The Gastropod Thais haemastoma in Georgia: T. h. floridana or T. h. canaliculata?

Abstract

In the southeastern United States, the snail Thais haemastoma has traditionally been subdivided into two subspecies according to the snail‘s locale, size of shell, number and size of spines, and depth of suture. Only Thais haemastoma floridana is supposed to occur in Georgia; since Thais haemastoma canalicukta is supposedly restricted to the Gulf of Mexico region. In Georgia, specimens fitting the description of both subspecies are common. The author concurs with the conclusions of Butler (1953) and Cunter (1979) in that the subspecies nomenclature is invalid and that they are merely ecological variants. In the southeastern United States, the genus Thais (Family Muricidae) is comprised of two subspecies: Thais haemastoma floridana (Conrad, 1837) and Thais haemastoma canaliculata (Gray, 1839) which are differentiated by locale, shell length, number and size of spines, and depth of the suture (Abbott 1974). Previous studies indicate that the Florida Rock-shell Thais haemastoma jloridana ranges from North Carolina to the West Indies and along the Central American coast to Trinidad; Hay’s Rock-shell Thais haemastoma canaliculata ranges from the west coast of Florida to possibly the northern coast of Mexico (Clench 1947). The drills differ in shell length, with T. h. floridana reaching 75 mm and T. h. canaliculata reaching 105 mm. T. h. jloridana has two or more rows of weak shoulder nodules, w’hile T. h. canaliculata has two rows of strong shoulder nodules. T. h. canaliculata has a more deeply channeled suture (Clench 1947). In Georgia, Thais haemastoma is uncommon (Walker 1981, Walker et al. 1980, Hoese 1969), although it may be common in specific areas (Hoese 1969). In a recent drill survey of Wassaw Sound, Georgia (Walker 1981), Thais haemastoma was found inhabiting oyster bars in two areas: approximately 500 meters north of the mouth of Cabbage Creek, Cabbage Island, and at the Deadman Hammock area, Wassaw Island. The snails were found preying on oysters at or near the mean low water mark in the more saline areas (S x > 18) of Wassaw Sound. Due to the rarity of the snail in Georgia, it does not represent any serious threat to the oyster fishery as it does in more southern waters (Butler 1953). According to locale, the Georgia population should be T. h. floridana because T. h. canaliculata is restricted to the Gulf of Mexico. However, many of the specimens found during the drill survey of Wassaw Sound best fit the description of T. h. canaliculata. For example, 11 out of 23 shells were greater than 75 mm in length, the maximum length Manuscript received April 9,1982; accepted April 27,1982. cited for T. h. floridana. Of these, five were over 80 mm, one was over 90 mm and one was 105 mm in length. The shoulder nodule(s) varied in size and number: 22% with two strong shoulder nodules, 26% with one strong and one weak nodule, 22% with two weak nodules, and 26% with only one weak nodule. The remaining shells were heavily infested with Cliona and not enough of the shell remained for adequate analysis. Furthermore, Hoese (1969) found in Georgia (N=62) 43 mais shells over 79 mm, nine over 89 mm and two over 99 mm. The largest reached 101 mm. Chesnut (1955) found Thais reaching 83 mm in North Carolina. Unfortunately, no descriptive characteristics of the shells were given by either Chesnut (1955) or Hoese (1 969). Specimens collected by the author from Shell Island, Florida, south of Mexico Beach, Florida, were also examined. Of these (N=7), 29% had two strong shoulder nodules, 14% had two weak shoulder nodules, 29% had one weak shoulder nodule and 29% had no shoulder nodules. According to locale, they should all be classified as T. h. canaliculata; however, most, due to the absence of two strong shoulder nodules on the majority of the shells collected, fit the description of T. h. floridana. Mayr (1963) defines a subspecies as “an aggregate of local populations of a species inhabiting a geographic subdivision of the range of the species and differing taxonomically from other populations of the species” (p. 672). The occurrence of shells resembling T. h. canaliculata in Georgia and shells resembling T. h. floridana in the Gulf of Mexico region show that the populations (Gulf vs. Atlantic) are not taxonomically different. Furthermore, Butler (1953) reports finding specimens of both subspecies in Pensacola, Florida, as well as in Barataria Bay, Louisiana. Gunter (1979) reports specimens of both subspecies in Apalachicola Bay, Florida. One can explain the presence of shells resembling T. h. canaliculata in Georgia in three ways: (1) that T. h. cana2iculata was introduced into Georgia waters from the Gulf region; (2) that the Georgia population being taxonomically distinct from T. h. floridana should be given another sub183