Experimental tests of snail grazing on turtle grass, Thalassia testudinum: evidence of resilience in short-term plant function to scarring of live tissues

In coastal ecosystems, the intricate interactions among grazers, microorganisms, and plants play a crucial role in shaping plant health and ecosystem dynamics. Recent work has shown that snail grazing on live seagrass tissue creates wounds that can facilitate fungal infection, suggesting a pathway by which invertebrate grazers may exert top-down control over seagrass growth. However, whether density-dependent snail grazing translates into measurable impacts on fungal infection and seagrass health under field conditions remains untested. To address this gap, we conducted field surveys and experiments to examine how fungal infection in Thalassia testudinum blades and seagrass health varies with different densities of the herbivorous smooth tegula, Tegula fasciata snail in Bocas del Toro on the Caribbean coast of Panama. Field surveys documented snail scarring on seagrass blades and that density of smooth tegula in seagrass beds was positively associated with both scarring prevalence and increased brown tissue (indicative of fungal infection) in seagrass blades. Our six-week experimental manipulation testing densities of tegula snails (0, 1, 2, 4, and 6 snails per 625 cm ² ) revealed no significant effects of snail density on seagrass metrics including biomass, productivity, shoot density, or degree of fungal colonization, despite visible emergence of radulation scars on blades. The only significant treatment effect was on total shoot number, where treatments with natural snail density (1 snail per 625 cm ² ) produced more shoots than treatments with the highest snail density (6 snails per 625 cm ² ). Although snail presence increased scarring on new growth blades relative to snail-free controls, tissue damage did not translate into detectable effects on seagrass biomass, productivity, or fungal colonization. These results suggest that T. testudinum can effectively compensate for snail grazing over short timescales, as it does with other grazers, such as turtles. Fungal biomass did not increase significantly with snail density or scarring presence, indicating that the grazer-pathogen facilitation pathway documented in other marine plant systems may not operate over short experimental timescales in turtlegrass. Together, these findings suggest a degree of ecological co-tolerance between T. testudinum and invertebrate grazers, though whether this resilience holds under sustained grazing pressure or compounding environmental stressors warrants further investigation.