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. , Okinawa, Japan. This is the first reported instance of a symbiosis between heterospecific ophiuroids. In the sense that the juveniles are brooded by another species, they may be regarded as "brood parasites. " There is no evidence that the symbionts physically damage the host, although they may steal its food. 0. 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 0. scolopendrina being involved. There is generally one juvenile, rarely two, on a host. The body size of the host may limit the maximum size of its symbionts. The ontogenetic transition of 0. annulosa from a symbiotic to a free-living life style involves marked changes in integument, arm spine morphology, tube foot structure, disk armament, and pigmentation pattern. Adults of Ophiocoma scolopendrina harbor conspecific juveniles as well, a relatively rare behavior among ophiuroids, but only juveniles of 0. annulosa occur in its bursae, the respiratory structures that in brooding and viviparous ophiuroids also serve as 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. Additional key words: brood parasitism, juvenile adaptations, Amphipholis squamata, Ophiomastix spp. , Ophiocoma spp. Meager information exists on the postlarval period of the ophiuroid life cycle, when, as in other marine benthic invertebrates, mortality commonly exceeds 90% (Hendler 1991; Gosselin & Quian 1997). Several studies of juvenile ophiuroids suggest that characteristics of their microhabitat, and in some cases the presence of adult ophiuroids, can be crucial to their survival (Hendler 1991). Some juvenile ophiuroids are abundant in algal "refuge" substrates, where the adults and juveniles of brooding species co-occur (Hendler & Littman 1986). Adult epizoic ophiuroids and their young often occupy the same sponge and coral hosts (Clark 1976; Hendler 1984). In addition, the juveniles of some broadcast-spawning species cling to conspecific adults and thereby become established in a suitable milieu (Hendler 1991). We describe an unexpected association between jua Author for correspondence. E-mail: hendler@nhm.org b Present address: Lake Biwa Museum, 1091 Oroshimo-cho, Kusatsu-shi, Shiga-ken 525-0001, Japan. veniles of Ophiomastix annulosa (LAMARCK 1816) and adults of Ophiocoma scolopendrina (LAMARCK 1816). 0. scolopendrina is abundant and widespread throughout most of the Indo-Pacific region. Its symbiont, 0. annulosa, has a narrower range, from the Maldives (Indian Ocean) to the South Pacific region (Devaney 1970; Clark & Rowe 1971). 0. scolopendrina differs from other Ophiocoma and Ophiomastix species in its restriction to sheltered intertidal habitats (Sloan et al. 1979; Liao & Clark 1995). We report that the symbiotic stage of 0. annulosa is also intertidal, although the adults and those of its congeners typically occur subtidally (Devaney 1978; Liao & Clark 1995). This is the first account of heterospecific symbiosis between ophiuroids. The relationship between 0. annulosa and 0. scolopendrina is more intimate than the association between the adults and conspecific juveniles of 0. scolopendrina, and it may be regarded as a form of brood parasitism. We illustrate the ontogeny of the juveniles, which are unlike the adults in appearance, and discuss several morphological features This content downloaded from 157.55. 39.45 on Fri, 02 Sep 2016 05:31:33 UTC All use subject to http://about.jstor. org/terms Brittle star symbiosis that may be related to their symbiosis. In addition to the Ophiocoma-Ophiomastix association, we document the presence of juveniles of Ophiomastixjanualis LYMAN 1871 on adults of Ophiomastixflaccida LYMAN 1864 and report the association of juvenile conspecifics with the adults of Ophiocoma aethiops LUTKEN