Densities of Large Marine Gastropods in Seagrass, Oyster Reef, and Sandy Habitats in Tampa Bay and along the Gulf Coast of Florida

Abstract

Marine gastropods in Florida are harvested by the marine-life trade, tourists (for shells), and traditional commercial fisheries, but stocks are not formally assessed for most species. The main goal of this project was to estimate biological variables that may be used to evaluate the Species of Greatest Conservation Need status of the banded tulip Cinctura lilium and six other large marine gastropod species that are commonly encountered in the targeted habitats. The principal target, C. lilium, was listed in the 2012 State Wildlife Action Plan. The densities and size structures of C. lilium and six co-occurring species of large marine gastropods in Tampa Bay were examined to evaluate population dynamics, seasonal variation, and habitat preferences of these species. Long-term trends in the densities of large, predatory marine gastropods in 10 regions from 2009 to 2017 were included. In Tampa Bay, strong habitat preferences were observed for all species studied: C. lilium and Melongena corona were most likely to be observed on oyster reefs, and Fasciolaria tulipa was found only in seagrass, whereas Strombus alatus, Triplofusus gigantea, Sinistrofulgur sinistrum, and Fulguropsis spirata were more common on soft sediments. Seasonal variations in densities were weak; seasonal differences in size structure were more pronounced. The smallest individuals of C. lilium appeared in spring and summer; F. spirata, S. alatus, and S. sinistrum reached a detectable size in fall. The smallest detectable M. corona appeared in winter. Regionally, there were more gastropods in southwestern bay grass beds (Tampa Bay, Sarasota Bay, and Pine Island Sound) than in coastal seagrass beds (Big Bend, Citrus, Hernando, Pasco, and coastal Pinellas) or northern bays (St. Joseph Bay and St. Andrew Bay), and each region had characteristic species. As a group, tulip snails exhibited modest local declines in densities at two sites, whereas the whelks and T. gigantea varied annually but did not exhibit consistent trends during 2009 to 2017. The present study establishes a baseline of abundance for seven species of common marine gastropods and recommends expanded monitoring in multiple habitats to improve future analyses of abundance trends.

Age and growth of one of the world’s largest carnivorous gastropods, the Florida Horse Conch, Triplofusus giganteus (Kiener, 1840), a target of unregulated, intense harvest

“…giganteus , which fell from a peak of 14,511 individuals in 1996, to 6,124 in 2000, 1,461 in 2015, and just 67 in 2020 [ 10 ], despite an increase in the number of fishing trips over roughly the same period [ 6 ]. Average densities of individual horse conchs surveyed along the west Florida coast from 2015 to 2017 were also lower than in Yucatán, Mexico, where the species is already considered to be overfished and in danger of local extinction [ 10 , 11 ]. Even in northwestern Florida, where horse conch densities are highest and populations appear to be stable [ 11 ], there is evidence for a substantial, post-1960s decrease in mean size [ 12 ].…”

“…Average densities of individual horse conchs surveyed along the west Florida coast from 2015 to 2017 were also lower than in Yucatán, Mexico, where the species is already considered to be overfished and in danger of local extinction [ 10 , 11 ]. Even in northwestern Florida, where horse conch densities are highest and populations appear to be stable [ 11 ], there is evidence for a substantial, post-1960s decrease in mean size [ 12 ]. Size decline is a common early warning signal of population collapse in overexploited fisheries [ 13 ].…”

“…Based on its large size, T . giganteus has been predicted to have a life span of many decades [ 11 ]. Given that an individual female can also produce up to 400 capsules, each with 70 hatching juveniles per capsule [ 14 , 15 ], high lifetime reproductive output may help buffer the species from harvest pressure [ 11 ].…”

“…giganteus has been predicted to have a life span of many decades [ 11 ]. Given that an individual female can also produce up to 400 capsules, each with 70 hatching juveniles per capsule [ 14 , 15 ], high lifetime reproductive output may help buffer the species from harvest pressure [ 11 ]. However, some large, tropical marine gastropods, such as the Queen Conch Aliger gigas (Linnaeus, 1758), grow rapidly and reach sexual maturity at just over three years in age [ 16 , 17 ] and have an average maximum age of around a decade [ 18 ].…”

“…giganteus specimen from the Florida Keys found that its 340 mm length shell recorded just six annual δ 18 O cycles [ 21 ]. Given that the record size for the species is a shell 604.8 mm long measured from apex to siphonal canal tip and, like many organisms, had slow growth in the latter part of its life, the previous lifespan estimate of “many decades” [ 11 ] is not unreasonable.…”