To investigate the response of the fish community structure to a natural disturbance in their habitat, fish abundance, biomass and species composition were analysed in relation to temporal variability of environmental conditions in a seagrass Zostera marina bed. A total of 3024 fishes belonging to 46 taxa (22 families) were collected by quantitative sampling for 10 years from 2007 to 2016 in the Seto Inland Sea, south-western Japan. Seagrass shoot density decreased to less than 1/20 of its original density after disappearance of vegetation caused by heavy rain in the autumn of 2011 and the area did not recover for the next five years. In order to analyse temporal changes of fish community, the fishes were divided into three groups depending on their habitats or lifestyles: pelagic or migratory species (PM), sand or mud bottom-associated species (SM) and seagrass (Z. marina) – or substrate (rocky bottom including macrophytes) – associated species (ZS). Multiple regression analysis showed seagrass shoot density had the most significant effect on biomass of ZS among the three groups, with higher fish biomass under higher seagrass shoot density. Fish community composition changed after the disappearance of the seagrass vegetation coverage with an increase in abundance of SM during the five years of the post-disturbance period. Seagrass vegetation was concluded to affect temporal change of fish community structure through a stronger influence on fish species that are more dependent on seagrass beds as habitat.

We collected 138 Kumamoto like oyster specimens in and around the Nakatsu tidal flat to monitor species diversity. The mitochondrial large subunit of ribosomal DNA (LSrDNA) and the nuclear ribosomal internally transcribed spacer-1 (ITS-1) region were amplified by polymerase chain reaction and restriction fragment length polymorphism analysis, leading to the discovery of the Kumamoto oyster Crassostrea sikamea in the Nakatsu tidal flat. A more accurate species identification using the nucleotide sequences for cytochrome c oxidase subunit I and ITS-1 confirmed the presence of the oysters. These results provide the first evidence of C. sikamea in the Seto Inland Sea of Japan.