FACTORS IMPACTING LUCINID BIVALVE AND SEAGRASS SYMBIOSES WITHIN THE SANIBEL-CAPTIVA AREA, SW FLORIDA

Bivalves of the family Lucinidae have been shown to exist within ecologically important symbioses between seagrass, the lucinid bivalve, and the bivalve’s endosymbiotic sulfide-oxidizing bacteria. The roots of the seagrass support the clams through root radial diffusion of oxygen, while the lucinids introduce dissolved oxygen for sulfide oxidation through burrowing in the sediment. In addition, the lucinids’ chemosynthetic endosymbionts oxidize otherwise potentially phytotoxic levels of pore-water sulfides while also providing their host with a supplemental energy source.

Because seagrass beds of Thalassia testudinum, Syringodium filiforme, and Halodule wrightii play an important ecological role within Pine Island Sound, SW Florida, as well as the highly anthropogenically disturbed Caloosahatchee estuary east of the islands of Sanibel and Captiva, a survey of seagrass biomass, bivalve abundance, and sulfide porewater throughout 19 sites in the region was conducted to evaluate the importance of this symbiotic relationship. Due to the initial trend of a north-south gradient in lucinid abundance, and the importance of a north-south salinity gradient to the local ecosystem, lab salinity tolerance experiments were also conducted. These tests concluded that Lucinisca nassula (Family Lucinidae) is intolerant to very low salinities (5 PSU), but L. nassula is capable of tolerating salinities of 20 PSU and higher for time spans of at least two weeks. No significant relationships were found between pore-water sulfide levels and lucinid abundance or seagrass biomass. Seagrass bed lucinid abundances ranged between 0 and 226 individuals per square meter. The analysis of samples throughout the study area showed a positive correlation between seagrass biomass and lucinid abundance. This finding provides the possibility of lucinid bivalves being used as an indicator of seagrass health in the future and further strengthens evidence for a positive symbiotic relationship between seagrass and lucinid bivalves.