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The Impact of Crown Conch on Intertidal Oyster
Populations in Mosquito Lagoon

By: Casey Craig, Courtney Buck, Chelsea Landau, and Jordan Filipponi
Mentor: Dr. Linda Walters

Introduction

Background

Crassostrea virginica, the eastern oyster, is a gregarious, estuarine invertebrate species (Manis et al. 2015). The historical distribution of C. virginica ranges from the Gulf of Mexico to the Atlantic coast of the United States as well as the Caribbean (Byers et al. 2015). Oysters form reefs in subtidal and intertidal habitats in the United States (Johnson & Smee 2014), and both reef habitats fill an important ecological niche in estuarine ecosystems (Manis et al. 2015). Oysters improve water quality, promote denitrification, and obtain nutrients via filter-feeding (Byers et al. 2015), while oyster reefs aid in shoreline stabilization by minimizing sediment erosion and by trapping sediments (Manis et al. 2015). In addition, by providing habitat and prey for a myriad of commercially and recreationally important species, oyster reefs also help to maintain high biodiversity levels in estuaries (Byers et al. 2015).

Both vertebrates and invertebrates can be found in abundance on oyster reefs (Solomon et al. 2014, Volety et al. 2014). Wading bird species, such as the American oystercatcher, and invertebrate species including blue crabs, stone crabs, and the Atlantic oyster drill, a small gastropod snail, are all predators of C. virginica (Carroll et al. 2015; Carlson-Bremer et al. 2014; Harding et al. 2007). Oysters are also an essential food source for Melongena corona, the Florida crown conch (Bowling 1994). Melongena corona is an intertidal gastropod, identifiable by the prominent, protruding vertically–curved spines on the whorls of its shells (Ruppert & Fox 1988). Shell coloration appears in thick bands and varies from brown to purple–grey (Ruppert & Fox 1988). This gastropod can grow up to 200 mm in shell length (Kaplan 1988). Its distribution includes Alabama and the entire coastline of Florida (Karl & Hayes 2012). Individuals are temperature sensitive and are vulnerable to mortality during winter months due to decreasing air and water temperatures (Loftin 1987). It is hypothesized that M. corona bury into the sediment during winter to escape frigid temperatures (Hathaway 1957). Conch will also bury partially into the sediment during low tide to avoid exposure, only surfacing when the reefs are inundated (Hamilton 1996). The thin stripe hermit crab, Clibanarius vittatus, seeks refuge in M. corona shells once the gastropod has died. These soft-bodied creatures with no shell of their own need gastropod shells–for protection and survival (Tricarico & Gherardi, 2006). Hermit crabs are attracted to conch shells via chemical cues signaling a nearby injured gastropod (Diaz et al. 1995).

Research Question

Commercial oyster harvesters throughout Florida have long complained that M. corona competes with them for harvestable-sized oysters (Edwards 2011). This is especially true in Mosquito Lagoon where between 1943 and 2009 oyster reefs have declined by 24% (Garvis et al. 2015). Mosquito Lagoon, the northernmost region of the Indian River Lagoon system, is an estuary located on the east coast of central Florida (Parker et al. 2013). It is 54 km long and 4 km wide with two narrow outlets connecting to the Atlantic Ocean: Ponce de Leon Inlet to the north and Haulover Canal, built in 1887, to the south (Gold & Richardson 1994). This water body separates the barrier island from the mainland. A large portion of the barrier island houses Canaveral National Seashore. Mosquito Lagoon is often hypersaline in contrast to other estuaries, having salinity values equivalent to or higher than that of the Atlantic Ocean (Parker et al. 2013). This estuary contains many habitat types, primarily salt marshes, mangrove stands, seagrass beds, and intertidal oyster reefs.

In Canaveral National Seashore (CANA), shellfish licenses have been historically available to commercial and recreational oyster harvesters. For commercial harvesters, licenses cost $150 per year and allow them to collect a daily quota of 14 bushels of oysters, over 75 mm in length, per person per day. Licensed recreational harvesters can collect two 5–gallon buckets per day. Although the impact of oyster harvesting has not been explicitly studied in Mosquito Lagoon, oyster fisheries in the United States have resulted in an overall 99.7% decline in oyster population after harvesting practices began in the late 19th century (Wilberg et al. 2011).

Oyster harvesters have observed population declines in Mosquito Lagoon and have suggested to the CANA Research Management Specialist that these declines were the result of M. corona, rather than overharvesting, and that predation by large numbers of M. corona is the cause of the observed systematic declines in local oyster reef populations. To test this claim, we conducted a three-part experiment to determine the impact of M. corona on oyster populations in Mosquito Lagoon. Our study focused on 1) abundance and distribution, 2) feeding preferences, and 3) movement of M. corona.

Materials and Methods >>