NO. 6 ECHINOID DISTRIBUTION AND HABITS — KIER, GRANT 59 



strate, only small and medium-size individuals were congregated in 

 groups on the sand. Therefore, a fossil assemblage consisting of large 

 D. antillamm, along with Echinometra viridis and perhaps Eucidaris 

 tribuloides, would suggest a hard substrate, possibly a reef or rocky 

 shore. Echinometra lucunter, on the other hand, lives only in the 

 intertidal zone, so its presence would suggest either a shoreline 

 environment, or a shoal. 



Factors controlling echinoid distribution. — Three major factors 

 seem to control the distribution of the echinoid species in the area 

 studied. These are depth, substrate, and distance from shore. Other 

 possible influences, such as light penetration, wave agitation, current 

 direction, water temperature, and food supply, either are functions of 

 the three major factors, or were influences which we were unable to 

 evaluate. 



Species controlled by depth. — Echinometra lucunter inhabits rock 

 just below low tide, whether along the shore of Key Largo, or on 

 exposed parts of the reef several miles from shore. Although it in- 

 habits rocky substrates, it is absent from rocks at depths greater than 

 about 10 feet. 



Species that inhabit turtle grass are controlled indirectly by the 

 depth of water. Turtle grass does not survive at depths greater than 

 about 40 feet; Lytechinus variegatus, Tripneustes ventricosus, and 

 Clypeaster rosaceus seem to be confined to waters shallower than that 

 depth. Turbidity seems not to have had great influence, as these spe- 

 cies were found in clear water as well as in the extremely murky 

 waters of Hawk Channel. 



Astropyga magnifica was found only deeper than 75 feet. Its dis- 

 tribution may depend on other factors, such as the nature of the sub- 

 strate, but depth also seems to be a major factor. 



Species controlled by substrate. — The sand dwellers, Encope 

 michelini, Leodia sexiesperforata, Clypeaster subdepressus, Meoma 

 ventricosa, and Plagiobrissus grandis are confined by their necessity 

 to burrow. They must have relatively grassless sand, where they are 

 unobstructed by the tangle of roots. The few specimens of M. ventri- 

 cosa found in turtle grass were unable to burrow, and were living on 

 the surface of the substrate, covered by objects they held onto the test 

 in the manner of Clypeaster rosaceus. 



Brissus unicolor and Echinoneus cyclostomus were found only 

 under detached pieces of rock in areas of coarse sand, and perhaps 

 their distribution is confined to such areas. However, too few speci- 

 mens were observed to be sure of this relationship. 



