CARBONATE SEDIMENT PRODUCTION BY THE ROCK-BORING URCHIN 

 ECHINOMETRA LUCUNTER AND ASSOCIATED ENDOLITHIC INFAUNA 

 AT BLACK ROCK, LITTLE BAHAMA BANK 



Charles M. Hoskin and John K. Reed 

 Marine Geology Department 

 Harbor Branch Foundation, Inc. 

 Fort Pierce, Florida 33450 



ABSTRACT 



Bioerosion studies were conducted during four cruises in 1982-83 to Black 

 Rock, a 405 m long j_sland of carbonate eolianite on Little Bahama Bank. The 

 urchin population (X = 37 adults m~2, 92 x 10^ total) bores in a 6 m wide zone 

 at depths of 0.5-3 m. SCUBA divers using chisels collected rocks with urchins 

 in their bore holes and similarly sized rocks without urchins and placed them 

 separately in 20 1 buckets with 0.0625 mm screen-walls for 2 days (13 usable 

 measurements). Urchins produced spherical to elliptical pellets 1-2 mm in 

 diameter. Disaggregated pellets contained no particles > 1.00mm. The pellets 

 were 46% (wt) unimodal sand, with a modal grain size between 0.125-0.177 mm, 

 and 54% mud. Urchins produced an average of 242 mg sediment urchin"! day-1 

 (dry wt), which is equivalent to 8.9 g m~2 day~l, or 9 tons yr~l for the entire 

 population. Volume of individual urchin bore holes averaged 72 cm^. Calculating 

 from boring rates, the bore holes were excavated in an average of 2.9 years. 



Rocks without urchins (controls) produced an average of 0.5 mg organic- 

 free sediment cm _ 2 day"! (dry wt). These particles were produced by bioerosion 

 of the infauna, which included eunicid and sipunculid worms, sponges, Lithotrya 

 barnacles, pelecypods, and microborers. Inorganic sediment weight was correlated 

 (r = 0.968, p < 0.05) with surface area of the control rocks. Infauna from 

 control rocks produced 5.0 g m _ 2 day - !, equivalent to 6.5 tons yr _ l, for Black 

 Rock. 



INTRODUCTION 



Bioerosion has a two-fold role in coral reefs; hard substrate is removed, 

 creating cavities, and some of the former substrate becomes sedimentary particles 

 that may be transported to new environments (Hopley, 1982). We have been 

 studying the area surrounding Black Rock, a small Pleistocene eolianite island 

 on the southwestern Little Bahama Bank, in order to identify and measure sources 

 of sediment, to follow dispersal pathways, and to measure the rate at which the 

 sediments accumulate. 



Bioerosion by the urchin Echinometra lucunter (Linnaeus) has been studied 

 previously in Barbados (McLean, 1967), Bermuda (Hunt, 1969), and the Virgin 

 Islands (Ogden, 1977). Another species, E. mathaei , has been studied in Persian 

 Gulf reefs by Shinn (p. 39, in Hughes Clarke and Kei j , 1973) and Enewetak 

 (Russo, 1980). The purposes of this study are to determine the rate of sediment 

 production and cavity formation by E. lucunter using a new method, to characterize 



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