Table 2.--Bioerosion by Echinometra lucunter. 



Method Substrate Rate of Erosion Author 

 (g urchin"! yr'^) 



Gut contents Beachrock 18.6 McLean, 1967 



Gut contents Algal ridge on 44.0 Ogden, 1977 



coral reef 



Gut contents Eolianite 128.0 Hunt, 1969 



Bucket enclosure Eolianite 88.0 This paper 



teeth from the Aristotle's lantern of the urchin had abraded tips; most spines 

 were sharply pointed, but a few had broken and re-grown tips (fig. ID, E, F). 

 It is possible that E. lucunter may use its spines in addition to teeth in 

 substrate erosion (McLean, 1967; Hunt, 1969). The rate of spine re-growth may 

 be so rapid, averaging 0.3 mm day -1 at 20°C (Davies, et al . , 1972, fig. 1, p. 

 874), that damage to spine tips is not ordinarily observable. 



Volume of bore holes for individual urchins averaged 72 crn-^ (range = 17 - 

 126). Using a measured density of 2.13 g cm~3 for the eolianite and the average 

 rate of substrate erosion by the urchins, we estimated that an average of 2.9 yr 

 was required to excavate a bore hole (range = 0.7 - 10.3 y). 



DISCUSSION 



Previous studies (McLean, 1967; Hunt, 1969; Ogden, 1977) measured only the 

 amount of sediment within urchin alimentary tracts and assumed that frequency 

 of gut turnover was once each day. Disadvantages of the gut contents method 

 are that 1) gut turnover frequency is not verified, 2) the material in the gut 

 is not necessarily produced by urchin activity and may have been ingested after 

 transport to the urchin bore hole from an al lochthonous source, and 3) not all 

 particles produced by urchin tooth and spine movements may be ingested. 



The advantages of the bucket enclosure method are that 1) all sediment 

 produced by the urchin is captured, 2) rates of infaunal sediment production 

 and the grain size of their product can be determined, and 3) contamination by 

 sediment from outside sources is minimized. There is, however, the possibility 

 of loss of some mud-sized particles through screen windows in the buckets, the 

 exclusion of drifting algal bits, and reduced water flow inside the buckets. 



Measurements of bioerosion by E. lucunter from various places are summarized 

 in table 2; there are too many variables to resolve the different rates reported. 

 To place our measurements in perspective, a comparison with other erosion 



158 



