LIFE HISTORIES AND GROWTH OF CORALS OVER A DEPTH GRADIENT 



Terence Hughes 

 Dept. of Earth & Planetary Sciences, 

 The Johns Hopkins University, 

 Baltimore, Maryland 21218, 

 and 

 Discovery Bay Marine Laboratory, 

 Discovery Bay P.O. Box 35, 

 St. Ann, Jamaica. 



ABSTRACT 



Results from long term monitoring of coral populations from -10m to -35m show 

 that several important life history parameters are strongly correlated with depth. 

 Typically, coral colonies tend to settle in greater numbers in shallow water, where 

 they are also more likely to be injured or killed compared to deeper sites. Colony 

 extension rates were found to be very weakly dependent on depth, with some of the 

 fastest growing colonies at -55m and -35m growing faster than many -10m specimens. 

 The net growth rate of shallow corals may be limited by higher rates of injuries. 



INTRODUCTION 



One of the best studied features of coral reefs is the variation in species 

 composition and area associated with depth gradients (see review in Stoddart 1969). 

 Less well understood is how such changes in relative abundance are brought about. 

 The number of colonies in a coral population is determined by the balance between 

 sexual (and, in some species, asexual) recruitment and colony mortality, while 

 colony size is a function of time since settlement and rates of growth and injury. 

 In the present study, these aspects of the population dynamics of five species of 

 foliaceous corals ( Agaricia agari cites , A. lamarcki , Leptoseris cucullata , Po rites 

 astreoides , and Montastrea annularis ) were studied on the north coast of Jamaica 

 over a wide depth range (-10m to -55m). I present here some of the major findings; 

 a fuller account, including species comparisons, will be presented elsewhere 

 (Hughes and Jackson, in review). 



METHODS 



2 

 Twelve lm quadrats were tied permanently to the reefs off the Discovery Bay 



Marine Laboratory between -10m and -20m ("shallow") and at -35m ("deep"). The 

 quadrats, which enclosed more than 1,000 foliaceous coral colonies were photo- 

 graphed each year from 1977 to 1980. Analysis of the photographs yielded popul- 

 ation structures, as well as rates of mortality and sexual recruitment. Coral 

 growth was measured in situ using fixed reference points (nails) and a plastic tape 

 measure, and at -55m by collection of Alizarin stained colonies. All colonies in 

 the quadrats were measured regardless of colony condition, size or ease of access. 



RESULTS 



Many important population parameters were strongly dependent on depth. The 

 size-frequency distribution of colonies of all species were dominated by small 

 corals over their entire depth range, but especially in shallow water. Large 

 colonies greater than 200cm in area constituted only 8.9% of the total colony 

 counts from -10m to -20m, compared to 17.4% at -35m. 



Depending on species, the mortality rate of whole colonies for all size classes 



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