by Timothy Pearce
The land snail family Haplotrematidae is widespread in North America. They are omnivorous, eating other snails as well as plants. Western North America hosts most (16 of 18) species of this family that occur in the
United States and Canada. Some species are found under sword ferns where they might gain protection by mimicking fern fiddleheads (see figure of Haplotrema vancouverense beside a fiddlehead).
Two related species, Ancotrema hybridum and Ancotrema sportella, are sometimes difficult to separate. Their beautiful shell sculpture includes ridges radiating outward like bicycle spokes, and finer spiral grooves cutting across the tops of the ridges, looking like beads (see figure of Ancotrema with beaded sculpture). The beaded sculpture extends to the end of growth in A. sportella, but the sculpture becomes smooth on the last, largest whorl in A. hybridum. They are easy to tell apart until you find one that is smooth on only half, or a quarter, of the last whorl.
Two things made us suspect that they might really be one species. First, some shells were difficult to classify. Second, the ranges of both species coincide from northern California to Alaska.
To address whether they are two species or one, we examined 311 museum specimens. If they are two species, we expected to see a two-humped curve (bimodality) in amount of smooth sculpture on the last whorl. We expected
many specimens without smooth sculpture at the end, many specimens with smooth sculpture on the entire last whorl, and very few specimens having smooth sculpture on just half the last whorl.
Instead, we saw continuous variation, with no hint of bimodality. That result is consistent with their really being one species. Furthermore, we discovered on every shell the sculpture became smooth around whorl number 5. If shell growth stopped before whorl 5, then it resembled A. sportella. If it grew beyond whorl 5 before becoming adult, then it gained the smoother sculpture of A. hybridum.
Next, we looked for reproductive differences. When new species arise, reproductive structures are sometimes the first to change. These changes might help individuals to recognize the correct mate. We found no consistent differences in the reproductive parts.
These (and most land snails) are hermaphrodites (one individual is both male and female), so we know we were not looking at male – female differences. Also, we knew we had adults only because the upper lip dips
downward at the end of growth, so we were not comparing adult – juvenile features.
Finding continuous variation in the feature traditionally used for separating the two species, no differences in the reproductive parts, coincidental geographical ranges, and discovering that the sculpture always
diminished about whorl 5, all led us to conclude that they are one species. A. sportella was named first, so by the law of priority, that is the name we will use.
Scientists get more glory for naming new species, not sinking a name as we did here, but this taxonomic cleanup work is important, too.
For more details, please review Pearce, T.A. & Fields, M.C. 2015. Shell and genital morphology fails to separate Ancotrema hybridum (Ancey, 1888) and A. sportella (Gould, 1846) (Gastropoda: Haplotrematidae). Malacologia, 59(1): 21-32.
Tim Pearce is assistant curator of mollusks at the Carnegie Museum of Natural History. He studies ecology and systematics of land snails.