The morphological characteristics of the zoeas and crabs of the false spider crab Elamenopsis ariakensis are described and their habitats are documented, with an emphasis on their symbiosis with the burrowing sea cucumber Protankyra bidentata. The 16 zoeas and 91 crabs were collected from the Ariake Sea and adjacent Yatsushiro Sea, Japan. Zoeas have no lateral spine, whereas the rostral and dorsal spines are extremely long. In all zoeal stages, the pleon consists of six segments, including the telson. Adult crabs have fused pleons, with five segments in males and three segments in females, including the telson, but six segments are found in young male crabs. All crabs were collected from P. bidentata burrows. In addition to E. ariakensis, seven commensal species were collected from P. bidentata burrows: two crabs, four bivalves, and one scale worm. The microhabitats in the burrow of P. bidentata were divided somewhat among species.