Previous outbreaks and current abundance of corallivorous gastropods in southwestern Shikoku

Abstract

Zooxanthellate scleractinian corals (hereafter referred to as "corals") are classified within the order Scleractinia, and are widely distributed in the shallow waters of tropical and temperate zones. The steric structures formed by corals provide sheltered habitats for a variety of organisms. Additionally, corals are an important local resource, serving as scuba diving and game fishing sites, while coral reefs act as natural breakwaters. Therefore, corals have high economic value and are a target for conservation activities. One conservation issue is damage by predators. Occasional outbreaks of corallivorous gastropods have damaged coral communities around the world. In Japan, the first outbreak of a corallivorous gastropods was recorded at Miyakejima Island in 1976. Subsequently, outbreaks were continuously recorded in areas along the Kuroshio Current, starting in Okinawa Prefecture. Control programs for corallivorous gastropods have been conducted continuously since 1989, after an outbreak was confirmed in southwest Shikoku. This study summarizes the efforts taken to control corallivorous gastropods in southwest Shikoku, and reports the status of the recent corallivorous gastropod outbreak. The total number of captured corallivorous snails per year was the key variable used to represent corallivorous snail abundance prior to 2014. In 2014, a method to obtain a population density index based on the number of snails captured per dive was adopted; snail density has since been monitored in this manner. The total number of captured snails peaked in 1998, while the density index decreased after 2014. These findings suggest that the last corallivorous gastropod outbreak has ended; the population of corallivorous snail reached a normal level in 2020 in southwestern Shikoku. We sorted and identified corallivorous gastropods collected during population control activities to determine the dominant species. In Ainan, Drupella fragum was monodominant in the 1991 outbreak; although it was less dominant from 2015 to 2017, it is still the most abundant species. These results suggest that the main species causing corallivorous gastropod outbreaks is D. fragum; monitoring this species is necessary to conserve coral communities in southwest Shikoku.