Paper Type

Master's Thesis


College of Arts and Sciences

Degree Name

Master of Science in Biology (MS)



NACO controlled Corporate Body

University of North Florida. Department of Biology

First Advisor

Dr. Eric Johnson

Second Advisor

Dr. Matthew B. Ogburn

Third Advisor

Dr. Adam Rosenblatt


In top-down regulation of ecological communities, predation of grazers is critical for maintaining the presence and growth of essential vegetation. The periwinkle snail (Littoraria irrorata) is a ubiquitous grazer in Atlantic salt marshes that can defoliate patches of Spartina alterniflora when populations are extremely dense. On the east coast of Florida, multiple predators could contribute to periwinkle population control maintaining this critical habitat. This study aimed to determine if the blue crab (Callinectes sapidus) is the primary predator of periwinkle snails in the salt marsh lower intertidal. Tethering was utilized to assess if periwinkle mortality is related to distance, predation strategy, and shell size. Groups of periwinkles (n = 5), were tethered in place at 0 m, 1 m, 3 m, and 5 m into the marsh for 24 hours. The presence or absence of each snail was recorded, and predated snails were classified by predation strategy (crushing, cutting, or extracting) using the characteristic patterns of snail remains. Key findings were: (1) Likelihood of predation for tethered snails declined exponentially as distance into the marsh increased, suggesting that the interior serves as a spatial refuge for periwinkles. However, predation was equally likely at 1 m, 3 m, and 5 m. The data also support that large crabs are limited to the marsh edge, while small crabs can forage within. (2) The crushing predation strategy was most prevalent across all distances, indicating that blue crabs are the dominant predator. (3) For all snails mortality was highest at the marsh edge, but across the interior larger snails generally had lower mortality than smaller snails. This size refuge favors larger snails and corresponds with blue crab size selectivity. Together, the observed trends in mortality, predation strategy, and size selectivity support the assertion that blue crabs are the dominant predator in the lower intertidal.