A sea without a shore
The Sargasso Sea is the only sea on Earth with no land boundaries. Instead, its borders are formed by four major ocean currents that create a massive, slowly rotating vortex called the North Atlantic Subtropical Gyre. The Gulf Stream marks its western edge, the North Atlantic Current its northern, the Canary Current its eastern, and the North Atlantic Equatorial Current its southern boundary. This gyre contains a vast, calm body of water roughly 3,200 kilometers long and 1,100 kilometers wide, covering about two million square miles of the open ocean.
The sea's defining feature is the large quantity of floating seaweed that gives it its name. The two dominant species, Sargassum natans and Sargassum fluitans, are holopelagic, meaning they spend their entire life cycle floating in the open ocean. They reproduce vegetatively by fragmentation, never attaching to the seafloor like other seaweeds. Small, berry-like gas bladders called pneumatocysts keep the algae buoyant, allowing it to form enormous floating mats. These mats, sometimes called the "golden floating rainforest," holds a total biomass estimated in the millions of tons. The water within the gyre is exceptionally clear, with underwater visibility up to 60 meters, and has a characteristically deep blue color.
A critical ocean nursery
The floating Sargassum is a unique and important ecosystem in the otherwise nutrient-poor open ocean. These golden mats provide habitat, shelter, and food for a wide array of marine life. Over 100 species of invertebrates and at least 127 species of fish have been recorded living within the seaweed. Several species, like the Sargassum crab (Planes minutus) and the Sargassum snail (Litiopa melanostoma), are endemic, found nowhere else on Earth.
One of the most specialized residents is the Sargassum fish (Histrio histrio), an anglerfish perfectly camouflaged to resemble the seaweed. It has modified, prehensile fins that allow it to climb through the tangled algae as it hunts for small fish and shrimp. The Sargasso Sea also is a nursery for juvenile sea turtles, including loggerhead and green turtles, which find protection and food among the mats during their formative years.
Perhaps most famously, the Sargasso Sea is the only known spawning ground for both the endangered American eel (Anguilla rostrata) and the critically endangered European eel (Anguilla anguilla). These catadromous fish spend their adult lives in freshwater rivers thousands of kilometers away before migrating back to the Sargasso Sea to reproduce and die. After hatching, their tiny, transparent larvae, called leptocephali, drift on ocean currents for one to three years to reach the coasts of North America and Europe.