The Hatta-mimizu Earthworm Derby

By Hiroyuki Watanabe (Professor emeritus, Kyoto University)

The earthworm Hatta-mimizu (Drawida hattamimizu) is said to reach five feet in length and is the largest worm in Japan. This worm was first collected at the Hatta village near Lake Kahokugata (Ishikawa Prefecture) along the Japan Sea coast and described as new species in 1930. At present, in addition to lake Kahokugata, three sites; Lake Biwa, Lake Yogo (Shiga Pref.) and Mikatagoko (Fukui Pref.) are recognized as its habitat. This worm mainly inhabits the ditches of paddy fields near the lakes. Farmers dislike this worm because of burrows into ditches causing leaking, but fishermen use it as bait for eel fishing in the lakes. A characteristic of this worm is that it lengthens rapidly when suspended.

The Lake Biwa Museum (LBM) held a competition named “Hatta-mimizu Derby” in 2013, a size contest, inviting contestants to provide photos of long worms held against a measuring tape for judgement by size. The winner proved to be 80 cm long in this contest.

After this event, the Lake Kahokugata Research Institute (LKRI) insisted that the largest worm would surely be found at Lake Kahokugta, as the type locality. Thus, LBM and LKRI agreed to hold the “Derby” again for one year from June 2015. The event kickoff was widely reported by TV and newspapers to bring attention of this worm to a wider public audience. As a scientist and the chairman of the committee of the derby, I look forward to seeing larger worms from unknown habitats.