Physiological response of symbiotic polychaetes to host saponins

Two species of the genus Arctonoe occur as host-isolated populations in the tidal and subtidal areas of Puget Sound. A saponin has been isolated from Evasterias troschelii (Stimpson) which has deleterious physiological effects on Arctonoe pulchra (Johnson), a species not associated with the asteroid. This same substance has no adverse effect on the viability and only a slight positive effect on the respiration of Arctonoe fragilis (Baird), the normal companion of Evasterias. These results suggest a possible mechanism for the strict isolation of these polychaetes and provide additional evidence for the biological significance of asterosaponins.