Aegagropiles

Aegagropiles^[a] are dense balls of fibrous marine material. Aegagropiles found on coasts of the Mediterranean Sea are formed from decayed leaf fibres of the seagrass species Posidonia oceanica, though other variants worldwide are made of different organic materials.

Aegagropiles have been used for millenia, such as for making shoes, ancient medicine, or to caulk ships, though their formation process has only become clear in recent years. Potential modern applications make use of the properties of aegagropiles which are rare in nature; for example, aegagropiles have been shown to be a good soundproofing material. Aegagropiles would also be a more environmentally-friendly substitute for some synthetic materials.

In recent years, plastic waste, especially microplastics, has been interfering in the process of aegagropile formation; studies and people have found plastic in many aegagropiles in the Mediterranean. Consequently, aegagropiles have been suggested as an indicator of plastic pollution in the sea, with concludingly positive first results.

Aegagropiles formed in the Mediterranean Sea have a variety of names. The name aegagropilae comes from the Greek αίγαγροσ (wild goat) and πῖλος (fur).^[1] They are also known as Neptune balls,^[2][3] egagropiles,^[4] egagropili,^[5] Posidonia oceanica spheroids,^[6] pilae stagnales, pilae marinae, pili marinae, sphaerae marinae, globuli marinae, and sphaerae thalassiae.^[7] Historically, they have been known as okr elbahr and annabdti.^[7][8] Aegagropiles are known as pelotes marines or pelotes-de-mer in French,^[7][9][b] Zeeballen in Dutch, motolini in Italian at Lago, Calabria, Italy, and Seebälle in German.^[7][c]

Historically, the term sea balls^[d] has been used synonymously with Mediterranean aegagropiles and its alternative names.^[12] More recently, sea balls has been used to describe Mediterranean aegagropiles,^[5] other marine material spheroids,^[13] and even fossils of sea balls.^[8] According to German botanist and phycologist Bruno Schröder, there are two types of sea balls: non-real,^[e] which means sea balls formed by wave action made of discarded plant matter and encompasses Mediterranean aegagropiles, and real,^[f] which means sea balls that form through spherical plant growth.^[7] Schröder noted that the genus of algae Aegagropila is an example of plants that form real sea balls.^[7] Canadian botanist W. F. Ganong also separated algal sea balls from sea balls formed by waves.^[14]

Posidonia oceanica is a seagrass species endemic to the Mediterranean Sea, whose remains form the aegagropiles found there.^[1] In autumn, Posidonia oceanica loses its leaves.^[15] The leaf sheaths remain attached to the rhizome when leaves shed, and are slowly buried by sedimentation in the "matte", an accumulation of dead rhizomes and roots.^[15][16] During the burial process, leaf sheaths, which are rich in lignin, are eroded, releasing constituent fibres.^[2][15] The sea rolls the fibres into a spherical shape, creating aegagropiles.^[1][g] Then, ocean currents can dislodge aegagropiles from the sea floor; some drift into deeper waters and some are washed ashore.^[2][15] Aegagropiles wash up on beaches especially frequently during storms, though details on where they travel during their relocation are not known.^[3]

Dry aegagropiles are light brown.^[7] Aegagropiles usually have a radius less than 8cm long; a study found that the mean radius is between 1.14cm and 2.68cm.^[1][h] The study also found that the average mass of aegagropiles is 0.212g.^[1][i] Another study described aegagropiles as having "diameter values from millimeters to centimeters up to 20 cm".^[5] Aeagropiles can be spherical, ellipsoidal,^[17] or in-between.^[7]

Aegagropile fibres are "relatively smooth", except for places in which salt crystals form following sea water evaporation.^[1] Fibres on the outside of the aegagropiles are often "broken into a disordered bundle of smaller fibers" because of the transport to the shore; however, this is unlikely to impact the formation process.^[1] Leaf sheath cells in Posidonia oceanica have thin and lignified walls, so fibres provide the needed stiffness to form aegagropiles;^[15] they are composed of 32% lignin, whereas Posidonia oceanica has 19% lignin.^[17] The stiffness also comes from the density of aegagropiles.^[1]

Aegagropiles have been suggested as a soundproofing material, as they have similar absorption to that of mineral wool and polyester fibre.^[18] Extracted lignin from aegagropiles has been suggested as a material for reinforcement.^[17] The poor flammability and good thermal insulation of aegagropiles make them excellent for building insulation;^[5][19] aegagropiles have been sold as a building insulation material.^[1] Aegagropiles would have less of an environmental impact because Posidonia oceanica absorbs carbon dioxide,^[18] and they would replace plastic fibres, which is unsustainable as it requires oil extraction.^[17] This would also reduce energy use and organic waste.^[17] Procuring aegagropiles for insulation uses up to thirty times less energy than procuring mineral wool, rock wool, or petroleum-based foams.^[19] However, Anna Sanchez-Vidal of the University of Barcelona argues against removing aegagropiles from beaches as they are part of beach ecosystems.^[2] Aegagropiles bring humidity to beaches,^[2] protect beaches against erosion, provide nutrients for dune plants, and feed beach arthropod communities.^[15]

Aegagropiles formed from Posidonia oceanica can only be found on coasts of the Mediterranean Sea as Posidonia oceanica is endemic to there.^[1] Posidonia oceanica agegagropiles have been found on the coasts of Spain, Algeria, Tunisia, and Turkey.^[17] It is estimated that between 13 and 50% of initial Posidonia oceanica may have been lost since 1960,^[15] reducing aegagropile numbers. WWF estimated that 34% of Posidonia oceanica meadows have been lost in the past 50 years.^[20] One cause is that seagrasses like Posidonia oceanica are at risk from heatwaves and industrial pollution.^[2]

On the east coast of Africa and on the coast of Australia, aegagropiles made of Posidonica australis have been found.^[7] Posidonia australis is also in decline.^[2] Non-real sea balls made of marram grass have been reported at Dingle Bay, Ireland in 1996 and 2006.^[13] These varied in shape; some were spherical and had a diameter of 7cm, whereas others were elliptical and over 25cm long.^[13] Non-real sea balls made of fragments of marsh plant stems, rhizomes, and roots have been reported in Florida, United States.^[21] Non-real sea balls have also been found in lakes and on the coast of Nova Scotia, Canada, the coast of New England, United States,^[11] as well as on coasts of the North Sea.^[7] Canadian botanist W. F. Ganong speculated that non-real sea balls occur "all over the globe".^[11]

The earliest known use of aegagropiles was in ancient Egypt, where they were used to make footwear.^[22] The earliest known dated description of aegagropiles was in 1216 in Andalusia, Spain, where they were known as annabdti. Abul Abbas, a Moorish scholar, mentioned them in his book of travel, Kitab ar-rihlat, for supposedly having medicinal properties.^[7][8] An Arabic pharmacologist, Ibu el Baither, called aegagropiles okr elbahr in his book Kitab al-Dschamie al-Kabir, noting supposed medicinal properties in dentistry.^[7][8] Venetians used dry Posidonia oceanica leaves to pack glass, ceramics and fish.^[22] Italian naturalist Giacinto Cestoni believed that aegagropiles are an aggregate of seaweed fibres.^[23][24] He did not know where they originated from.^[23][24] The Encyclopédie, published between 1751 and 1772, defined aegagropiles as "a substance usually an oblong, rounded or spherical ball shape, [and] fist-sized".^[12] The Encyclopédie noted that aegagropiles are formed from fibrous plant substances.^[12] In the Oeconomische Encyclopädie, published between 1773 and 1858, aegagropiles were also described as having medicinal properties.^[25] Botanists in France have know that Mediterranean aegagropiles are formed from Posidonia oceanica remains since at least 1872, though theories that they are formed from the remains of pine cones existed in the 19th century.^[11] In 1877, Hugh Algernon Weddell reported that aegagropiles found on beaches around Hyères are made of Posidonia caulini,^[10][11] another name for Posidonia oceanica.^[26] He disproved the theory that aegagropiles are made of algae; thus aegagropiles were removed from the algae genus Aegagropila.^[11] In 1897, Génau de Lamarlière reported that aegagropiles found on the coast of Spain are made of Posidonia oceanica, though other variants can be made of algae or pine needles.^[27]

A 2018–2019 study published in Scientific Reports investigated the role of aegagropiles in trapping and extracting plastic debris from sea and carrying them to shore,^[2][15] which has been described as "mopping up ocean plastic from the seafloor".^[2] The study examined the accumulation of aegagropiles on different beaches on Mallorca, chosen for its extensive meadows of Posidonia oceanica and its large accumulations of plastic waste.^[15] The study found that:^[15]

Plastic debris in loose leaves (wracks) were found in 50% of the samples, with up to 613 plastic items per kg of dead leaves. Plastic items consisted mostly of fragments (61.29%) followed by pellets (33.67%) and foams (2.90%). The polymers were identified by spectrometry and included polyethylene (PE) (50.57%) followed by polypropylene (PP) (32.18%) and polyvinyl chloride (PVC) (6.90%). Plastic sizes ranged from 0.55 to 287 mm and averaged 9.08 mm.

Aegagropiles may collect up to 900 million pieces of plastic from the ocean every year.^[3] It is unclear whether the plastic damages the seagrass specifically.^[3] After the paper was published, Anna Sanchez-Vidal, a researcher at the University of Barcelona and lead author of the paper, received pictures of aegagropiles filled with plastic.^[2] In an interview with the BBC, she said that these examples contained larger plastics like "sanitary towels, tampons, [and] wet wipes".^[2] Sanchez-Vidal stated that the aegagropiles return "trash to us that was never meant to be on the seafloor".^[2] However, aegagropiles are not a solution for plastic waste problems, as removing them to reduce pollution would disrupt the beach ecosystem, and they would not work as a plastic filter.^[2]

A 2024 study by the Spanish Institute of Oceanography found that aegagropiles are an effective indicator for marine plastic pollution on the seafloor, especially microplastics, which were present in 62% of collected aegagropiles containing plastic. However, the study suggested that more research is needed to assess what fraction of plastics in aegagropiles are fibres, in order to assess differences between plastics found on beaches compared to on the sea floor.^[4] Additionally, the study found that aegagropiles may facilitate the movement of plastic from the seafloor to the surface, which could lead to increased consumption of plastic by sea life.^[4] The study concluded that the number of plastic items trapped in aegagropiles is dependent on the number of plastic items in the area rather than the size of the aegagropiles.^[4]

A 2026 study suggested that Mediterranean aegagropiles offer "promising opportunities for large-scale [plastic pollution] monitoring programs", although the method is still "in an early stage" and "[f]urther research is required" to standardise aegagropiles as an indicator of marine plastic pollution.^[6] The study analysed 1300 aegagropiles collected on 13 beaches of the Latium coast, and found that more than one-third contained plastic, with microplastics being the most common type.^[6]

• Marimo – Species of algae which forms spheres