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Various 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 in 13C 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.

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Previous studies of habitat suitability of sessile organisms on subtidal rocky substrata have been focused only one or two terrain attributes. In this study, we propose a new method to construct a centimeter resolution seafloor topographic model by using underwater photogrammetry to obtain multiple terrain variables and to investigate relationships between the distribution of sessile organisms and multiple terrain variables. Point cloud models of five square sections (11.3–25.5 m2) of the bedrock surface of Otsuchi Bay were reconstructed with a 0.05 m resolution. Using the 0.01 m resolution point cloud models, five terrain variables were calculated on each face of the mesh models: height above seafloor, topological position index, slope, aspect, and ruggedness. The presence/absence data of four species of sessile organisms (ascidian Halocynthia roretzi, barnacle Balanus trigonus, polychaete Paradexiospira nakamurai, and articulated coralline algae Pachyarthron cretaceum) were located on the mesh models. H. roretzi and B. trigonus were more abundant on vertical and high faces above the seafloor, and P. nakamurai were more abundant at high faces above the surroundings. In high position where the current velocity increases, the three sessile animals may have an advantage for their suspension feeding. In contrast, P. cretaceum, unlike the other three sessile animal species, occurred at various heights and on gentle slope faces suitable for photosynthesis.