The Gymnogobius heptacanthus and Chaenogobius annularis belonging to the Perciformes, Gobiidae, mainly distributed in the coastal waters of Korea, Japan and China. The gobid fishes is known to 212 genera, 1,875 species distribution in the worldwide, of which 27 genera, 59 species in Korean waters. This species occurred commonly in seagrass meadow.
Fish samples consisting of a total of 229 G. heptacanthus (from 1.4 to 2.9 cm SL) and 281 C. annularis (from 1.3 to 1.8 cm SL) were collected monthly during from May 2011 to August 2011 using RN80 net. All samples were fixed immediately with 5% formalin after collection. Guts were then removed, and the gut contents were identified under a dissecting microscope. Diets were quantified by frequency of occurrence (%F), percentage of number (%N) and volume (%V), and index of relative importance (IRI). Index of relative importance (IRI) is calculated by Pinkas et al. (1971) equation. To assess the feeding habits of G. heptacanthus, we used the graphical analysis proposed by Amundsen et al. (1996). A total of 229 G. heptacanthus examined, 208 individuals contained food items and 21 were empty (9.17%). Of the prey items consumed, the major food item is copepods constituted 98.08% in frequency of occurrence, 77.14% in numbers, 90.20% in volume and 96.74% in IRI. Barnacle larvae were the second largest dietary component. A total of 281 C. annularis examined, 250 individuals contained food items and 31 were empty (11.03%). Of the prey items consumed, the major food item is copepods constituted 68.40% in frequency of occurrence, 44.11% in numbers, 52.89% in volume and 61.61% in IRI. Barnacle larvae were the second largest dietary component. Graphical analysis of the diet composition showed that G. heptacanthus and C. annularis is an opportunistic and specialized predator characterized by strong individual feeding specialization. Copepods are located in the upper right corner of the diagram, suggesting a specialization for copepods by the G. heptacanthus population, and for copepods and barnacle larvae by the C. annularis.