Planktonic larvae of the rare spoon worm, Ikedosoma elegans (Annelida: Thalassematidae), collected from Tanabe Bay, Wakayama, Japan

Human activities, such as coastline development, water pollution, and the introduction of invasive species, threaten mudflat macrofauna. Many spoon worm species are endangered, possibly because of anthropogenic disturbance of mudflat environments. Understanding worm ecology is necessary for conservation. However, information regarding these species is limited, presumably because of difficulties in sample collection. In Tanabe Bay, Wakayama Prefecture, Japan, we attempted to collect and identify planktonic larvae instead of adults, using DNA barcoding to determine rare species. A larva of the rare spoon worm, Ikedosoma elegans, endemic to Japan, was collected. Thus, the existence of an unknown population of I. elegans likely exists. Furthermore, the use of larval monitoring for detecting rare marine invertebrates is demonstrated.