Field observations of the rapid, interchangeable colour patterns (known as physiological colour patterns — “PCPs”) displayed by juvenile bullethead parrotfish, Chlorurus sordidus (Forsskal) were made at three sites around Lizard Island on the northern Great Barrier Reef, Australia. A previous study of the PCPs observed in this species revealed these colour patterns to be influenced by a number of simultaneously interacting variables. Of particular importance were a juvenile's size, schooling and feeding behaviour. PCPs have traditionally been thought to function for cryptic purposes within heterogenous environments. However, the studies used to demonstrate so-called “background matching” were reliant upon the subjectivity of human perception of colour resemblance. Furthermore, they failed to confirm that in the natural environment, the physical background was an influential factor in PCP determination. By quantitatively measuring the natural structural heterogeneity of the coral reef background and dividing study sites into areas of differing structural complexity, the present study aimed to determine whether the physical background was influencing PCP determination in juvenile C. sordidus. In this way it was possible to determine whether certain PCPs were associated with certain background types. In addition, the log-linear modelling technique used for data analysis enabled the relative importance of the physical background to be ranked numerically alongside other variables previously demonstrated to be influential in determining the PCPs displayed by this species. In this way, although significant, the reef structural complexity was shown to be 29, 6 and 2 times less influential in PCP determination than a juvenile's size, schooling behaviour and feeding behaviour, respectively. Furthermore, the consistency in results between the two studies of juvenile C. sordidus strongly suggests the systematic field-based approach used to study PCPs is a reliable method for obtaining quantitative, biologically important data.
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