19 February 2007

How I determined this was Hawaiia minuscula

hminuscula
The pictured shell was 1.8 mm in diameter.

Hawaiia minuscula is a widespread land snail in the U.S. My specimens came from Monocacy Natural Resources Area in Frederick County, Maryland. Its small, more or less flat and mostly smooth shell with a wide umbilicus make H. minuscula easy to recognize until one realizes that there is another species, Lucilla inermis1, whose shell is almost identical to that of H. minuscula. According to Pilsbry2 the shells of the species in the subgenus Hebetodiscus "resemble Hawaiia minuscula somewhat, but are smoother and more depressed". I think the resemblance is a bit stronger than "somewhat", for after I examined the drawings in Pilsbry, I had no idea how tell if the shells I had were minuscula or inermis.

Then, I noticed in Pilsbry that there was a tiny difference between the anatomies of Hawaiia and Hebetodiscus: the latter has one retractor on its penis and another one on its epiphallus, while the former has a retractor only on its penis. Luckily, I had 2 alcohol specimens. So I dissected them. Now, if these were large snails, it would be quite easy to find their genitalia, but the snails I dissected were barely 1.6 mm across and what was worse was that they were apparently not adults; their genitalia were nowhere to be found.

As I was about to give up, Tim Pearce came to the rescue and pointed out that the teeth on the radulae of H. minuscula and L. inermis have different shapes, the most significant being the shapes of the marginal teeth, blade-like in minuscula but serrated in inermis (see the drawings below).


The teeth of Lucilla inermis (left) and Hawaiia minuscula (right). Notice the different shapes of the marginal teeth (#7 for inermis and #6 for minuscula). Drawings from Pilsbry.

No sooner did I read Tim's e-mail, than I set to work. I removed the radula of one of my specimens, transferred to a slide and put it under the microscope. The marginal teeth were blade-like (see photo below). I had H. minuscula.

minuscularadula
Flattened radula of Hawaiia minuscula from Monocacy Natural Resources Area. The capital M marks the column of median teeth; the red arrows point at the blade-like marginal teeth.

But, now there is another puzzle. Frank C. Baker3 gave the "most common" habitat of H. minuscula in Illinois as "woodlands of oak, hickory and sycamore". All of my specimens also came from leaf litter samples from woods. In contrast, this is what Hubricht4 said about H. minuscula: "A species of bare ground. I have never found it in leaf litter." Interestingly, he lists similar habitats for inermis: "open, grassy situations..." Go figure.


1I've been told that Helicodiscus (Hebetodiscus) singleyanus inermis (H.B. Baker, 1929) is now Lucilla inermis.
2Pilsbry, H.A. 1948. Land Mollusca of North America . Volume 2, parts 1 & 2.
3Baker, F.C. 1939. Fieldbook of Illinois Land Snails. p. 72.
4Hubricht, L. 1985. The distributions of the native land mollusks of eastern United States. Fieldiana #24.

5 comments:

Tim Pearce said...

I wondered why you chose to say Lucilla inermis instead of L. singleyana, which I tend to use (and I see Harry Lee called his cf. L. singleyana, too). My guess is that you used L. inermis because Pilsbry (1948: 635) illustrated the radula of that taxon.

Now I am curious why I use L. singleyana instead of L. inermis! I had thought it was the more common species, but Hubricht (1985: 110, 112) showed both species to be sparsely scattered throughout the eastern USA (absent from the northernmost part), and interestingly, with pretty much coincident ranges (a hint that they might be the same species?). Perhaps I use L. singleyana as a holdover from the days when L. inermis was a subspecies, and I just dropped the subspecies.

Yes, Turgeon et al. (1998) recognized (listed in genus Helicodiscus) both L. singleyana and L. inermis. Note, however, that many other species they listed should also be in Lucilla. Without meaning to pronounce species validity (!), I give the list of Helicodiscus in Turgeon et al. that should now be in Lucilla: L. aldrichiana, L. barri, L. hadenoecus, L. inermis, L. nummus, L. punctatellus, L. singleyanus, L. tridens, L. jacksoni. Many of those were named by Hubricht; somehow he could see differences we don't.

I agree with Harry Lee that this group is ripe for revision, and that molecules would be an important player in sorting them out. I am intrigued by Harry's suggestion that some or many of them could be non-native. That would be fitting, since it was a non-native population of their look-alike genus Hawaiia that was first discovered and named from Hawaii, even though they are native to North America.

AYDIN ÖRSTAN said...

I used inermis, because according to Pilsbry the shell of singleyanus has a spiral microsculpture, while that of inermis is smooth. My specimens also have smooth shells like those of minuscula.

AYDIN ÖRSTAN said...

Harry G. Lees's comment originally posted on Conch-L:

Dear Aydin,

Wonderful blog. Despite the tininess of the snail, you tackled a daunting taxonomic problem and worked it out your own (with a timely and properly acknowledged assist from Tim Pearce). Little land snails can be daunting at times!

You're not the only malacologist who has been vexed by this uncanny conchological similarity (Lucilla vs. Hawaiia). For a few years now I've been accumulating lots of "Hebetodiscus cf. singleyanus" from places like eastern KS (Tom Grace!), Haywood Co., NC, and various parts of FL (Duval, Levy, St. Lucie, and Lee Cos.). Today I
once again looked closely at these and concluded that there are three separate morphs involved, and none quite conforms to any of the known
American taxa. Most were collected in the same leaf litter sample as Hawaiia minuscula (A. Binney, 1840), from which I have been able to distinguish it (not so easy in juveniles). Various authors have considered this (a group of)
fossorial (burrowing) species, which habit, along with their small size, would account for sporadic detection, often small numbers per lot, and a paucity of live-collected material.

I found reference to the synonymy of Hebetodiscus and Lucilla in Roth and Sadeghian (2006), who cite Falkner et al. (2002: 114).

Here is a tentative taxonomy:
HELICODISCIDAE Baker, 1927 [? not + DISCIDAE Thiele, 1931]

Lucilla Lowe, 1852 [Type Helix scintilla Lowe, 1852] + Hebetodiscus H. B. Baker, 1929 [as a subgenus of Helicodiscus; type: Helicodiscus inermis H. B. Baker, 1929]

Lucilla scintilla (Lowe, 185) [as Helix; type by monotypy] Europe
Lucilla inermis (H. B. Baker, 1929) [as Helicodiscus (Hebetodiscus)] USA Lucilla nummus (Vanatta, 1899) [as Zonitoides] USA
Lucilla singleyana (Pilsbry, 1889) [as Zonites] USA

In Duval Co., FL, where I live, as well as elsewhere in the state, I have the impression that the little shells I am assigning to Lucilla have appeared in places that I don't think they were before although I cannot prove this assertion.In other words this Florida morph may be expanding its range, which is often the sign
of an alien introduction. Roth and Sadeghian (2006) report Lucilla singleyana as a non-native snail in California, and the citations of this taxon in Europe make me (and some other authors) wonder about what's native to where. Is one (or more?) of the American species a synonym of Lucilla scintilla? Has one or more of these taxa a trans-Atlantic introduction? Is the group truly
native to both continents (Holoarctic)? This may be a job for a molecular systematist.

Harry

Falkner, G., T. E. J. Ripken, and M. Falkner. Mollusques Continentaux de France, Liste de
R馩rences annot饠et bibliographie. -
patrimoines naturels, 52. Paris, Mus鵭 National d'Histoire Naturelle. 1-350.

Roth, B. and P. S. Sadeghian, 2006. Checklist of the land snails and slugs of Californian. Second
edition. Santa Barbara Museum of Natural History Contributions in Science 3: 1-82. Oct. 20.

Frank Anderson said...

This may be a job for a molecular systematist.

If we can get plenty of relevant live (or alcohol-preserved, or frozen) specimens, a little money and an interested student, we could test some of these hypotheses in my lab without too much trouble (famous last words...).

AYDIN ÖRSTAN said...

Andy, you supply the money, I will supply the specimens.