Meet Marimo, the World’s Most Charismatic Algae

The words beloved and algae don’t often show up in the same sentence.* But Aegagropila linnaei is different. Velvety balls of the green algae are revered as national treasures, celebrated in religious ceremonies, and even kept as pets. 

A. linnaei is a woolly algae that grows in lakes all over the Northern Hemisphere. In most regions the algae forms dense mats on the lake bed. In a few special places it grows into colonies of huggable emerald-green orbs. The movement of a lake’s waves gently turns the algae balls as they grow, ensuring that every inch is exposed to sunlight.

Image credit: Alice, Wikimedia Commons // CC-BY-ND 2.0

In Japanese, the lake balls are called marimo (literally “bouncy ball plant”); in Ainu, the language spoken by the native people of Japan on the island of Hokkaido [PDF], they’re torasampe (“marsh monster”). In Icelandic, they’re known as kúluskítur, or “round s**t,” after the complaints of fishermen who have snared a ball in their nets.

Despite these epithets, the people of Japan and Iceland adore their lake balls. Hokkaido’s Lake Akan has been declared a national park in order to safeguard the lake's marimo colony. The marimo themselves are bigger than basketballs and hundreds of years old. More than 500,000 visitors each year make a special trip to the marimo observation center located on an island in the middle of the lake just to watch the marimo grow. (They grow very slowly—about 5 mm a year—so this can't be an action-packed event.)

Because it’s Japan, the lake ball craze has spawned a weird regional mascot. Hokkaido’s cartoon representative is Marimokkori, an anthropomorphic marimo with his very own algal erection. Yes, for real. The mascot’s name is a portmanteau of marimo and mokkori, a slang term for an erection.

Other celebrations of the marimo are more reverent. Members of Hokkaido’s native Ainu community have staged a marimo festival every year since 1950. The festival grew out of concern for the then-polluted lake and its marimo, which were being removed from the lake and kept as pets. The community held a national Marimo Amnesty Day that resulted in the recovery of 48 lake balls. An Ainu priest blessed the marimo before returning them to the lake. Now every year a number of marimo are taken from the water, cleaned, blessed, and carried through the streets. At the end of the ceremony, a priest in traditional Ainu dress rows out onto the lake and lovingly releases the marimo back to their watery home. 

Ghost shrimp on marimo. Image credit: Flickr user mobile_gnome // CC-by-ND 2.0

The kúluskítur of Iceland’s Lake Mývatn were not so lucky. Mining operations in the 1960s released phosphorus and nitrogen—two of bacteria's favorite foods—into the lake. Existing colonies of bacteria exploded into massive blooms, clouding the water and blocking sunlight from reaching the marimo on the bottom of the lake. Mining at the lake ended in 2004, but by then it was too late. By 2013, Lake Mývatn had lost its marimo. The loss may not be permanent; some scientists believe the damage can be reversed, and have begun devising plans to return the lake to its former chemical balance.

These days, you don’t have to travel to meet marimo. Aquarists have embraced smaller versions of the fuzzy algae balls as décor and oxygen sources for their fish tanks. Marimo have also made a comeback as pets, and it’s no wonder—they’re cute, fascinating, and low-maintenance. They’re also much easier to find now than they used to be. Rather than poaching national treasures, you can just buy your own on Etsy or Amazon.

*Outside of the phycology community, anyway.