Ocean warming is the key filter for successful colonization of the migrant octocoral (Ehrenberg, 1834) in the Eastern Mediterranean Sea.

Climate, which sets broad limits for migrating species, is considered a key filter to species migration between contrasting marine environments. The Southeast Mediterranean Sea (SEMS) is one of the regions where ocean temperatures are rising the fastest under recent climate change. Also, it is the most vulnerable marine region to species introductions. Here, we explore the factors which enabled the colonization of the endemic Red Sea octocoral (Ehrenberg, 1834) along the SEMS coast, using sclerite oxygen and carbon stable isotope composition ( O and C), morphology, and crystallography. The unique conditions presented by the SEMS include a greater temperature range (∼15 °C) and ultra-oligotrophy, and these are reflected by the lower Cvalues. This is indicative of a larger metabolic carbon intake during calcification, as well as an increase in crystal size, a decrease of octocoral wart density and thickness of the migrating octocoral sclerites compared to the Red Sea samples. This suggests increased stress conditions, affecting sclerite deposition of the SEMS migrating octocoral. The Osc range of the migrating indicates a preference for warm water sclerite deposition, similar to the native depositional temperature range of 21-28 °C. These findings are associated with the observed increase of minimum temperatures in winter for this region, at a rate of 0.35 ± 0.27 °C decade over the last 30 years, and thus the region is becoming more hospitable to the Indo-Pacific . This study shows a clear case study of "tropicalization" of the Mediterranean Sea due to recent warming.