Pinna nobilis is the largest bivalve mollusk in the Mediterranean, present since the late Miocene, belonging to the Pinnidae family. This organism can reach dimensions of 120 cm and we can meet it between 0.5 and 60 meters depth, mostly on a sandy substrate like in seagrass meadows.

Since ancient times, Pinna nobilis has been a resource for mankind, but today’s pressure from mankind is much more relevant than in the past. In fact, this organism is threatened by illegal trawling, coastal construction, anchoring, illegal mining and pollution. All these activities degrade the habitat and compromise the vitality of the eggs and larval stages, accelerating the decline of populations in the Mediterranean. Last but not least, environmental threats that range from climate change to the invasion of alien species must also be considered. The invasion of alien species can be considered both a direct consequence of climate change and deriving from human action (ship traffic, opening of channels connecting the basins).

Precisely for this reason this species is protected by several directives: the European Council Directive 92/43/EEC (EEC, 1992), in the ANNEX II of Barcelona Convention and by local law in all European Union Mediterranean countries.

The species is common in Posidonia oceanica meadows, where it lives with the tapered anterior third of the shell buried in the substratum for anchoring. Attachment is achieved by byssus threads, glued to different type of susbstratum like sand or the root and rhizomes of P. oceanica.

The triangular shape of Pinnidae and their anisomian condition (rudimentary or absent anterior adductor muscle of the shell) are a consequence of the bond to the substrate assisted by the byssus. The large dimensions reached are due to the large extension of the posterior mantle and shell, a consequence of life in semi-infaunal habitats of mobile substrates.

The ecological role of these specimens sees them particularly involved in the filtration process, precisely because of their large size. By doing this, they are able to retain a large amount of detritus, promoting higher water quality. In fact, Pinna nobilis preferentially feeds on detritus (95% of the ingested material), phytoplankton, micro- and mesozooplankton and pollen grains.

P. nobilis is shown to be a type of solid substrate in a habitat characterized by a mobile substrate. This helps colonization by benthic flora and fauna. Having a high longevity (about 27 years) it is also a stable substrate over time of colonization.

These abilities contribute to the increase of biodiversity in an area that in their absence would probably be poorer.

These are organisms defined as successive hermaphrodites, with asynchronous maturation of the gametes, which do not carry out self-fertilization. Sexual maturity is reached at 2 years of age and the release of the eggs occurs in late summer. Larval life is pelagic and lasts for up to 10 days.

The main patterns of the gaping activity are shell opening during the day, and shell closing during the night. Night opening also occurs when the moon is in the sky and sufficiently (more than 50%) illuminated.

Furthermore, storms with strong hydrodynamics affecting the seabed also alter gaping activity, reducing the maximum time the shell remains open and promoting shell closing.

Hydrodynamics is also able to determine the distribution of size, population, orientation of the shell and growth, establishing the availability of food and reducing the survival of the larvae due to the wave motion.

The type of substrate, depending on the hydrodynamics, also determines the degree of resuspension and the stress inflicted on the organisms. Precisely, for this reason, Pinna nobilis is found in the Posidonia oceanica meadows which are able to influence the net force of the wave motion by drastically reducing the flow compared to the areas outside the meadows. P. oceanica, being characterized by an interlacemet of roots and rhizomes, helps anchor the byssus, traps sediment and particles, which are food for P. nobilis. Furthermore, P. oceanica, serves as a refuge especially for the most vulnerable juvenile stages.

There is therefore a close relationship, at times, fundamental to the survival of the populations of Pinna nobilis.

THE THREATS

CLIMATE CHANGE

The Mediterranean is warming up and doing so at twice the rate of the rest of the oceans. The changes in temperature and the consequent shortage of food, due to the temporal displacement of phytoplankton peaks, as well as changes in local hydrodynamism, could be the cause of the decline of these organisms already endangered. No less important is the decrease in pH, due to the increase in atmospheric CO₂, which can negatively affect the metabolism and the calcification process.

POLLUTANTS

Considered a threat due to the fact that these bivalves cannot escape neither terrestrial nor marine pollution. This induces oxidative stress leading to an overstimulation of the antioxidant enzymes and observation of oxidative damage on the lipidic membranes.

LOSS OF HABITAT AND ANCHORAGE

The degradation and fragmentation of Posidonia oceanica meadows affect the population dynamics of P. nobilis. The anchorage in turn is able to undermine the meadows and cause direct and indirect damage to the populations of Pinna nobilis.

FOOD NETWORK ALTERATION

Octopus vulgaris is the major predator of Pinna nobilis. Predatory pressure on P. nobilis can increase due to cascading effects on the food web. In fact, the population of Epinephelus marginatus, the major predator of Octopus vulgaris, is decreasing in the Mediterranean due to commercial and recreational fishing. It makes sense to the increase of predatory pressure by Octopus vulgaris (on Pinna nobilis) which no longer suffers, as it once was, the predatory pressure by E. marginatus.

INVASIVE SPECIES

Global climate change is favoring the spread and establishment of invasive species around the world. For P. nobilis, the spread of a macroalga, Lophocladia lallemandii, which can alter the potential food sources for P. nobilis and associated fauna, proves to be dangerous to its survival. L. lallemandii is also able to cause physiological effects on the bivalve, due to bioactive alkaloids called lofocladine. When we are in the presence of that algae, we can also observe an increase in anioxidant enzymes which are indicators of a condition of oxidative stress, that is particularly visible at the histological level, in concomitance with the gills and digestive gland.

Among the invasive species we cannot fail to mention a parasite, Haplosporidium pinnae, which generates heavy inflammatory responses and various dysfunctions in the organism. This parasite, since 2016-2017 has been a promoter of mass mortality events of this bivalve.

In fact, the extreme virulence of this parasite will probably lead to the decline of P. nobilis populations, except in those areas where the environmental conditions go beyond the tolerance limit of the parasites, or in cases where there are resistant organisms to parasite.

The high genetic homogeneity found throughout the Mediterranean certainly does not favor the survival of P. nobilis in this “pandemic” condition.

Haplosporidium pinnae was, in fact, the cause of the disappearance of the population of Pinna nobilis within the Porto Cesareo Marine Protected Area where it is possible to observe what are called “cemeteries” of Pinna nobilis. When the organism dies, the shell continues to act as a colonization substrate even if the decalcification rate gradually increases.

Nature has its laws, sometimes a difficulty makes much more stronger and resistant as we have often seen. Other times the extreme fragility of an ecosystem, of an organism sees the total loss of it. And with it, you lose all the ecosystem services it provided, and even the simple possibility of losing your breath for a second and observing them in their magnificence. We are losing as mankind, we are losing big.

P.S. Thanks to the guys of the Porto Cesareo Marine Protected Area who allowed me to observe this issue closely and for having been at their side even for a short time.

Maria Bruno