Invertebrate Grazing on Live Turtlegrass (Thalassia testudinum): A Common Interaction That May Facilitate Fungal Growth

Background/Question/Methods

In terrestrial systems, grazers can have large and diverse effects on plant communities. Studies across multiple habitats have demonstrated grazers’ ability to influence primary production, leaf abscission rates, nutrient content in leaves, flowering rates, colonial propagation, as well as plant diversity and succession. Mechanisms of control can be direct, through the consumption of plant tissue, and/or through myriad indirect pathways. One powerful indirect pathway of grazer control on plant growth is facilitation of microbial infection in live plant tissues via grazer wounding. In coastal wetlands and tropical reefs, snail grazers have been shown to similarly regulate foundation species by feeding on marsh grasses and hard corals. In many cases, their impacts are amplified because they facilitate microbial infection in grazer-induced wounds. However, whether snails commonly graze live plants and facilitate microbial growth on plants in tropical seagrass systems is less explored. Given the importance of seagrass as a foundation species in the Caribbean and evidence that invertebrate grazers can have cryptic but strong top-down control in other marine systems, we investigated if commonly abundant snails graze live turtlegrass (Thalassia testudinum) at multiple, dispersed Caribbean sites and if so, whether resulting scars promote fungal infection in live seagrass blades.

Results/Conclusions

Our initial survey showed the occurrence of snail-induced scarring on live turtlegrass blades was common, with 57% of live leaves scarred. Feeding trials demonstrated that two of five common snails (Tegula fasciata–smooth tegula and Smaragdia viridis–emerald nerite) grazed unepiphytized turtlegrass blades and that smooth tegula abundance had a positive relationship with scarring intensity. Subsequent surveys at three Caribbean sites (separated by >150 km) also showed a high occurrence of snail-induced scars on turtlegrass blades. Finally, simulated herbivory experiments and field observations of a turtlegrass bed in Florida, United States suggests that herbivore damage could facilitate fungal growth in live seagrass tissue through mechanical opening of tissue. Combined, these findings reveal that snail grazing on live turtlegrass blades in the Caribbean can be common. Based on these results, we hypothesize that small grazers could be exerting top-down control over turtlegrass growth directly via grazing and/or indirectly by facilitating microbial infection in live seagrass tissue. Further studies are needed to determine the generality and relative importance of direct and indirect effects of gastropod grazing on turtlegrass health.