Image Analysis of Coral Skeletons for Extension Rates, Calcification Rate, and Density

Measurement of coral extension rate is facilitated by x-radiography of medial slabs of coral skeletons which contain annual density banding. Densitometer analysis of film optic density with calibration to skeletal density can also provide calcification rate and density, but is time consuming and restricted to thin transects equal in width to the densitometer beam. Coral density and calcification data are useful because they provide skeletal growth information additional to extension rate. A microcomputer system has been assembled and a BASIC program has been written to rapidly and accurately obtain extension, density, and calcification measurements from x-radiograph revealed coral skeletal growth bands. The hardware consists of: an IBM-PC microcomputer, internal digitizing board providing resolution of 256 x 256 picture elements (pixels) by 256 grey levels, closed circuit TV camera, and an evenly distributed back-light source. The software (BASIC program) is written to incorporate actual scale of the x-radiograph. Initially, film optic density is calibrated to skeletal density by digitizing the image of an aluminum wedge included in the x-radiograph. Appropriate formulas (Buddemeier, 1974) and mass absorption coefficients are utilized. Next, the coral x-radiograph is digitized and a transect perpendicular to growth band boundaries is defined. The transect dimensions are selectable by the operator. Pixel values of optic density within the transect are converted to skeletal density through the wedge calibration. These values are averaged parallel to band boundaries and graphed to produce a chart of peaks and valleys representing high and low density portions within each band. The program finds midpoints of each peak and valley along the graph. Linear distance between these points for each couplet gives annual extension rate. Density is found by integration of the area under peaks and valleys. Mass or calcification rate is calculated by the product of extension and density. Extension, density, and mass are also available for subannual band portions. Depending upon skeletal growth rate, the program can analyze up to 12 years on a single digitization. The image (and transect) can also be printed on a dot matrix printer making it possible to overlap and continue transects along the entire portion of the coral desired. Conventional image analysis systems cost in the $40,000 price range. The microcomputer system described here can be implemented for less than $5,000. The method provides relatively rapid, accurate, and objective measurements of the three skeletal growth parameters: extension, density, and calcification (mass). Investigations continue into the precision and reproducibility of the technique as well as for hardware to obtain greater resolution