Southward range expansion of Siderastrea spp. into Ilha Grande Bay: evidence of tropicalization in the Southwestern Atlantic

The ocean is not a static backdrop; it is a living, shifting system where the boundaries of life are drawn by temperature. For centuries, scientists have understood that marine species have specific thermal limits, much like plants that can only survive in certain climates. When the water gets too cold, tropical creatures cannot survive; when it gets too warm, temperate species struggle. As the planet warms, these invisible lines on the map are moving. This phenomenon, known as tropicalization, describes the process where warm-water species expand their territories toward the poles, entering ecosystems that were once too cold for them. In the Southwestern Atlantic, the southeastern coast of Brazil serves as a critical meeting point between the warm tropical waters of the north and the cooler temperate waters of the south. This region is a natural laboratory for watching how climate change reshapes the sea, as even small shifts in temperature can determine whether a coral reef thrives or disappears.

In this context, a new study reports a significant shift in the southern boundary of a specific group of corals. Researchers have documented a living population of Siderastrea corals in Ilha Grande Bay, a location approximately 230 kilometers south of where these corals were previously known to exist. For decades, the southern limit for this genus of coral in Brazil was considered to be the region around Arraial do Cabo. The discovery of a healthy, reproducing population so far south suggests that the ocean is warming enough to allow these tropical organisms to establish themselves in waters that were once too chilly. This finding is not just about a single species moving; it is a tangible sign that the thermal boundaries of the Southwestern Atlantic are shifting, potentially reshaping the entire community of life along the coast.

The researchers, Guilherme Luiz Garcia Pereira and Luís Felipe Skinner, focused their investigation on Ilha Grande Bay, a transitional zone where tropical and temperate waters mix. To understand the significance of their discovery, they first looked at the history of the water temperature in the area. Using a continuous record of sea surface temperatures spanning forty-five years, from 1982 to 2025, they observed a clear and steady warming trend. The average temperature of the water has risen significantly over this period, increasing at a rate that, while seemingly small in a single year, accumulates to a substantial change over decades. This long-term warming provides the necessary environmental context for the coral's arrival, suggesting that the water has finally become warm enough to support a population that was previously restricted to the north.

During field surveys conducted in June 2026, the team explored eight different locations within the bay, diving to depths between two and five meters. They found that the corals were not scattered randomly but were concentrated in a specific shallow rocky reef at a place called Sítio Forte Cove. In this single location, they counted 295 individual coral colonies. The density of these corals was surprisingly high, with nearly twelve colonies found in every square meter of the reef. This is a robust population, far denser than what has been recorded in other parts of the Brazilian coast, indicating that the conditions at Sítio Forte are not just suitable for survival, but for thriving.

The structure of this new population offers clues about its history. The researchers measured the size of every colony they found and discovered that the vast majority were quite small. More than sixty-five percent of the colonies were less than four centimeters in diameter, which corresponds to the size of young, juvenile corals. The size distribution was heavily skewed toward these smaller individuals, with very few large, old colonies present. This pattern suggests that the population is relatively new. If the corals had been there for a long time, the researchers would expect to see a mix of sizes, including many large, mature colonies. Instead, the dominance of small colonies points to a recent establishment, likely occurring within the last decade or so, coinciding with the period of most rapid warming in the region.

The environment where these corals live also tells a story. The reef at Sítio Forte is covered mostly by turf algae and sand, rather than the massive coral structures seen in tropical reefs further north. The new Siderastrea colonies are growing alongside a type of sea anemone called Palythoa caribaeorum, which is common in shallow, warm waters. The researchers noted that the corals are growing in areas where the water is shallow and where sediment from nearby streams and mangroves settles. This is significant because Siderastrea corals are known for their ability to tolerate sediment and variable conditions, traits that would be essential for surviving in a transitional zone like Ilha Grande Bay. The fact that they are establishing themselves in such a dynamic environment suggests they are well-adapted to the challenges of this new frontier.

While the study confirms the presence of the genus Siderastrea, the researchers are careful to note that identifying the exact species requires further genetic analysis. However, based on the known distribution of the species in Brazil, it is highly probable that these are Siderastrea stellata, a species known for its resilience to colder upwelling waters. The historical record of this species in the region ends at Arraial do Cabo, making this new population a clear extension of its range. The study also addresses a historical record from 1971 that mentioned the presence of these corals in a nearby area, but that old record was vague and lacked evidence of a living population. The current findings are different: they document a dense, living community that has clearly taken root.

The connection between the warming ocean and the arrival of these corals is strong, though the study does not claim to prove a direct cause-and-effect relationship with absolute certainty. The timing aligns perfectly: the long-term rise in water temperature matches the window of time when the coral population likely established itself. The researchers suggest that the warming waters have removed the thermal barrier that once kept these corals from moving south. As the ocean continues to warm, it is likely that this population will persist and potentially expand further, bringing with it a suite of tropical species that could alter the composition of the local marine communities.

This discovery serves as a concrete example of how climate change is actively rewriting the map of marine life. It is not a theoretical prediction but a visible reality occurring in the waters off Brazil. The presence of nearly three hundred coral colonies in a bay that was once too cold for them is a testament to the power of environmental change. As the Southwestern Atlantic continues to warm, the boundaries between tropical and temperate ecosystems will continue to blur, and the communities of life within them will change in response. The story of the Siderastrea in Ilha Grande Bay is a single, clear chapter in a much larger narrative of a warming world, where the ocean's inhabitants are forced to move, adapt, or disappear.