Seasonal fluctuation in food sources of herbivorous gastropods in a subtropical seagrass bed estimated by stable isotope analysis

Abstract

AbstractVarious herbivorous invertebrates in seagrass beds are considered to be generalists in food use and their diets may temporally fluctuate according to the availability of food sources. We assessed whether food sources of herbivorous gastropods vary in a subtropical seagrass bed in Nagura Bay, Ishigaki Island, where coexisting seaweeds grow densely in spring but minimally in summer. Abundant gastropods and their possible food sources were collected in spring and summer of 2013 and 2015, and their stable carbon and nitrogen isotope ratios were measured. Between the two seasons, each possible food source had similar isotopic values, but all the herbivorous gastropod species in summer were more enriched in13C than the gastropod samples in spring. The mixing models in SIAR (Stable Isotope Analysis in R) showed that the total contribution rates of seaweeds, i.e. rhodophytes, phaeophytes and chlorophytes, for all herbivorous gastropod species decreased from spring to summer; in contrast, the contribution rate of seagrasses increased. Linear Mixed Models showed that the seasonal variation in δ13C of the herbivorous gastropods was larger than that of the possible food sources, adding further evidence to the seasonal change in food sources of the herbivorous gastropods. This seasonal change in food use appears to correspond to the change in seaweed biomass, suggesting that herbivorous gastropods flexibly change their diets depending on food availability.