Cladophora - Marimo Moss Ball.
A rather unique Algae, originally native to Lake Akan in Japan, where it is revered and held in folklore as the love plant.
Their unique shape is achieved by the rolling movement produced by the water current, and the Marimo will rise and fall within the water depending on light levels to photosynthesise.
These are truly a fish keepers dream, although they can easily be kept as a plant of interest in a glass container.
In the home they do best in clean, cold water with low lighting. Every 1 or 2 weeks change the water and gently rinse to remove any build-up of debris. Tap water is fine, and a gentle roll on your palm will help to keep the spherical shape. Average room temperatures, below 25c, and avoid direct sunlight, which will scorch them.
Make sure to rotate them regularly, so that all sides of the ball are evenly exposed to the light.
Marimo can live for many years, and are often passed on from generation to generation as a living heirloom.