200 THE BIOLOGY OF HYDRA : 1961 



HAND: I'd like first just to applaud this work and say this is 

 exactly what I was asking for, except that you must find out what 

 these nematocysts are! 



CROWELL: About how long did you wait before you retested? 



ROSS: A few hours. The anenomes, when you strip them off, 

 take at least an hour to open up and relax. The other anemones will 

 also take about an hour to settle securely on the shells. One has to 

 wait until all are open and all are settled. So several hours always 

 elapsed between the two sets of observations in our reversal experi- 

 ments. But we did several such experiments, and each time ob- 

 tained clear evidence of big differences in the threshold of nema- 

 tocyst discharge as measured by "stickiness" of tentacles to shell. 



GOREAU: There is a matter which may be important in connec- 

 tion with what Ross just said. Not too long ago we observed at a 

 depth of about 70 feet a large anemone, probably Bartholomea 

 annuhta, which has living amongst its tentacles a small red fish and 

 several shrimp of the genus Periclimcnses. This shrimp moves 

 freely amongst the tentacles, climbs around on them or hovers just 

 in front of them, waiting for small fish to come along. As soon as 

 a fish is in position, the shrimp climbs aboard and proceeds to 

 remove ectoparasites from the head and mouth. Once finished with 

 the job, the shrimp returns to its host anemone. Neither the shrimp 

 nor the commensal fish living among these tentacles excite any 

 sort of feeding reflex on the part of the anenome. The questions I'd 

 like to ask are these: "What protects these commensals against the 

 nematocysts of the host anemone? Do the nematocysts fail to dis- 

 charge into the animals at all, or are they immune to the action of 

 the nematocysts? " The observations made by Ross seem to indicate 

 that there is complete failure to discharge any menatocysts. In other 

 words, commensal animals living among the tentacles of anemones 

 can probably do so because they somehow inhibit nematocysts 

 discharge and do not trigger off any sort of feeding reflex. That's 

 the thing I don't understand, because I know that such anemones 

 react instantly to any bits of meat dropped on the tentacles. This 

 immediately sets off a feeding reaction resulting in flexion of the 

 tentacles and opening of the stomodeum. 



