Linear skeletal extension rates in two species of Diploria from high-latitude reefs in Bermuda

X-radiography was used to study annual linear skeletal extension rates of the reef-building scleractinian corals Diploria strigosa and Diploria labyrinthiformis from the high-latitude reefs of Bermuda. Coral samples for X-radiography were collected from seven localities of varying biotopes and depths around the Bermuda platform and band couplets were measured. Mean extension rates of both species were highest on inshore and nearshore reefs, gradually decreasing towards the edge of the Bermuda platform and onto the fore-reef slope. Extension rates of D. labyrinthiformis were statistically higher than those of D. strigosa at three localities, while at the other four, the rates of both species were not statistically separable. extension rates of d. labyrinthiformis were statistically higher than D. strigosa within depths of 20 m and 32 m but not statistically separable at 3 m and 6 m depths. Extension rates of both species decreased significantly with increasing depth (r²=0.92, P<0.03 for D. labyrinthiformis and r²=0.95, P<0.02 for D. strigosa). Each species showed an inverse curvilinear relationship between extension rate and depth, the rate of change (i.e. slope) being the same for each species. Comparison of extension rates of each species from Berumuda with published rates of these species from lower latitudes showed an inverse relationship between extension rate and latitude.