Babysitting brittle stars: heterospecific symbiosis between ophiuroids (Echinodermata)

Juveniles of Ophiomastix annulosa live on adults of Ophiocoma scolopendrina , a confamilial broadcast-spawner, in the intertidal zone of Sesoko Is., Ok inawa, Japan. This is the first reported instance of a symbiosis between he terospecific ophiuroids. In the sense that the juveniles are brooded by ano ther species, they may be regarded as "brood parasites." There is no eviden ce that the symbionts physically damage the host, although they may steal i ts food. O. annulosa of less than 4.5 mm disk diameter primarily occupy the bursae, and also cling to the arms or disk of their hosts. The association is seen year round, usually with 4.2% or fewer of O. scolopendrina being i nvolved. There is generally one juvenile, rarely two, on a host. The body s ize of the host may limit the maximum size of its symbionts. The ontogeneti c transition of O. annulosa from a symbiotic to a free-living life style in volves marked changes in integument, arm spine morphology, tube foot struct ure, disk armament, and pigmentation pattern. Adults of Ophiocoma scolopend rina harbor conspecific juveniles as well, a relatively rare behavior among ophiuroids, but only juveniles of O. annulosa occur in its bursae, the res piratory structures that in brooding and viviparous ophiuroids also serve a s brood chambers. An association between adult and juvenile individuals is documented for a Panamanian population of Ophiocoma aethiops, and evidence is presented of a heterospecific symbiosis between juveniles of Ophiomastix janualis and adults of Ophiomastix flaccida in the Philippines.