Rabdotus
Along the I35 corridor between Dallas and San Antonio, we get a lot of posts for Rabdotus, but there is often uncertainty about whether these are R. dealbatus dealbatus (whitewashed rabdotus) or R. mooreanus (prairie rabdotus) (there are other Rabdotus species in the state too, but they are, for the most part, easier to identify and don't occur in this area). I thought that it might be useful to summarize a bit about these species, based on descriptions and discussion from Pilsbry, 1946 (Land Mollusca of North America (North of Mexico)) and Fullington and Pratt Jr., 1974 (The Aquatic and Land Mollusca of Texas, Part III).
With fresh shells, R. dealbatus is 'rather wide, globose conic, thin, profusely streaked and mottled with opaque white on a gray or brownish gray ground' while R. mooreanus is 'ovate-conic; white above, coffee-with-cream colored below the periphery, or all white, the basal tint absent; sometimes varied with waxen or dark gray streaks, and often showing scattered gray dots which are translucent by transmitted light.' So, based on coloration, fresh shells should be pretty easy to tell apart. But what about long-dead shells where the color is worn and the entire shell has become opaque?
Both shells are generally smooth. R. dealbatus dealbatus is 17 -33mm with a diameter 57 - 65% of height whereas R. mooreanus is 20 - 30 mm with a diameter 47 - 62% of height. So some difference but lots of overlap.
R. dealbatus dealbatus generally inhabits more wooded areas while R. mooreanus generally inhabits more open grasslands, but 'they broadly overlap in their ecological preferences and at many localities their colonies form a mosaic.'
Along the western edge of the I35 corridor, we can also run into R. dealbatus ragsdalei, but this shell is typically strongly ribbed-striate rather than smooth.
My conclusion is that we can often distinguish between the two species, but not always, particularly in the case of sub-adult and/or very old shells. I would love to hear any additional tricks that people have to distinguish between the species.
Comments
I struggle with these two species. Their range overlaps as does their morphology so there is always a group of specimens that could be either. I am wondering if there is any literature on microhabitat differences between the two species? Sometimes when you have sympatric species like this, they settle into slightly different niches. Is there any evidence of this being the case where the species overlap?
The only bit that I'm aware of is what I quoted from Fullington and Pratt about "broad ecological overlap". They go on to say that the species can hybridize, mainly in floodplain areas where individuals of both species may wind up together even though R. mooreanus generally prefers more open grasslands and R. dealbatus more wooded areas.
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