NO. 6 ECHINOID DISTRIBUTION AND HABITS — KIER, GRANT 55 



Predation of Plagiobrissus grandis by species of the helmet conch 

 Cassis has been noted above in the discussion of that species. One 

 observation led us to believe that Cassis preys upon other species of 

 echinoids as well. A specimen of Clypeaster subdepressus at station 

 57 on sand in 18 feet of water was crushed as though it had been 

 stepped upon by a heavy foot. About 6 feet away, on the surface of 

 the sand, was a live specimen of Cassis madagascariensis Lamarck 

 about 7 inches long. Possibly the large gastropod had eaten the urchin, 

 and had broken up the test by action of its foot, which was about the 

 same diameter as the echinoid. If this happened, it contrasts strongly 

 with the manner in which C. madagascariensis drills into the fragile 

 test of P. grandis by means of its radula, leaving the test unbroken, 

 and only the small circular drill-hole as evidence of its predation. 

 However, that process occurs beneath the surface of the sand, with 

 both the gastropod and the victim remaining buried. The C. subde- 

 pressus was on the surface, near where it normally lives, and the 

 Cassis shell was smooth and clean of encrusting algae except for an 

 area of about 4 square inches on its highest dorsal elevation, indicating 

 that it was out of its normal buried habitat. Until this type of preda- 

 tion by Cassis is actually observed, it must be regarded merely as an 

 inference from circumstantial evidence. 



Dead tests of the small burrowing spatangoid Brissus unicolor 

 also had the small circular hole that indicates predation by a gastro- 

 pod. As C. madagascariensis is an efficient burrower, and a known 

 predator of burrowing echinoids, possibly it also feeds upon B. 

 unicolor. 



Several dead tests of Clypeaster subdepressus were collected, in 

 which the ventral surface was almost completely excavated, and the 

 remaining rim marked by numerous short radiating scratches (pi. IS, 

 fig. 8). The organism that preyed upon the urchin was not observed, 

 but presumably it was a fish. 



RELATION OF TEST SHAPE TO LIVING HABIT 



Thin discoid species such as Leodia sexies perforata and Encope 

 michelini normally inhabit the upper part of the sand substrate. They 

 burrow to a depth of only about a quarter of an inch under normal 

 circumstances, although in some areas they were found slightly deeper. 

 Hyman (1955, p. 556) cites studies that indicate that sand dollars 

 dig deeper in stormy weather. The thicker and less completely flat- 

 tened Clypeaster subdepressus also lives very near the surface of the 



