Russian Moss Balls, Chladophora aegagropila, are found in the UK, Iceland, Sweden and Japan.
This is a velvet like algae and can grow up at 30 cm in diameter. Once thought to be extremely slow growing (perhaps taking up to 200 years to reach the size of a tennis ball), recent studies indicate that growth can be accelerated by providing more nutrients.
In Japan, these algae balls are called “Marimo” and most commonly found in Lake Akan. They have been declared a “Special Japanese Natural Treasure”.
In captivity it requires very strong light and will do best in water with a high carbonate hardness (>7dKH).