Marking earthworms for release–recapture studies using the trace element rubidium

A classic method to assess animal populations is to mark a population, release them into the wild, and make measurements on individuals that are captured after a period of time. The objective of this study was to determine whether earthworms can assimilate and retain sufficient rubidium (Rb) in their tissues to differentiate marked and unmarked earthworms. Fifty adult and 50 juvenile earthworms (Aporrectodea turgida (Eisen)) were placed in individual pots with soil containing 500 mg Rb kg−1 for 1 week. Earthworms assimilated Rb at rates of 23–26 μg Rb g−1 earthworm fresh weight day−1, and the Rb concentration in earthworm tissue was 100-fold greater than in unmarked earthworms after 1 week. When we transferred marked earthworms to clean soil, they eliminated about 50% of the Rb in their tissues within 3 days. The Rb concentration declined exponentially during the elimination period, but remained 10 times greater in marked earthworms (78.6–112.4 μg Rb g−1 oven-dry tissue) than unmarked earthworms (on average, 5.7 μg Rb g−1 tissue). These results indicate that marking earthworms with Rb may be an effective way to track individuals and differentiate marked earthworms from indigenous populations in ecological release–recapture studies.