Metabolic Gradients Along a Branch of the Reef Coral Acropora Palmata

Gradients of respiration, primary production and zooxanthellar cell division rates were measured as a function of distance from the growing tip in the dominant Caribbean shallow water reef building coral Acropora palmata. The terminal 5 cm of a branch exhibits significantly higher respiration rates (1.8×) and significantly lower net primary productivity rates (0.04×) than regions of the branch proximal to 10 cm from the tip. The section of the branch 5-10 cm proximal to the tip has metabolic rates intermediate between the distal 5 cm and the rest of the branch. Cell division rates of zooxanthellae are significantly affected by distance from the tip of the branch, the orientation of tissue in which they live (top or upward facing side of branch vs. bottom or lower facing side), and the interaction ofdistanee from tip and surface, but not by time of day (noon vs. midnight). The highest mitotic indices (x¯ = 14.7, SD = 1.4, N = 6 on the top vs. x¯ = 8.8, SD = 0.45, N = 6 on the bottom) were found in the terminal 2.5 cm of the branch decreasing to a minimum of about 4 on both surfaces 22.5 cm from the tip. Density of zooxanthellae increases with distance from tip, with values measured 2.5 cm from the tip of 0.8 × 10⁶ cells · cm⁻² on the bottom and 1.6 × 10⁶ cells · cm⁻² on the top, to those measured 22.5 cm from the tip of 2.1 × 10⁶ cells · cm⁻² on the bottom and 2.2 × 10⁶ cells · cm⁻² on the top.