Oculina patagonica, commonly known as the Mediterranean encrusting coral, is a species of hard coral belonging to the family Oculinidae and the order Scleractinia. The genus name Oculina comes from the Latin ‘oculus‘ (eye), possibly referring to the shape of the corallites, while patagonica refers to the Patagonia region, although, paradoxically, its presence in the South Atlantic is anecdotal. Originally described from Atlantic waters, its expansion in the Mediterranean Sea has sparked debate over whether it is an introduced or native species. Recent genetic studies (Leydet & Hellberg, 2015) have shown that Mediterranean populations are genetically distinct from Atlantic ones, suggesting it is not a recent introduction but a native species undergoing recent expansion. Today, Oculina patagonica is common throughout much of the Mediterranean, particularly in the southern and eastern basins, and has also been recorded in adjacent Atlantic areas such as the coast of Huelva (Spain). It colonizes both natural and artificial habitats, often in shallow zones between 0 and 10 meters, though it can be found at depths of up to 200 meters.
Morphologically, Oculina patagonica forms encrusting colonies, more or less circular in shape, with a brownish-green color due to the presence of symbiotic zooxanthellae. Colonies may cover just a few square centimeters or extend over several square meters, with variable thickness, usually greater at the center. Its corallites are small, ranging from 1 to 2.5 mm in diameter, and are closely packed. They have 24 short, thick tentacles arranged in two crowns. A distinctive feature of this species is that the corallites may be completely immersed in the calcareous matrix, especially in the thicker parts of the colony. This trait distinguishes it from similar species such as Cladocora caespitosa, which has larger and more prominent corallites, or Madracis pharensis, with more widely spaced and pentagonal corallites. Other Mediterranean species like Phyllangia americana mouchezii or Polycyathus muellerae may also be confused with it, but they exhibit longer tentacles and more developed or individualized calcareous structures.
In terms of feeding, Oculina patagonica combines two trophic strategies. On one hand, it maintains a symbiotic relationship with dinoflagellate algae of the genus Symbiodinium (especially S. psygmophilum), which produce nutrients via photosynthesis that are used by the coral. On the other hand, as a carnivorous coral, it uses its tentacles with stinging cnidocytes to capture small planktonic prey such as larvae and tiny crustaceans. This dual feeding mode allows it to adapt to various environmental conditions and make the most of available resources.
The reproduction of Oculina patagonica can be both asexual and sexual. Asexually, it reproduces by budding, resulting in clonal colonies. Sexual reproduction, which is less frequent, typically occurs at the end of summer, coinciding with the full moon and a drop in temperature after the summer peaks. The species is gonochoric, meaning there are separate male and female colonies. Fertilization is external and produces planktonic larvae that later settle on a substrate to form new colonies. This strategy enables it to rapidly colonize new surfaces, acting as a pioneer species.
A remarkable aspect of Oculina patagonica is its high ecological tolerance: it can survive in environments with high variability in salinity, temperature, turbidity, acidity, and pollution. This explains its growing presence in port areas and artificial structures, where many other species fail to thrive. Although it is a photophilic species that primarily inhabits well-lit areas, it can also be found in shallow caves, adapting to environments with low light and fewer zooxanthellae. During extremely warm summers, bleaching (expulsion of zooxanthellae) has been observed, such as in Israel, but unlike many tropical corals, Oculina patagonica shows a remarkable capacity for recovery. This resilience, combined with its opportunistic nature, has allowed its distribution in the Mediterranean to surpass 20,000 km² and continue expanding.
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