Allopeas gracile (Hutton, 1834)

     One of our back yard snails, Allopeas gracile was said by the late H. A. Pilsbry to be perhaps the most widely-distributed terrestrial snail in the world. There are certain biologic similarities between Corbicula and this ten-millimeter Terebra-look-alike which may aid in our understanding of why this is so. Besides being an South American native, Allopeas is also hermaphroditic, but then so are the majority of the terrestrial snails in most of this range. Also important in the spread of these creatures through the tropics and temperate zones is that notion of "exoticism" we mentioned earlier. For centuries man has produced and witnessed, usually to his dismay, successful introductions of living things across once-natural barriers. As we look around Jacksonville, stark example: are numerous: water hyacinths (South America), pigeons and starlings (Europe), Fire Ants (South America), and Garden Slugs (Europe). Why is it such creatures succeed so strikingly? Firstly, they are extracted from an environment in which the population was held in balance by interaction with natural enemies. For mollusks like certain slugs and our Allopeas, these included predators such as birds, rodents, beetles, and other mollusks on the one hand, and parasites such as flat worms and protozoa on the other. When a Allopeas is brought into the United States on an South American plant, its predators are left behind, and chances are that the full array of parasites responsible for its place in its native ecosystem don't make the trip with him. Such was probably the situation when the giant African snail Lissachatina fulica (Bowdich, 1822) was introduced into the Hawaiian Islands as a food source in 1936. A few years later, its establishment there had biologists importing predators including snails like Gonaxis and our own Euglandina rosea (Férussac, 1821) and even pondering potential parasites to curb the unbridled growth and spread of this snail. It seems that Hawaii had no such creatures of its own awaiting the ill-contrived arrival of Lissachatina. I believe that most of our introduced gastropods found things in about the same shape as did Lissachatina. Fortunately nobody need fret over most of them (including our Allopeas) because they don't create the disturbance that the up-to-eight inch Lissachatina snails did (and this includes auto accidents from the slimy remains on highways).

     Another advantage enjoyed by Old World introductions from starlings to slugs was a head start in adaptation to the urban habitat. Certain animal species such as the English Sparrow and Escargot were able to adapt to human-altered environments in Europe. After centuries, perhaps millennia, of adaptive coexistence with man there should be little surprise that, when brought here, they hook up that same environmental niche. Native American species simply were at an evolutionary disadvantage. This "urbanophilia" (or city-dwelling habit) is typical of most exotic land gastropods and has probably (repeatedly) helped Allopeas gain a foothold.

     Now at last to our offshore triton, C. parthenopeum. Club President Charlotte Lloyd indicated that the type locality is recapitulated in the name, an archaic equivalent of Naples, Italy. The irony of geographic reference notwithstanding, what is a Mediterranean mollusk doing on the local sunken vessel Casablanca? We'll address that shortly, but first let's look at the entire range of this mollusk. Numerous sources including correspondence, published works, and, of course, specimen shells themselves allow this synopsis: north-and southwest Mediterranean through the entire west African coast, through South Africa, East Africa, India, Taiwan, Japan, at least three fourths of Australia, New Guinea, north island of New Zealand, Hawaii, Baja California to the Galapagos and Cape Hatteras to Uruguay. A range across this staggering expanse of sea is matched only by a few pelagic snails like Janthina, which drift about at the mercy of the elements. A like characteristic of the Neapolitian Triton in fact accounts for its extensive distribution. Larval marine gastropods (veligers) usually take only a brief swim before settling into the sedentary life with which we usually identify them. Just a few years ago, however, Scheltema reported that the veligers of C. parthenopeum swam about for three months. That's enough to cross the Atlantic (as the author indicates). In fact, that probably is long enough to almost explain the unparalleled distribution of this snail. The passage of larvae between the tropical Western America and other portions of its range must not occur often - considering the distances involved and the prevailing currents of that region. Perhaps this has allowed sufficient isolation of the Panamic C. parthenopeum to develop subspecific characteristics as is the contention of experts like Beu and Keen. It is conceivable, but not too likely, that C. parthenopeum reached the western New World by crossing the Central American isthmus in Miocene time. We know that Pliocene (say 5,000,0000 year-old) fossils of this triton so closely resemble the modern descendants that they are considered conspecific. Honorary club member Dr. William Clench has pointed out that this retarded rate of conchological evolution must be considered in evaluating the cosmopolitan ranges of the tritons (Ranellidae). Although C. parthenopeum is undisputedly the most eclectic of the lot, the great majority of our other Florida tritons are also found in the Indo-Pacific province. Such is not true, however, of the western American species, and this is a most interesting discrepancy, one which may be discussed in a subsequent Shell-O-Gram.

     It is obvious from the previous discussion that a variety of biologic characteristics play a role in the distributions of mollusks: reproductive habits, life history, natural enemies, and adaptability to the environment in addition to the mechanical transport. Some are of general importance whereas others operate only in special cases. Certainly these features are not unique to the mollusks, but it is clear from the record that they have exploited them as well as other living creatures, and better than most. These humble organisms have encircled the globe and remained in residence for one-half billion years, a feat achieved by very few other living things. Little wonder that the study of how the mollusks do "their things" is such a fascinating pursuit.

*Taxonomy in the above article has been updated from that originally published in 1976.