STUDIES IN THE PHYSIOLOGY OF COMMENSALISM. 2. THE POLYNOID GENERA ARCTONOË AND HALOSYDNA

¹ University of California, Santa Barbara College, and David Starr Jordan Junior High School, Burbank, California

1. Water from an aquarium from which a host Evasterias troschelii has been removed 24 hours before no longer possesses an attraction for commensal Arctonoë fragilis. Similar results were obtained five hours after removal.

2. Water from an aquarium from which a host Evasterias has been removed with difficulty, so that tube feet were torn from it, has no attraction for commensals almost immediately after removal of the host. Such water appears to repel them.

3. Water from an aquarium from which a host Evasterias has been removed with comparative ease (involving less disturbance to the starfish) retains its attraction for commensals immediately after removal, but shortly thereafter the attraction appears to weaken.

4. If a host Evasterias is suspended in a test aquarium in a dialyzing bag for two hours, at the end of this time not enough attractant has passed through to attract commensals.

5. Experiments to determine whether attractant will diffuse through a dialyzing bag in effective concentrations overnight may be unsuccessful because the abnormal conditions inside the bag affect the host in such a way that the response of commensals is negated even after the host has been released from the bag into the test aquarium.

6. Washed eviscerated integument of Stichopus californicus does not appear to attract its commensal, Arctonoë pulchra.

7. Washed eviscerated integument of Evasterias does not appear to attract its commensal.

8. Washed viscera of Stichopus appears to repel its commensal.

9. Ground-up washed preparations of viscera of Evasterias appear to repel its commensal.

10. Immature Evasterias do not exert as strong an attraction on commensals as adults, even after compensation has been made for difference in weight.

11. No chemical attraction for its commensal, Halosydna brevisetosa, paralleling that observed in specific echinoderm-annelid partnerships can be demonstrated in the terebellid Amphitrite robusta, in spite of the fact that the scale worm is limited to this and one other terebellid host.

12. Commensal Halosydna are neutral to light or mildly negatively phototactic. A positive tactile response to the tentacles and body of the host was found in these worms.