridescent interior structure, its shells were used as an ornament. In many regions, it was a traditional delicacy. Sometimes, small irregular pearls were found inside the pen shells.
With the increase in awareness of the need for its conservation, as well as its designation as a strictly protected species, pen shell populations in many areas of Croatia have developed dense settlements since the beginning of the 21st century.
During the autumn of 2016, Spain recorded a death of this species. It was probably caused by a disease, which spread to the Croatian part of the Adriatic Sea in 2019, resulting in its mass mortality. The exact cause of this disease is unknown and it spread to the entire Mediterranean Sea, almost leading to the extinction of the noble pen shell as a species.
At the beginning of 2024, only about 20 living specimens were known in the seabed of Croatia. However, their survival and the discovery of live specimens in some other areas of the Mediterranean, such as the Marble Sea, offer hope that noble pen shell will again spread in our seabed.
Posidonia (Posidonia oceanica) is a seagrass with dark green, meter-long leaves that grow in tufts from the tips of its intertwined branches. The branches grow only one centimeter per year and are often covered in sediment, sometimes in meters-long layers. Such branches began growing thousands of years ago, making Posidonia one of the longest-living organisms on Earth!
The scientific genus name Posidonia was given after Poseidon, the Greek god of the sea. The species name oceanica was given to it by the renowned biologist Carl Linnaeus because he thought it lived in oceans. However, Posidonia is an endemic Mediterranean organism and its closest relatives inhabit the seas of Australia.
Like terrestrial flowering plants, Posidonia reproduces through the process of flowering, which includes both male and female flowers. It blossoms at regular annual intervals and the unattractive green inflorescence blossoms during the autumn. Sometimes few can be found, while at other times, the inflorescence can be found in each of the hundreds of tufts on the surface of a square meter. After fertilization with thread-like and sticky pollen, which is carried by the sea, a fruit that looks like an olive develops. In the spring the fruit ripens, detaches itself from the stalk and floats on the surface for several days. When it bursts, a seed falls out and sinks to the bottom. If it sinks to the bottom at depths shallower than 40 meters which has enough light for the development of Posidonia, it will begin the process of slow growth. Through creeping growth and the branching of branches, over the course of decades, a settlement with a diameter of only a few meters will develop. Over the centuries, it may expand to encompass part of a smaller bay. It will take millennia for the settlement to reach the proportions that Posidonia has in the Croatian seabed today. Because of its extremely slow growth, any destruction of its smallest settlements is irreparable within the timeframe of a human lifespan.
The settlements of Posidonia are inhabited by numerous organisms and are considered areas with the greatest biodiversity of the Mediterranean Sea as well as areas with the greatest organic production. Its leaves purify the water column and after they fall off, they accumulate on the shore as lažina (accumulated dry sea grass on a sandy shore) where during the winter they protect sandy beaches from erosion caused by waves. In its settlements in Croatia, a fish named picarel has been traditionally caught for centuries. Its leaves are even today used for making mattresses and pillows and its buried branches permanently store carbon, consequently reducing the impact of global warming. When you are near the sea, take a deep breath because you'll inhale the oxygen produced by the most important plant of the Mediterranean Sea: Posidonia.
Dr. sc. Ante Žuljević
