Modulus carchedonius vs Modulus modulus, Does carchedonius occur in Florida Keys?

In another topic the question arose as to whether a Modulus found in Key Largo, Florida Keys is M. carchedonius or M. modulus. The discussion is move here to its own topic for greater exposure and to solicit comments and help.

Member Wayne Wilson identified a Key Largo Modulus as carchedonius. Member marlo commented:

"I'm also curious about Modulus carchedonius. I think the rest of us are calling this Modulus modulus. I can't find carchedonius as a synonym nor is it listed in Common and Scientific Names of Aquatic Invertibrates from the United States and Canada: Mollusks, 2nd Ed., 1998, American Fisheries Society Special Publication 26, Bethesda, Maryland."

Wayne replied:

" Marlo, I will check into those shells and try to take pictures if I can figure out how to send them to this site. For Modulus carchedonius I used Humfrey's Sea Shells of West Indies as a referrence. It does not look like a modulus. I have pictures, poor as they are, and I will get back with more info when I get more time. Here's the picture of the shell I think is Modulus carchedonius. Let me know if I am correct. Wayne

Wayne,

Regarding Modulus carchedonius (MC) vs Modulus modulus MM). I don't have Humfrey in my library. However, I believe he placed MC only in Caribbean locations. I do not have any MC and have never seen one. I did find one photo on the web. Assuming it's accurate I attached it below. It's also pictured (poorly) in Abbott's Compendium of Seashells and Eisenberg's Seashells of the World. I know of no confirmed findings of it in Florida and, as I previously noted, it is not listed in Common and Scientific Names of Aquatic Invertibrates from the United States and Canada: Mollusks, 2nd Ed., 1998, the most comprehensive listing of reviewed and verified mollusks identifications found in the USA. Nor does Redfern report MC in Bahamian Seashells. Both MM and MC are pictured and described in Abbott's American Seashells, 2nd Ed. Abbott also places MC in the Caribbean. Abbott reports MC as "uncommon", whereas the Modulus I collected in Key Largo was plentiful (virtually in every sample taken) and is found everywhere I've collected among seagrasses in the Keys. Shell characters can be extremely variable, but Abbott notes one character that I believe confirms the Key Largo specimens as MM - the columella tooth of the live-collected adult specimens is colored brown to purple.




In MC the columella tooth is not colored. It's impossible to distinguish among many Modulus when juvenile. Only comparison of a series of adult shells would permit a valid distinction. Because of the adult size and amount of animal involved I usually include only one adult Modulus in a vial. Did you get one? Check out the columella tooth.

Wayne replied:

Marlo, I understand what you are saying about the color of the tooth with Modulus modulus but as a birder and birding we use the general gist of the bird to aid in its ID. These two shells look different to me. In Humprey's Sea Shells of the West Indies the author separates these two species. With Modulus modulus, "Sculpture of strong radiating ribs and numerous spiral cords; outer lip with a number of sharp ridges within....The colour variations of this shell may cause some difficulty in identification; the basic shape and sculpture always remain constant however." The specimen of Modulus modulus that I have in my collection look different than do these micros sent me. Humprey separates Modulus carchedonius, "with dotted spiral cords. The base of the shell is always of a lighter colour, with brown dots on all the spiral cords. Sculpture of several small spiral cords, witha sharp angled periphery,......The shell is not unlike M. modulus, but can easily be separated from it by the sharp periphery and lack of axial ribs." Animals do expand ranges. Where does the the Carribean start? Is there a clear division? Are the keys not part of the Carribean. I am sending a copy of the picture from the book. It is not clear but then I am not clear with the separation of these two species. Wayne

Without a fair number of both from diverse locations it's hard to judge whether some of Humprey's distinctions arise from having compared limited populations or if one species or the other has greater or lessor variability. Then there's the question of whether he created his own descriptions for all shells portratyed or if they are reprints of the author's descriptions, which may have been based upon limited observations or populations from other locales. M. modulus from along both sides of Florida shows a lot of variability in shape, coloration, radial ribbing, etc. Humprey ackowledges that color variations cause difficulty in identification of both, and both have spiral ridges with brown flecks and outer lip with sharp ridges, which on more mature shells also reflects ridging in the aperture. He emphasizes the lack of axial ribs in MC, but some northern Florida MM lack such ribs. His comment about the sharp periphery could be valid if we had more MC to examine. Note also that less mature MM from northern Florida have much sharper peripheries than mature specimens. However, I still believe the Key Largo specimens should be classified as MM for several reasons, the most prominent of which is Humprey's own comment that MC's columella tooth is always white. The tooth on the Key Largo specimens is brownish purple. While MM's tooth can range from white to colored, MC's is always white. Note also that the Largo specimens where collected off thalassia (no algae). For the Largo specimens to be MC, then Humprey has to be wrong on the tooth and habitat, and he may be. But, without sufficient comparison specimens to determine if the presents or absence of axial ribs is a truly distinguishing character or if the sharp periphery is also, then the only definitive character both Abbott and Humprey distinguish is that MC always has a white columella tooth, and the ones from Largo do not.

Anyone else out there have sufficient experience with these two species to shed additional light?

Since no responses have occurred, I posted the following on Conch-L:

"We’re trying to resolve some ambiguity as to whether Modulus carchedonius is restricted to the Caribbean south of Florida or if it has been substantiated as occurring in the Florida Keys and/or north. ........ So, my questions are:

Has anyone documented Modulus carchedonius from the Florida Keys or elsewhere in Florida?
Do those of you familiar with Modulus modulus and Modulus carchedonius agree with Humfrey’s distinctions? Is Modulus modulus always without a “sharp periphery” or never lacking axial ribs?

The following responses were received:

Dick Petit wrote, "You should consult the monograph of Modulus in Johnsonia. It is in Volume 1, no. 14. It is an authoritative monograph, written by Tucker Abbott."

Bill Fenzan wrote, "Since I am primarily a cone collector, I do not know if anyone has documented the collection of Modulus carchedonius from anywhere in Florida.

On your second question, I went to the reference Dick supplied in Johnsonia. Here is what Abbott says about separating the two species:

"It (Modulus charchedonius) shows very little variation in its characteristics and may best be separated from M. modulus Linné by its angulate contours, lack of nodules, neatness of its spiral sculpturing and, especially in the young, by its nearly closed umbilicus. The columellar tooth in carchedonius is never colored, whilie in modulus is (sic) is frequently touched with purplish brown."

This seems to be enough to enable separation of these two species.

Have you considered that you may have specimens of something other than M.modulus or M. carchedonius? Petuch (1988), discusses and illustrates 3 species of Modulus from East Florida he named in 1987 (lindae, kaicherae & pacei). He also describes M. calusa as a new species from Rabbit Key Basin, off Rabbit Key, Florida Bay, Everglades National Park, Florida.

The description of this species appears to match specimens you describe from Key Largo. Here is the reference:

Petuch, E. J., 1988. Neogene History of Tropical American Mollusks, Biogeography & Evolutionary Patterns of Tropical Western Atlantic Mollusca. The Coastal Education & Research Foundation, Charlottesville, Virginia. pp. 217."

HerbC, upon reviewing the Key Largo Modulus material concluded as follows:

re: Modulus modulus v. Modulus carchidoneus - I have two distinctly different species. The ones I've marked as M. carchidoneus meet all the criteria described in Abbott's description. I've been following your inquiries on Conch-L and was very careful to differentiate these two lots before ID'ing them. If the second is not M. carchidoneus, it is at least not M. modulus.