Diffraction patterns as a tool to recognize copepods

Abstract

Photographs of three species of calanoid copepods were digitized, binarized and edited to remove the debris present in the images and to simulate different degrees of segmentation. Twentyeight segmented images of the three species were generated after pre-processing to obtain their digital Fourier transform or diffraction pattern. Ouster analysis was carried out for the images of the copepods and for their diffraction patterns to determine which set of images discriminated genus, species and sex of the selected organisms. The dendrograms of the diffraction patterns had more power of discrimination and 50% less variability. This method is promising, but more research is necessary in order to implement an automated system for plankton identification in the near future. Introduction Even for the most competent plankton taxonomist, the correct identification of a marine microorganism can take a long time (Ortner et ai, 1979; Omori and Ikeda, 1984). In recent years, there has been a decrease in the number of competent plankton taxonomists (Simpson et ai, 1992), which could be the reason for some of the reported synonymies for calanoid copepods. Bjornberg (1981) reports that Acartia lilljeborgii is the same as Acartia denticomis and Nannocalanus minor is the same as Calanus vulgus, and Trujillo-Ortfz (1990) reports that Acartia californiensis is the same as Acartia bacorehuisensis. A broad group of researchers have been trying to implement automated systems in order to identify and count organisms in a faster and more accurate way. These systems include the Coulter-Counter (Sheldon and Parsons, 1967), the in situ Coulter-Counter (Herman and Dauphinee, 1980; Herman and Mitchell, 1981), silhouette photography (Ortner, 1979) and the video camera (Liacopoulos, 1983; Latrous, 1984; Rolke and Lenz, 1984). However, none of these systems ensure complete effectiveness of the taxonomic classification due to the orientation of the organism or the resolution of the image. Another group of researchers have begun to use coherent optical processing for the recognition of planktonic organisms (Cairns et al., 1972; Almeida et al., 1976; Jeffries et al., 1980,1984). A very important part of these techniques is the acquisition of a Fourier transform or diffraction pattern of the organism. The rapid development of computational sciences has made image-processing software available to almost everyone in order to obtain diffraction patterns and other image-processing tools. Coherent optical processing techniques were introduced by Vander Lugt (1964). The objects they used to test their systems were images of O Oxford University Press 1471 V.A.Zavala-Hamz el al. SELECTION OF ORGANISMS AND PHOTOGRAPHS DIGITAL BINARY CONTOURS AND SCANNING DIGITAL DIFFRACTION PATTERNS CORRELATIONS AND DENDROGRAMS OF ORGANISMS AND DIFFRACTION PATTERNS Complate Linkage