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Posted: May 22 2011

 Lithopoma americanum (Gmelin, 1791), American Star Shell

Lithopoma tectum (Lightfoot, 1786), West Indian Star Shell

I am of the opinion that Lithopoma americanum (Gmelin, 1791) is a form or morph of Lithopoma tectum (Lightfoot, 1786).  See discussion below.

Abstract: Opercula of L. tectum and L. americanum were compared and no consistent distinguishable differences could be ascertained. Sculpture of the base was also found to vary with overlapping characters that would not permit consistent distinction. Spire sculpture presented fairly consistent differences, but probably not sufficient to confirm speciation versus polymorphism without DNA confirmation. It is probable that these two forms are a single species; namely, Lithopoma tectum (Lightfoot, 1786).

Part 1 – The Operculum

When I first presented L. tectum and L. americanum, I presented them as separate species largely upon the erroneous perception of differing opercula (see the presentation charts dated May 2011, retained below). The external features (teleoconch and base) of many univalve species that populate widely varying habitats can be dramatically influenced by the differing habitat and/or environmental conditions in which they develop and, as a result, can vary dramatically in appearance. External features are also often susceptible to more rapid evolutionary habitat adaptation. However, the operculum for many species is a far more conservative anatomical feature and is little, if at all, in form and/or function, influenced by environment. Similarly, relatively, the operculum would be expected to display a more conservative evolutionary change than external features, especially where the principal or only function of the operculum is to close the aperture to resist drying out and predator protection. In other words, while the external features of the shell of a species may vary considerably due to environmental (ecomorphs, ecophenotypes), polymorphic (within species genetic variation) or more rapid adaptive change, the operculum is far more conservative and less likely to be so influenced in many species. Therefore, despite the presence of variable external features within many species, the operculum remains constant and can be a reliable basis for identification and concluding the same species is at hand. So, when I initially believed L. tectum and L. americanum had distinctly differing opercula, I concluded they were indeed separate species. As it turned out, I was erroneously relying upon a specimen of L. tectum preserved with a mismatched operculum.

My May 2001 presentation of L. tectum relied upon a single specimen collected in 1982. However, as several astute readers noted after reviewing my presentation, the operculum I presented looked far more like the operculum of L. caelatum than of L. tectum or L. americanum. After a far more in depth search on the net, it appeared (and I finally concluded) that the wrong operculum had been erroneously paired with the L. tectum shell back in 1982 when I was a novice collector. Tom Watters, Curator of Molluscs, Museum of Biological Diversity, Ohio State University, who had first questioned the operculum I presented, offered to send me the museum’s collection of Lithopoma for examination. I accepted and presently had several L. tectum and more L. americanum to examine.

In total, my review included the following material from my collection and that provided by Tom (all with opercula):

L. tectum: Bahamas 9; Honduras 6; Belize 6; Grenada 5; Puerto Rico 2; Upper Florida Keys 2; Middle Florida Keys 1; and Brazil 1
L. americanum: Bahamas 3; Biscayne Bay, Florida 5; Upper Florida Keys 10; Middle Florida Keys 14; and Lower Florida Keys 4

My review first focused on a comparison of opercula. Immediately to the point, as the below slides illustrate, I could not distinguish any meaningful differences between the L. tectum and L. americanum opercula. I requested Harry Lee to review my photos and he agreed. Because I wanted the images to be as large as possible so detail could be readily seen, I am presenting two sets of several slides each in different sizes. As a result, there is a bit of repetition.

The following detailed description fits all of the opercula of both L. tectum and L. americanum I examined.

Part 2 – The External Features

One suggestion was that L. tectum and L. americanum might be ecophenotypes - two forms of the same species with differing adaptive external physical characters resulting from environmental or ecological factors rather than genetic expression. Another was that they were polymorphs - genetically driven differing phenotypes of the same species occurring simultaneously in the same randomly mating population. I believe the illustrations comparing L. tectum and L. americanum taken from the same specific locations/habitats on the same collecting occasions (slides 1.21 and 1.25) argue against ecophenotypes and for the likelihood they are polymorphs. My limited comparison of Bahamian material (slide 1.23) also suggests this. However, I do not have sufficient comparative material of this type (different phenotypes from same locations) or any for the West Indian and South American locales to be able to draw a firm conclusion. If it is the case that these two are polymorphs, it does appear that the L. tectum form is far more dominant in the West Indian and South American locales, that L. americanum dominates in Florida, and it may be a mixed bag in the Bahamas. More extensive comparative specimens with good collecting data would be needed to draw convincing conclusions. And, we solicit reports, photos and specimens (loan only for photography – all to be returned) to contribute to a larger sample for addressing this issue from a morphological perspective. Obviously, a comprehensive DNA-based study could accomplish the same objective with a much smaller sample. Unfortunately, a reliable hand-held DNA analysis device is not yet available for us collectors to take with us into the field. I keep hoping it’ll be developed while I’m still able to get into the field.

To ascertain the possibility of consistent morphological differences of the shells I compared external features of shells labeled L. tectum and L. americanum from two perspectives – the base and the teleoconch.

Part 2.1 – The base

Considering the descriptions I had of L. tectum and L. americanum in several publications, comments on hand from other collectors and the one L. tectum I had available, my initial descriptions of these shells took the position that the sculpture of the bases differed in one consistent aspect as noted in slide 2 (from initial presentation) presented here.

I compared the bases of my expanded population of material and found the degree of expression of sculptural features of the base was quite variable (especially at differing stages of maturity), but otherwise quite consistent, including the small spiral ridge at the base of the

columella crossed by  more elevated axial knobs.

While this latter character was consistently

present in all L. americanum in my sample, and

was not present in some L. tectum, it was indeed

present in more L. tectum than not. The following

slides present six  comparisons illustrating the

variable, but consistent expression of characters

on the base, which, coalescing various

descriptions, has been described as flattened

with several spiral cords separated by moderately

deep grooves; the cords crossed by numerous

thin imbrications (most evident in juveniles) that

are more knob-like in older shells; and having a

small spiral ridge at the base of the columella

crossed by more elevated axial knobs.

Part 2.2 – The teleoconch

I compared the teleoconchs of my expanded population of material and found that spire sculpture presented fairly consistent differences. Not really a surprise. After all, that’s exactly the issue. Because the most apparent, gross morphological sculpture is most often relied upon to differentiate, and when it occurs and a differential label is applied, then those who come after with similar shells apply the differential label. But, are they really different because they look a little different and have a different label? It may well be that the differences in spire sculpture reflected in two polymorphs of the same species have given rise to two differential labels and been the basis used by collectors to group shells either L. tectum or L. americanum. The two are essentially the same except that shells labeled L. tectum display fewer, more widely spaced axial ribs that are stronger over their entire length, are more pronounced at the abapical end, often project over the suture, and terminate in open spines. L. tectum is also often described as larger. My opinion is that these differences reflect two polymorphs and that enough shells with “mixed” or “cross-over” characters exist to demonstrate a continuum and not speciation. Only far more of a broadly geographic sample or a comprehensive DNA study will resolve this matter. The following slides present direct comparisons of shells labeled as L. tectum or L. americanum. However, in my opinion, with heaviest reliance on the examination of the opercula and failure to find any distinguishing differences, I believe all these shells should be considered one species; namely, Lithopoma tectum (Lightfoot, 1786).

The following slides present various specimens for comparison or illustration of the elements I've discussed. 

The following slides discuss L. tectum versus L. olfersii (Philippi, 1846).

The following slides provide a detailed description that should be considered to apply to both forms of Lithopoma tectum (Lightfoot, 1786), the L. tectum americanum (Gmelin, 1791) morph and the L. tectum tectum morph.



Marlo posted on Conch-L:

"L. tectum has been treated as subspecies of L. americanum and also as a valid species. I believe there should be no doubt that the two are separate. I also doubt that, but for an occasional isolated instance, L. tectum does not occur in Florida.

I would encourage those with Florida shells they believe might be L. tectum to reexamine them in light of this presentation; and, we would like to hear the results."

5/23/11 Harry Lee, author of Marine Shells of Northeast Florida 2009, responded:

"That's a thorough, persuasive, and attractive presentation.

I especially appreciate the discussion of the opercula of the two taxa. I had never noticed that previously.

If the relationship is subspecific or conspecific, L. americanum (Gmelin, 1791) is subordinate to L. tectum (Lightfoot, 1786) - not vice-versa. However, your presentation makes those contingencies appear less likely.

It will be of value to hear from other collectors - especially with regard to intergradation of these two morphologies, including the opercula.

We're appreciative,"

Steve Rosenthal, collector from New York, wrote:

"I took a quick look at my shells and its initeresting. All my FL and Bahamas shells that I collected myself are fairly small, and along with the others from there, now that i look DO seem to fall into the one grouping with the sculptured ridge around the umbilicus. I may have to change some labels. ie americana.

But my two big "tecta" are confusing. I bought a real big one from Randy Allmemand at a show (almost 58mm, very high and narrow), from the Dominican Republic. But using your features it looks like americana based on the same sculptured ridge and the operc, its really tall and narrowly angled at the apex. That seems pretty big for americana??

My other big tecta has no operc, its from a friend from St Vincent and is the only of my shells that lacks the distinct umbilical ridge and thus may the only "true" tecta in the bunch.

Do you think that americana gets that big and is in the DR?

I cant recall if you talked about ranges at all other than FL and Bahamas which of course are the primary geographical areas of interest."

Marlo replied:

"Femorale has L. americanum from E. Panama and Brasil, the live animal photo in my presentation is from Grand Cayman, and it's present in Texas. I suspect it's present throughout the Caribbean.

Here's what Malacolog has to say.

Maximum Reported Size: 40 mm
Distribution: USA: North Carolina, Florida: East Florida, West Florida, Florida Keys; USA: Texas; Mexico: Veracruz, Campeche State, Yucatan State, Alacran Reef, Quintana Roo; Belize, Panama, Cuba: Havana Province; Cayman Islands: Grand Cayman Island; Virgin Islands: St. Croix."

Tom Watters, Curator of Molluscs, Museum of Biological Diversity, Ohio State University, questioned the validity of the operculum presented in my presentation:

"Intrigued by your Lithopoma posting and the two different opercs, I went through my collection. I have numerous lots from Florida to Brazil, and the southern ones in particular seem to be perfectly good tectum tectum based on the shell – long, numerous digitations – quite beautiful. But not a single one of them has the non-sulcate operculum. Your operculum looks at lot more like that from caelatum. I’m curious to see if anyone else has a shell with your operc."

Marlo replied:

"Your comment about the similarity to caelatum is insightful and apropos. I am working on a presentation of caelatum and in the process have noted the same similarity. I also reviewed several photos online of tectum from Brasil and Panama and those with operculums do indeed reflect a sulcate operculum. My shell is self-collected (from 1982) and I feel the perc is from the shell (esp. since it fits the aperture). But, that was 30 years ago when I first started collecting and I do recall collecting caelatum at the time on the same trips (different locations in the Keys). I’m beginning to wonder, as the evidence is starting to indicate the tectum perc may be sulcate. Hopefully, I’ll hear from more collectors, esp. Marcus Coltro.

Did you also examine the bases for the presence/absence of the ridge at the base of the columella?"

Regarding my question about the ridge,
Tom replied:

"The ridge seems to vary greatly even within a population, from absent to nodulose."

Ken Piech, collector from Tennessee, also questioned the operculum in my presentation:

"Harry & Marlo,

Perhaps you two have received other, off-list responses to Marlo's excellent presentations of L. americanum (Gmelin, 1791) and L. tectum (Lightfoot, 1786). During this Memorial Day weekend, I have had a little time to review our collection and the shells of these two taxa and am now able to give you some feedback, as "suggested" in Harry's email below. I will comment only on one aspect of Marlo's presentations, the opercula. In our collection we have three lots of Lithopoma tectum, one specimen from Virgin Gorda, BVI (1987), six specimens from Ave de Barlovento (Las Aves), Venezuela (2003) and two specimens from La Blanquilla, Venezuela (2005). All nine opercula have morphologic features similar to the opercula Marlo has pictured for the L. americanum and similar to the picture in Abbott, #482 (plate 2). I have not personally found a specimen of L. tectum north of the BVI. However, the operculum that Marlo has pictured for the L. tectum from Seven Mile Bridge appears to me to resemble closely the operculum for L. caelatum (Gmelin, 1791), which we find in abundance in the Keys.

I hope this information is helpful. In two weeks Alice and I will be back in the Florida Keys collecting for several weeks. Because of this discussion, I will certainly be looking for Lithopoma. I'm sure that I will see lots of L. americanum and L. caelatum, but I will be especially looking for L. tectum."

Marlo replied to Ken:

"I really appreciate the examination you did. You are not the first to do so and reply. Tom Watters, Curator of Molluscs, Ohio State University, also reviewed their collection with the same results and he too noted, “But not a single one of them has the non-sulcate operculum. Your operculum looks at lot more like that from caelatum.” As a result I reviewed as many online photos as I could find. All have the sulcate operculum. As a result, as I wrote Tom, “Your comment about the similarity to caelatum is insightful and apropos.”

"I’m beginning to believe, as the evidence apparently indicates, that the tectum perc is sulcate and I mixed up percs back in 1982. I’m now pretty convinced that back in 1982 when I really wasn’t that aware or careful about these things, I probably did not track operculums to shells very carefully and it’s most probable that after a collecting trip and cleaning all the shells, I matched Astraea percs to shells by size in ignorance that the perc of each species could be different. This is embarrassing."

"Harry, I’m expecting several lots of L. tectum and had planned to wait until I saw them and confirm, before I wrote you, but now that several have questioned my presentation, I think we’re back to the drawing boards as to whether L. tectum is a valid species. All the other characters may be too variable to be definitive. We may have to wait until someone does the DNA thing."

Harry responded to Ken's comments:

"Thanks for the objective reporting. Yours is the only response of which I am aware.

I only have one lot of material that meets Marlo's criteria for L. tectum (self-collected) and can be vouchsafed authentic (self-collected). Of the six constituent specimens, four still have opercula - two with animal attached to it and the shell. Each of these four opercula has the deep groove discussed by Marlo.

Abbott and Femorale show similar shells/opercula in L., tectum shells."

"This is science; it doesn't get any better!!"

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