Stramonita canaliculata

History of Spread

The native range of Stramonita canaliculata (Channeled Rock Shell) is somewhat uncertain, because of confusion with S. floridana (Florida Rock Shell). Both species were formerly treated as subspecies of S. haemastoma, and often simply recorded as the latter species. Stramonita canaliculata was believed to be confined to the Gulf of Mexico, while S. floridana occurred on the Atlantic coast from Florida to North Carolina (Sieling 1960; Abbott 1974). However, S. canaliculata has been reported from Georgia (Walker 1982; Hoese 1969, cited by Butler 1985). Claremont et al. 2011, found considerable geographical overlap between the two species, with S. canaliculata occurring on the Florida Atlantic coast at least as far north as the Indian River Lagoon and S. floridana occurring at locations in the Gulf to the Texas-Mexico border, and on the Yucatan Peninsula, although S. canaliculata was more common in the Gulf. They suggest that the two species may be differentiated more by ecological preferences, with S. floridana preferring more open, oceanic environments, and S. canaliculata favoring more continental, estuarine shorelines (Claremont et al. 2011).

The northern limit of the Western Atlantic range of 'S. haemastoma' was believed to be Oregon Inlet NC, just north of Cape Hatteras, but in 1955-1956, living animals and recent shells were found in Chincoteague, Hog Island, and lowermost Chesapeake Bays (Sieling 1960; US National Museum collections). Stramonita populations in these bays had been identified as S. floridana (Sieling 1960; Counts and Bashore 1991; Harding and Harasewych 2007), but Claremont et al. (2011) has re-identified them as S. canaliculata. These northern populations are considered to be probable introductions (Sieling 1960; Counts and Bashore 1991; Prezant et al. 2002), although natural dispersal of larvae from southern waters cannot be excluded. The source of these introductions could be either the Gulf of Mexico or S. canaliculata populations on the Atlantic coast.

Invasion History on the East Coast

In 1955, living specimens of Stramonita were found in Hog Island and Chincoteague Bays, extending the northward range of the genus by 240 km. Sieling (1960) considered the morphology of the shells to match shells from Louisiana, presumably S. canaliculata. Old shells, perhaps 80-100 years old were found in excavations in the now-closed Green Run Inlet, suggesting an early introduction, probably before 1883, when the inlet became unnavigable (Sieling 1960). However, this species was not found in a thorough 1913 survey of Assateague Island mollusks (Henderson and Bartsch 1914; cited by Sieling 1960). Sieling (1960) suggested that cold winters in the late 1800's and early 1900's may have reduced or eliminated early populations. In that case, the specimens found in the 1950's may have resulted from oyster transplants. In a 1988-1989 survey, S. haemastoma was rare 'with the decline of oyster populations' (Counts and Bashore 1991), but they were found again in 1994-1996 survey (Prezant et al. 2002).

Stramonita has not been reported from Chesapeake Bay proper, until recently (Andrews 1956; Wass 1972), but U.S. Natural History Museum collections include one worn specimen found at Virginia Beach in 1955. In 2005, four specimens were collected from Deep Creek, Newport News VA, on the James River, and in 2006 13 specimens were collected from Back River, a York tributary near Williamsburg VA (Harding and Harsewych 2007). Harding and Harsewych (2007) used morphological and genetic evidence to identify snails as S. floridana, but Claremont et al.(2011) re-analyzed their data, and identified the Chesapeake and Atlantic bay specimens, and their Indian River Lagoon reference specimens, as S. canaliculata.

Although larvae of the S. haemastoma complex are capable of long-distance transport, and adults have been found on logs far at sea, natural dispersal northward is unlikely because the prevailing currents near the Bay mouth run southward, while the Gulf Stream further offshore moves eastward (Bumpus and Lauzier 1965). Therefore, coastal shipping or oyster transplants seem the likeliest source of the populations in the eastern bays.

Invasion Comments