Sexual dimorphism in the red morwong, Cheilodactylus fuscus
1994
Schroeder, A. | Lowry, M. | Suthers, I. (New South Wales Univ., Kensington (Australia). School of Biological Science)
The red morwong, Cheilodactylus fuscus Castelnau (Pisces : Cheilodactylidae), is a conspicuous fish on the rocky reefs of the eastern Australian coast. Visual determination of the sex of individuals in the field is necessary to remove sex bias in home-range and associated bioaccumulation studies. A sample of 61 fish (30-50 cm fork length) was collected from spear-fishing competitions in the Sydney area, and from each fish 16 morphometric measurements were taken. A multivariate analysis revealed that, in comparison with females, males are generally larger, with a significantly larger orbit-tubercle - or horn - situated in front of each eye, a smaller eye diameter and a smaller head length. A discriminant function analysis derived from these four features correctly identified 87 percent of an independent sample of 40 fish. Visual appraisal of fish on the basis of the proportional length of the horns misidentified less than 10 percent of fish less than 30 cm and less than 30 percent of fish 20-30 cm fork length. The obit-tubercle in males is between 2 percent and 4 percent of the fork length and is composed of bone. According to this simplified visual technique, red morwong aggregations in the Sydney region have consistently biased sex ratios.
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