Posted: Mar 24 2011
Stramonita haemastoma floridana (Conrad, 1837), Florida Rock Shell
Update: Aug 2011
Ref: Claremont, M., Williams, S. T., Barraclough, T. G., and Reid, D. G. 2011. The geographic scale of speciation in a marine snail with high dispersal potential. Journal of Biogeography U. Biogeogr. 38: 1016-1032.
Referenced paper utilized the Stramonita haemastoma complex of shells to investigate the geographic scale of speciation in a marine snail with a long open ocean larval stage. Although not specifically focused on a comprehensive analysis to ascertain morphological and genetic speciation within the complex, the study did utilize a combination of molecular and morphological methods to establish geographical ranges and reconstruct phylogenetic relationships. As a result, the authors found support for six species within the Stramonita haemastoma complex. A pertinent finding was that, although S. rustica was validated as a species and has a wide range, that range did not include Florida. The authors found that only S. canaliculata and S. floridana occurred in Florida and, contrary to prior findings, these two species occurred on both sides of Florida with the former predominant on the Gulf side and the latter on the Atlantic. Within the Stramonita haemastoma complex of six species, the authors found that only two had a distinctive morphological feature that would distinguish it from the others. The channeled suture found in S. canaliculata was one, and the white aperture of S. rustica was the other.
Since the work reflected in the referenced paper needs to be further substantiated by a more comprehensive sampling and molecular analysis of live-collected material, I have not yet revised my presentation. However, these findings should be kept in mind; esp. regarding the absence of S. rustica in Florida. And, when dealing with a specific Stramonita from the Gulf or Atlantic coasts, if it has a channeled suture, more than likely, it may be S. canaliculata.
The above presentation did not include comparative analysis between the three Florida Stramonita because of the lack of consensus regarding distinguishing morphological characters and the uncertainty as to geographic distributions, despite information published by past authors. (And, also because I cannot claim with confidence that any of the shells in my collection are indeed S. rustica.) Following is a brief discussion of what I believe to be the best, most current findings regarding Stramonita found in Florida waters.
Stramonita haemastoma floridana, Stramonita haemastoma canaliculata and Stramonita rustica
It is rather clear from my readings and the comments I’ve received over the years that distinguishing Stramonita haemastoma floridana (SF), Stramonita haemastoma canaliculata (SC) and Stramonita rustica (SR) in Florida is difficult to impossible. The “impossible” particularly applies to juvenile shells when it comes to distinguishing SR from the others, but also applies to SC versus SF at all stages of maturity. Sculptural variation and overlap among these shells is so common that several researchers have indicated that separation is often possible only via molecular examination. Some geographic separation may help, but even this is somewhat unclear. As many researchers have commented, more definitive answers require a comprehensive survey (yet to be published). Following are some of the data and comments I’ve received applicable to these three shells.
Abbott (1974), regarding SC: “… characterized by its large size (up to 4 ½ inches in length), strongly indented suture and rugose sculpture with a row of double, strong nodules on the shoulder…” “T. haysae Clench, 1927, is a synonym.”
Harding and Harasewych (2007) in discussing SF and SC pointed out that “Several authors … had reported that differences in shell morphology are neither consistent nor concordant with geographical patterns and concluded that subspecific distinction was not warranted.”
More recently than the “Several authors” referred to by Harding and Harasewych, Liu, Foltz and Stickle (1991) also found that characters of shell and radula could not be relied upon to taxonomically distinguish SF and SC, but "A high degree of genetic differentiation between canaliculata-like and floridana-like collections was observed..." Liu et al studied shells collected from 2 south Texas, 2 Louisiana, 2 Mississippi, and 3 Florida (Pensacola Bay, Ponce Inlet and Marineland) localities. Mixed (or hybrid) populations were identified in three locations (one in Louisiana and at the two NE Florida sites). All 120 specimens from Pensacola were canaliculata-like. Although six shell characters were recorded (including suture indentation and two measures of prominance of shoulder nodules), none were consistently concordant with either of the two genetic groups and "shell morphology overlapped considerably ... between the two groups." Liu et al's geographic sample was rather disjunctive (especially for Florida), but both SF and SC were identified at some location in the northern Gulf and NE Florida and cohabitating in three. Liu et al concluded that the two “maintained their genetic differences in areas of sympatry, although rare hybrid individuals occurred.”
Harding and Harasewych (2007), in discussing records of Stramonita from Chesapeake Bay, found that morphologically these shells (larger size, deeply channeled suture, and the presence of strong, rugose shoulder nodules) matched SC, but their molecular analysis revealed they were “genetically very similar to, and in one case indistinguishable from, a population of Stramonita haemastoma floridana from southeastern Florida.” Since their comparative analysis included specimens from the northern Gulf of Mexico, they were also able to conclude that based upon their “bar-coding” data, SF was distinguishable from Azorean Stramonita haemastoma haemastoma and, in turn, the SF populations along the eastern USA were distinguishable from populations from the northern Gulf of Mexico (SC), as advocated by Liu et al.
Abbott (1974) and others distinguish SR as “always has a white aperture.” However, Redfern (2001), in discussing SR, notes that “The occurrence of shells with white or orange apertures within a single colony suggests that the interior color is an unreliable diagnostic feature.”
Harry Lee (email to Conch-L; 3/22/11): “I agree that the conchological differentiation, if it can be made, needs some refinement.”
Personal correspondence from British research group: “The species of Stramonita are exceedingly variable; each can occur in smooth or strongly nodulose forms. The ridge in the aperture is, I think, simply a sign of approaching maturity (termination of growth), as are the ‘teeth’ within the aperture. The white aperture of adults, black and white spots on primary ribs, and thick shell with prominent internal teeth are the characters that best distinguish rustica, we believe. Juveniles may simply not be identifiable.”
It appears that separation of SR from SF and SC based upon morphological characters combined with location data is possible with some reliability in mature adults. However, separation of SF and SC does not appear to be reliably possible based upon morphological characters alone, and that location data may only be partially helpful. None of the published studies have been comprehensive or specific enough in detail to ascertain where or to what degree the distributions of SF and SC overlap. Additionally, as related in personal correspondence, current molecular research (soon to be published) by a British group found “S. rustica does indeed occur in E (ie Atlantic) Florida and Bermuda (and probably Bahamas too); we have no records from W Florida and, from what we think we know of its preferences, it is unlikely to occur there.* In Florida, S. canaliculata is mainly on the W coast, floridana mainly on the E, but there are exceptions to both generalities.”
*See "Update: Aug 2011" at very start of presentation. The British group has now published and reported that although found in Bermuda, they did not document S. rustica in the Bahamas or Florida.
Abbott, R. Tucker. 1974 American Seashells, Second Edition, Van Nostrand Reinhold Company.
Harding, J. M. and Harasewych, M. G. 2007. Two new modern records of the southern oyster drill Stramonita haemastoma floridana (Conrad, 1837) in Chesapeake Bay, USA. The Nautilus 121(3): 146-158.
L. L. Liu, D. W. Foltz and W. B. Stickle. 1991. Genetic population structure of the southern oyster drill Stramonita (=Thais) haemastoma. Marine Biology, Volume 111, Number 1, 71-79.
To me on treasure coast they are rare beach finds thou you can find them as common live individuals on rocks of Jettys. To see them you have to go at low tide on thin sandy beaches then search the underside of large rocks they are the only snails of large size commonly three inches in size. The only other common snails on rocks in jettys are nerites and a tiny perwinkle that's often overharvested live.