Sexual Dimorphism of Flathead Mullet (Mugil cephalus) from Northern Mindanao Rivers Using Geometric Morphometric Analysis

Sexual dimorphism is biologically important for determining the morphological traits of marine species. Flathead mullet (Mugil cephalus) was examined based on the patterns of sexual dimorphism from the two rivers in Northern Mindanao. The samples were collected from Cagayan de Oro River and Mandulog River to investigate phenotypic divergence and sexual dimorphism in two different populations of wild-caught M. cephalus using geometric morphology based on landmark basis. A total of 17 landmarks were identified to describe the left and right body shapes of the specimen using the TpsRelw (version 1) to obtain the warp scores of each population. Results of the Relative Warp Analysis show no significant variation in the body shape within the sexes in the respective sampling sites. Results of the discriminant function analysis (DFA) show that in Cagayan de Oro River there is no significant variation in the body shape of mullets between sexes suggesting no sexual dimorphism in the flathead mullets in Cagayan de Oro River. In contrast , Discriminant Function Analysis show that there is significant variation in body shape between sexes in Mandulog River in Iligan City suggesting presence of sexual dimorphism in the area which can be due to the presence of abundant and highly diverse predator species in Mandulog River. Sexual dimorphosm can be used by fishes as an adaptive mechanism to maximize predator-escape performance and survival capacity. Furthermore, there is a significant shape variation between populations of mullets in Cagayan de Oro and Mandulog River which can be interpreted as due to geographic isolation which serves as physical barrier on the gene pool. In conclusion, many factors could account for variation in body shape of Mugil cephalus including predation and biogeographical barriers.