Posidonia oceanica (L.) Delile (PO) is an aquatic plant that is a dominant and endemic sea grass of the Mediterranean basin belonging to the
Posidoniaceas family [
1], and therefore, it is not an alga, despite still being frequently and wrongly defined as such in numerous scientific papers [
1]. PO is a slow-growing plant that might live for millennia, and it forms huge underwater meadows that are estimated to cover more than 2.0% of the Mediterranean seabed (for a total of more than 12,000 km
2) and that could extend from the sea surface up to 40 m of depth [
1,
2]. PO mainly spreads itself by sexual reproduction, although asexual reproduction might also occur by stem extension. The sexual annual reproductive cycle starts in the fall with pollination, and after a period of six to nine months, its mature fruits release seeds that reach the seabed and can develop roots and produce a new plant [
3]. In only one year, the meadows of PO produce a huge amount of debris as leaves (leaf blades and sheaths), rhizomes, and roots that could constitute a biological sediment on the seabed and, eventually, might be either degraded by macro-organisms, microorganisms, and abiotic factors, or being subjected to the hydrodynamic action of the sea and rolled on its floor or ripple marks, or transported by the waves on the coasts where they accumulate [
3]. The leaves, in particular the portions called leaf blades, fall from the plant after 5–8 months of existence and, generally, at higher rates in the autumn, a lower amount of sunlight reaches the sea, and many strong storms occur. Conversely, the leaf sheaths usually remain attached to the rhizomes. The leaves, that reach the seashore transported by the waves and deposited on the beaches by the winds, usually form huge banquettes with thickness ranging from a few centimeters up to 2.5 m [
3,
4]. The annual total amount of PO leaves that reach the coast is in the range of 5 to 50 million tons, and nowadays the banquettes are estimated to cover about 50,000 km
2 of sandy shores in the Mediterranean areas [
1,
2,
3,
5]. They are generally composed of wet and dried brown PO leaves, and they play an important ecological role in preserving the ecosystem and the biodiversity. In fact, they represent a favorable habitat for many species, promote sediment entrapment and stabilization, regulate the CO
2 absorption of the sea and of the atmosphere, as well as water oxygenation, and protect the coasts from erosion by acting as a barrier [
2,
4,
5,
6]. PO roots and rhizome fragments, instead, might naturally be entangled by the constant rolling of the sea motions and aggregate as ball-shaped materials that are then delivered by the waves on the coasts, where they dried under the sun and the wind action [
2,
4]. These brown dried fibrous balls are generally known as PO egagropili (POEG), also spelled egagropilia, egagropoli, or aegagropiles, and reported as sea balls, sea rissoles, sea potatoes, beach balls, Neptune balls, or Kedron balls. The name “aegagropiles” derives from the ancient Greek words of αίγαγρoς (wild goat) and πῖλoς (fur), as the shape of these sea balls is reminiscent of the ones that are generally regurgitated by goats [
4].
Figure 1 shows a representative image of some POEG samples collected by the scholars on the beach of Marzamemi, Sicily, Italy; 36°44′34″ N, 15°7′1″ E, and
Figure 2 indicates the map of the sites in the Mediterranean Sea [
2,
5]. As with the leaves, every year millions of POEGs are delivered on the beaches by the winds, mainly in the period between October and March and especially after strong sea storms [
3,
5]. In some cases, POEG deposition could constitute a characteristic geomorphological feature of the landscape, such as along the southeastern Gulf of Sirte, in Libya, near the coastal town of Brega (30°26′06″ N, 19°40′01″ E) (
Figure 2). Here the POEGs are deposited by the action of westerly winds, while the hot and arid wind of Ghibli, from the south, carries huge amounts of Sahara sand, thus forming peculiar POEG sandy sheets and dunes that are considered paleoenvironmentally interesting to study in Holocene era [
5]. Indeed, POEGs have been frequently studied in integrated archaeological and geological investigations as a sign of the coastal barrier evolution in different Mediterranean areas as well as of the stratification and of the climate changes during the Holocene (e.g., the studies on the Mistras coastal barrier system in central Sardinia, Italy [
7]).