l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



was bound in places by patches of algae. The vegetation was sparse, 

 and the topography unbroken except by small sponges and corals, and 

 by the burrows of fish which piled pebbles, shells, and echinoid tests 

 around the entrances of their burrows. 



Too few samples were obtained to establish the range of this 

 species in this area, but extensive searches at depths near 65 feet, and 

 briefer searching at 110 feet revealed no specimens. After the first 

 specimen, a juvenile, was found on the 85-foot terrace, a return to 

 that depth produced a good collection, so we infer that the species 

 at least does not live in shallower depths ; Mortensen's report indicates 

 that it does inhabit deeper waters. 



This species was found at stations 17a and 36. 



Behavior. — Mortensen (1940, p. 207) emphasized the extreme 

 rarity of Astropyga magnified in collections made by dredge hauls, 

 and correctly deduced that its distribution was spotty because indi- 

 viduals grouped together and traveled over the surface of the sand 

 in small groups. He was precisely correct. This species is highly 

 mobile; it was observed moving across the sand flats single-file in 

 groups of two to five, at a speed of approximately 3 feet per minute. 

 Only one isolated individual was found, a juvenile at station 17a ; all 

 others were in groups. 



This species moves about on its relatively short ventral spines, with 

 all other spines radiating rather uniformly, although waving slightly 

 with motion of the animal. Small fish, identified by J. E. Randall as 

 a species of the cardinal fish Apogon, swim among the spines, keeping 

 up with the echinoids as they move along (pi. 1, fig. 4). The anal sac 

 is fully inflated (pi. 1, figs. 1-5), and the iridescent blue spots along 

 the edges of the ambulacra reflect enough light to appear to glow 

 (pi. 1, figs. 2, 3, 5,). Upon the approach of danger, presumably 

 sensed by changes in light intensity, the spines group together into five 

 cone-shaped bundles, thus producing a strong and formidable defense 

 (pi. 1, figs. 1-3, 5). 



The gregarious habits, acute sensitivity to changes in light, waving 

 and then bundling of spines, are modes of behavior that recall 

 Diadema antiUarum. That species is much less mobile during the day, 

 normally remaining in groups on the open sea bottom or hidden in 

 niches in coral, sponge, or rock. D. antiUarum is active at night, 

 however, and then its behavior is yet more strikingly similar to that of 

 A. magnifica (Schroeder and Stark, 1964, p. 133). 



