The snail Smaragdia bryanae (Neritopsina, Neritidae) is a specialist herbivore of the seagrass Halophila hawaiiana (Alismatidae, Hydrocharitaceae)

The endemic Hawaiian gastropod Smaragdia bryanae is a specialized marine herbivore that uses the endemic seagrass Halophila hawaiiana as both food and habitat. These small neritids, their grazing scars, and their egg capsules are found year-round on seagrass leaves, where they feed on protoplast contents released as the sharp outer-lateral teeth of the snail's radula puncture leaf epidermal cells; the contents of these cells are likely swept into the mouth by the long, wispy cusps of the marginal teeth. Structural differences from the typical neritid radula include elongated outer-lateral teeth with two sharply pointed cusps, delicate marginal teeth reduced in both size and number, and a compressed central section. Snails grazed on leaves of H. hawaiiana steadily in laboratory culture, and grew and reproduced on this diet. In laboratory choice experiments, snails did not graze the thalli of any of six macroalgal species growing near seagrass where snails were collected, and strongly preferred occupying seagrass. Seagrass samples from five field sites on Oahu and one on Maui showed from 30% to 94% of leaves damaged, with 11% of the total leaf standing area grazed. Snails are smaller (mean length 2.74±0.32 mm, mean width 2.15±0.17 mm, n=217) than the width of the leaves of H. hawaiiana (mean 3.24±1.26 mm, n=790). The snails associate constantly with their host, despite the scattered distribution, small patch size, and variability of the seagrass resource, demonstrated by a sevenfold range in the leaf area index (mean 1.11±0.61 cm² blade surface cm⁻², n=31) among samples. Damage on grazed leaves (mean 8.21±7.05 mm² per leaf, or 16.5% of leaf surface, n=511) is concentrated in the apical and central epithelia between the midrib and the marginal veins, where snails may access cells with thinner walls and few fibers. Details of the grazing interaction between these extant species in Hawai'i shed light on the ecological specialization of members of the genus Smaragdia to seagrasses over geological time.