What is the world’s largest single celled organism?

The green algae Caulerpa gets the title of largest single celled organism. Its can grow up to a meter long, making it nearly 10,000 times larger than most single celled organisms. Most plants this size are made up of thousands of cells.

Caulerpa are actually pretty complex. It grows as a thin filament along the sea floor and then leaf like sprouts extend upward and roots (rhizoids) sprout downward. No cell wall separates the leaves and roots from the main filament. It’s all one long cell. If you turn one upside down, within 24 hours the roots start growing downward and the leaves start growing upward. They grow fast, up to a centimeter a day.

You’ve may have seen one or more of the several species of Caulerpa. Because they are hardy and can handle colder water they are commonly used in saltwater fish tanks. Some species have fan like leaves where others have grape like buds.

In their natural environment Caulpera are kept in check by animals unfortunately they are a serous problem when introduced elsewhere. Infestations, likely caused by aquarium owners dumping into to the ocean, have been reported in the Mediterranean Sea, Australia, and California. With no natural predators they become the dominant plant life and hurt the diversity of fish and other wildlife in the area.