Eastern Tropical Pacific (Playas del Coco, Costa Rica), it was noted 

 that Diadema mexicanum often foraged during dark hours and spent 

 daylight hours in precisely the same individual locations in crevices, 

 holes or spaces under rocks. These observations were based on 13 D. 

 mexicanum , tagged with individually numbered FD-67 Floy Tags, over 

 an 18-day period (1-18 March, 1970). During our stay in Tektite, 

 we were interested in determining how far Diadema antillarum travels 

 at night, in what kind of path and whether consistent patterns of 

 movement could be discerned. We could then find areas possibly 

 ungrazed by Diadema and compare the algal flora with that in grazed 

 areas. We also cleared two 16 m areas of urchins, one on the reef 

 itself and the other in a patch reef area, in hopes of producing 

 changes in the flora experimentally. 



Twenty Diadema antillarum were individually numbered _in situ with 

 FD-67 Floy Tags on the second day of the mission. We were sur- 

 prised to find, in contrast to the data for Diadema mexicanum , no 

 strong evidence for a homing tendency. After 18 days, 3 of the 

 Diadema had returned to the same hole or crevice for each daylight 

 observation. Within their respective crevices, however, their 

 location varied greatly, e.g., the upper left wall, the rear right 

 lower corner, the lower left just inside, etc. At night these 

 urchins were seen to range as far as 50 cm from their respective 

 crevices, but no pattern could be discovered in the direction or 

 distance traveled with any of the urchins. A fourth Diadema re- 

 turned to the same hole for four days, disappeared for two days, 

 and was back for the next four days, after which it was never 

 seen again. Three Diadema each spent the first day in the crevice 

 in which they were tagged, then spent the next day in holes under 

 coral 21 cm, 28 cm and 30 cm away, respectively. Two disappeared 

 by the next day but the individual which had traveled 28 cm the 

 first night spent the next two days in the same hole, spent a third 

 day 10 cm away from the new hole, and was then never seen again. 

 The thirteen remaining tagged Diadema were not found after intensive 

 searching. 



Sixteen square meter areas of coral reef and patch reef were each 

 cleared of Diadema antillarum , A total of 137 and 61 were removed, 

 respectively. Within 7 and 5 days 61 new Diadema had invaded the 

 coral reef area while 19 had invaded the patch reef. This con- 

 stitutes 44.5% and 32,1% of the original population densities (cf., 

 Randall et al . , 1964). This further indicates that Diadema does 

 not return to the same crevice each night in the Caribbean. 



Similar observations were made for Eucidaris tribuloides and 

 Echinometra viridis . Individuals of these species would leave their 



VI-66 



