Unprecedented finding: Posidonia oceanica meadows respond to Mediterranean warming with bloom and pseudoviviparity

A recent study led by the Institut Mediterrani d'Estudis Avançats has revealed a link between the warming of Mediterranean waters and the blooming of Posidonia oceanica, offering new perspectives for the protection and conservation of this keystone species.

Seagrass meadows, vital for both the environment and the economy, are declining worldwide due to human activities. However, in some areas where protective measures, such as improving water quality and regulating anchoring and trawling, have been implemented, a recovery has been observed. Now, global warming and marine heat waves pose a new threat that is accelerating their degradation and loss.

Posidonia oceanica, a species endemic to the Mediterranean, is very vulnerable to warming, especially considering its low resilience, as it is a very slow-growing plant with very infrequent sexual reproduction (flowering). However, in 2022, during a period of unprecedented warming in the Western Mediterranean, a massive flowering episode was observed in the Posidonia oceanica meadows of Mallorca.

This event seems not to be isolated, as intensive blooms have been reported on the French coast and in Sardinia after the 2022 heatwave.

In addition to such flowering, and thanks to the contribution of citizen science (www.observadoresdelmar.es) it was observed that 64% of the meadows studied in Mallorca also showed pseudoviviparity, an extremely rare phenomenon in the plant world in which plants produce clones of the mother plant instead of flowers. Pseudoviviparity, a very unusual form of reproduction (present in about 50 plant species worldwide), can occur in response to extreme environmental conditions. This phenomenon has been observed in terrestrial plants inhabiting stressful environments, such as deserts and alpine regions.

This is the first study to provide morphological and genetic evidence for the existence of pseudoviviparity in Posidonia oceanica and to document this phenomenon at multiple sites for a marine plant.

‘This discovery has been really exciting, as it is a fascinating phenomenon that is largely unknown, not just in marine plants, but in plants worldwide, and it opens up a whole series of questions about the implications for the ability of Posidonia oceanica to recover from heat stress events such as heat waves.”  Dr Fiona Tomas Nash, author of the study.

Photo: Posidonia Oceanica meadow

The study also highlights the importance of considering pseudoviviparity when assessing seagrass resilience mechanisms in the face of climate change. Pseudoviviparity may have several implications for seagrass populations, including resilience to climate change, genetic diversity, population distribution and expansion, and population structure.

In summary, this study provides new insight into how seagrasses may respond to extreme environmental conditions and highlights the need for further research to fully understand the implications of pseudoviviparity and other adaptive responses in seagrass populations.