A study conducted by IMEDEA reveals that the spider crab endangers posdionia’s conservation

A study conducted by the Advanced Mediterranean Institute Studies (IMEDEA, CSIC-UIB) shows for the first time ever that the crab percnon gibbesi, also known as the spider crab, feeds itself from posidonia oceanica. This discovery reveals a new danger for posidonia meadows conservation, key ecosystems for marine biodiversity, carbon capture and coast’s stability.

The posidonia oceanica’s meadows, a marine endemic sea grass from the Mediterranean, create fundamental ecosystems for marine biodiversity. These meadows offer refuge and nourishment to numerous species, help alleviate climate change when setting carbon and contribute to the seafloor stability. However, its conservation is endangered by multiple pressures, from natural and human origin, such as invading species expansion.

A team from the Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB) has studied the interaction between Posidonia oceanica and the spider crab (Percnon gibbesi), a tropical herbivore that, since its arrival in the Mediterranean in 1999, has rapidly spread along the coastline. The aim of the study was to assess the potential impact of this species on Posidonia, a key element in the health of Mediterranean marine ecosystems.

Both species coexist in shallow coastal areas, which led the research team to closely examine the consumption of different Posidonia oceanica tissues by the crab.

To carry out the study, specimens of Percnon gibbesi were kept in aquariums under controlled conditions, simulating different environmental scenarios. After an acclimation period, the crabs were fed various Posidonia tissues, leaves, fruits, rhizomes, and entire shoots, to evaluate their feeding preferences. The experiments, conducted in tanks with and without crabs, allowed researchers to measure the net consumption of each type of plant tissue.

The results reveal that the spider crab shows a clear preference for Posidonia rhizomes and fruits, while leaves are the least consumed tissue. Although temperature did not significantly alter consumption levels, the fact that this invasive species feeds on rhizomes, a structure essential for the plant’s growth and anchoring to the seabed, poses a serious threat to the stability and regeneration of Posidonia meadows. Furthermore, the consumption of fruits could compromise the reproductive capacity of this marine plant, hindering its long-term conservation.

Researcher Daniel Puentes, a master’s student and first author of the study, emphasizes that “although Percnon gibbesi herbivory seems limited to shallow waters and rocky bottoms, its spread throughout the Mediterranean could turn this phenomenon into a significant ecological threat.” In a context where Posidonia oceanica is already facing pressures such as warming waters, pollution, and boat anchoring, this new source of impact deserves special attention.

Andrea Antón, IMEDEA researcher and co-author of the study, highlights “the urgency of continuing to investigate the effects of invasive species on marine ecosystems.” She also stresses the importance of conserving Posidonia meadows, recognized as an endangered habitat by the International Union for Conservation of Nature (IUCN). The team recommends strengthening monitoring of the interaction between the spider crab and this marine plant, as well as evaluating possible measures to reduce its impact on one of the Mediterranean’s most valuable ecosystems.