Cleaning mutualist or parasite? Classifying the association between the brittlestar Ophiothrix lineata and the Caribbean reef sponge Callyspongia vaginalis

Symbioses often exist along a mutualism-parasitism continuum, and the classification of any given relationship requires a careful examination of costs and benefits for both symbiont and host. It has been proposed that deposit-feeding by the obligate sponge-dwelling brittlestar Ophiothrix lineata on the surface of the tube sponge Callyspongia vaginalis may increase filtration efficiency resulting in enhanced sponge growth or reproduction while providing protection and food for the brittlestar. However, C. vaginalis produces large (0.5-1.4 mm) larvae that are brooded in chambers and released into the interior of sponge tubes year-round, and these larvae could be consumed by O. lineata. In laboratory experiments, brittlestars readily consumed sponge larvae. When larval traps were placed over sponge tubes in the field, fewer larvae per brood chamber were collected from sponge tubes containing brittlestars than sponge tubes that lacked brittlestars, supporting the hypothesis that brittlestars consume sponge larvae under natural conditions. Sponges with brittlestars exhibited no difference in growth or number of brood chambers compared to sponges without brittlestars after 8 months, indicating no positive effect of symbiont on host. Spatial and temporal variations in larval release by C. vaginalis likely decrease encounter rates of brittlestars with sponge larvae, reducing the negative impact on the sponge and helping to maintain the association. The available evidence suggests that, depending on the reproductive status of the sponge, the association between O. lineata and C. vaginalis ranges from commensalism to larval parasitism.