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Under the waters of the Mar Menor, a tiny army has just deployed on a pyramid of biodegradable clay bricks. There are 55,000 flat oysters —Ostrea edulis— born in a hatchery of the Spanish Institute of Oceanography (IEO-CSIC) and today converted into hope to filter and regenerate a system on the brink of collapse. The operation, carried out by the IEO team and the Association of Southeastern Naturalists (ANSE) with the logistical support of the WWF solar boat, It is the first experimental reef native oyster from the Mar Menor. A deep project. The project is called RemediOS-2, and it doesn’t come out of nowhere. Its first phase, RemediOS-1demonstrated in 2022 that it was possible to produce oyster seed from native specimens of the Mar Menor. In just four months, the IEO hatchery in Lo Pagán produced 60 million larvae from just 36 broodstock. Now, the second phase makes the leap to the open sea. The idea is simple, but ambitious: oysters are natural biofilters. A single oyster can filter five to ten liters of water a day, removing organic matter and nutrients. The researchers estimate that a well-established culture will retain up to 20% of the nitrogen that enters the lagoon each year, and that the entire oyster population would be able to filter the entire Mar Menor in just 23 days. But how does it work? The experimental reef is located near Isla del Barón, one of the most sensitive areas of the Mar Menor. There, 175 blocks of biodegradable clay designed by the company Oyster Heaven were anchored. Larvae settled on them, which now grow feeding on excess nutrients in the water. Each block functions as a “temporary home”: the material slowly degrades while the oysters attach themselves to the bottom and form their own natural reef. In total, the system occupies about 12 square meters of seabed, but represents a key experiment in ecological restoration. Scientific monitoring is carried out by ANSE under the direction of the IEO-CSIC. Technicians analyze the survival, growth and stress level of the oysters, in addition to measuring their sexual maturation and the accumulation of contaminants such as bacteria. E.coli or marine biotoxins. An extra help. To monitor this entire process they need even more hands. So the project counts too with technological support from the Polytechnic University of Cartagena (UPCT). Its researcher Francisco López Castejón used a remote underwater vehicle (ROV) to inspect the reef and see how this technology can monitor underwater habitats where diving is difficult. With science and technology working together underwater, the next question is inevitable. Why oysters in the Mar Menor? For decades, this sea has suffered from the accumulation of nitrates and phosphates from intensive agriculture. These nutrients feed an excess of phytoplankton that clouds the water and depletes oxygen, causing massive episodes of anoxia and death of fauna. The goal of RemediOS-2, in the words of the Department of the Environment of the Region of Murciais that the flat oyster acts as a natural regeneration tool. Its filtering helps reduce eutrophication and its shells, rich in calcium carbonate, contribute to carbon storage, an added benefit in the face of climate change. Beyond restoration. The project is also a test bed for a new blue economy. According to the Pleamar Program, The project aims to involve the local fishing sector, organize marine spaces for future restoration actions and demonstrate that regenerative aquaculture can be compatible with environmental recovery. The third phase of the project will include genetic studies to check whether local oysters are better adapted to climate change, with the aim of producing resistant “seeds” that can be reintroduced both into the lagoon and the Mediterranean. Forecasts. For now, oysters continue to grow under the gaze of researchers and underwater robots. The third part of RemediOS is already in planning. Perhaps these 55,000 oysters alone cannot save the Mar Menor, but they can demonstrate that environmental restoration can start with a mollusk, a handful of biodegradable bricks and a simple idea: let nature repair itself. Image | IEO Xataka | The reservoirs in the Segura basin are at their limit. The question is whether the new rains can save them