Field of Science

Wild Slug Chases (Taxon of the Week: Gastrodontoidea)

Daudebardia rufa (Oxychilidae) - note the small shell compared to the body. This species belongs to a subfamily, Daudebardiinae, that also happens to be carnivorous. Photo by Jiří Novák.

Before I get on to the subject of today's post - a bit of a gripe, and a bit of unsupported and unjustified speculation. I have a feeling that I'm not going to say anything of note or value here, so feel free to skip forward a couple of paragraphs while I ramble.

It should come as no surprise to anyone that, contrary to common assumption, not all public information is available on the internet - a lot of it is still locked up in those paper things that we call "books" (or printed journals, or what-have-you). This is becoming a serious issue in a time when, for so many people, a search for information begins and ends with Google and anything that a search engine doesn't find is treated as if it doesn't exist. That particular problem, however, is a topic for another day. The point that I specifically wanted to raise is that some topics remain unusually under-represented online. Malacology is one of those mysterious absences. Or, to be more accurate, framework-level malacology is*. Run an online search for, say, Mitridae or Fossarinae or Stomatella, and you'll get a ton of pretty pictures of their shells, but it's surprisingly difficult to find out what they actually are. What features, specifically, maketh a mitrid? What defines a Diodora? I've done a few Taxon of the Week posts on gastropod families or superfamilies by now, and to be honest I've usually ended up blabbing on madly about any old guff in a desperate attempt to hide the point that all I really know about the taxon in question is its name (not being a malacologist myself, I have very little idea where to start looking to find out**). So why is there this particular pot-hole in the information superhighway? These are not obscure, unfamiliar animals. They are large, often cosmopolitan taxa, familiar to many a beach-goer or amateur shell collector.

*If anyone knows of any useful sites, I'd be happy to know about them.

**So why do I write about these things if I don't know anything about them? Because, to be honest, I'm writing these posts for my own benefit and to teach something to myself, not to you. I may not have known anything whatsoever about clausilioids before I started writing a post on them, but hopefully I knew just a little bit more when I'd finished. And if any of you benefited from my learning curve, then that's just the gravy.

Mesomphix andrewsae (Gastrodontidae). Photo by slapcin.

I wonder if the nature of malacology itself is a factor. Perhaps more than any other branch of zoology (with the closest competitor probably being ornithology), malacology has been a field where the Interested Amateur been able to make a prominent contribution. Arthropods are too fiddly, vertebrates start to smell after a few days, but shells are accessible to all (and they're pretty!). This has had a couple of effects on how malacology has developed. One is that molluscan classification was up and running largely before other zoologists had even cottoned on to this new-fangled "family" concept. Another is that books rather than journal articles have held a lot more significance as malacological sources - encyclopaedic works such as Powell's New Zealand Mollusca (just to name one that I'm personally aware of) that were of great appeal (and accessibility) to professionals and members of the public alike. And the thing is, geriatric and dog-eared as some of these of these sources have become, they're still useful today. The reason so little of the information has been transferred online is probably just because, so far, not many people have seen the need to.

Anyway, enough of that, on to the actual topic of this post (and if you did slog through the last couple of paragraphs, a quick reminder that I have absolutely no idea about anything in there, and was almost certainly talking completely out of my khyber). Gastrodontoidea are an assemblage of land snails recognised by Hausdorf (1998) as including six families - Pristilomatidae, Chronidae, Euconulidae, Trochomorphidae, Gastrodontidae and Oxychilidae (the names of two families have been corrected to the names used by Bouchet et al., 2005). Previously these families had been included in the Limacoidea, which Hausdorf divided up into a number of superfamilies. However, he did not dispute the monophyly of the original extended Limacoidea, and many authors continue to use the larger grouping rather than recognising Hausdorf's subdivisions.

Geotrochus obscura (Trochomorphidae). It does not take much to realise how these creatures got their name, does it? Photo from here.

Hausdorf's analysis is not without its problems. For a start, he was coding families rather than individual species, which (a) assumes that the families you're using are monophyletic, and (b) usually requires the author to estimate the 'ground-state' coding for a family, which can be a hazardous exercise (just because the majority of members of a family possess a particular character state does not necessarily mean that state is ancestral for that family). Secondly, the characters that supported monophyly of Gastrodontoidea were reductions of the stimulator in the male genitalia (among other things, the stimulator is the part of the genitalia that produces the love darts in those snails that have them ) and of the venation of the lung. Not only are reductions or losses always somewhat suspicious as supporting characters - if it is easier to lose a character than to gain it, they will probably be prone to homoplasy - but each one of these characters was both homoplastic with other non-gastrodontoid limacoids, and had been reversed in some supposed gastrodontoids. Hausdorf's Gastrodontoidea was not monophyletic in the molecular analysis of Wade et al. (2006) (but pretty much no relationships within the extended Limacoidea were well-supported in that analysis), nor did it appear in the morphological tree of Barker (2001) (which, however, did not include support levels for any of its results).

Most 'gastrodontoids' seem to be rather small (often only a few millimetres in diameter). Many limacoids have small shells compared to their bodies, and are unable to fully retract into them (Bouchet & Abdou, 2001). The Microcystinae, a subfamily of the Euconulidae (though Hyman et al., 2007, suggested they may be closer to the Trochomorphidae), are ovoviviparous - that is, they incubate their eggs internally until they hatch out and are released as live young*.

*Is there a better way of putting this? I just realised that the phrase "live young" is a bit unfortunate - after all, it's not as if eggs are dead.

Euconulus fulvus (Euconulidae). Photo from here.

Gastrodontidae and Oxychilidae possess a cartilaginous love-dart, but other families are dart-less. Gastrodontidae also possess an internal duct between the male and female parts of the reproductive system (remember, all pulmonates are hermaphrodites), and are apparently able to fertilise themselves. Indeed, Barker (2001) lists gastrodontids among families of snail for which some individuals lack the male penis, so they are only able to fertilise themselves or be fertilised by others, not fertilise others. I find this intriguing, as explanations for the ways and means of hermaphroditism often seem to proceed on the assumption that, because of the different required reproductive commitments, 'tis preferable to fertilise than be fertilised (love-darts, for instance, are thought to have evolved to prevent one partner from fertilising the other, then taking off before it is able to be fertilised itself). Penislessness in gastrodontids would seem to go against that assumption, so why would it develop? Is it that, what receptive-only individuals lose in the ability to produce more offspring, they gain in having more control themselves over how those offspring are provided for?


Barker, G. M. 2001. The Biology of Terrestrial Molluscs. CABI.

Bouchet, P., & A. Abdou. 2001. Recent extinct land snails (Euconulidae) from the Gambier Islands with remarkable apertural barriers. Pacific Science 55 (2): 121-127.

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.

Hausdorf, B. 1998. Phylogeny of the Limacoidea sensu lato (Gastropoda: Stylommatophora). Journal of Molluscan Studies 64 (1): 35-66.

Hyman, I. T., S. Y. W. Ho & L. S. Jermiin. 2007. Molecular phylogeny of Australian Helicarionidae, Euconulidae and related groups (Gastropoda: Pulmonata: Stylommatophora) based on mitochondrial DNA. Molecular Phylogenetics and Evolution 45; 792-812.

Wade, C. M., P. B. Mordan & F. Naggs. 2006. Evolutionary relationships among the pulmonate land snails and slugs (Pulmonata, Stylommatophora). Biological Journal of the Linnean Society 87 (4): 593-610.


  1. ...that is, they incubate their eggs internally until they hatch out and are released as live young*.

    *Is there a better way of putting this? I just realised that the phrase "live young" is a bit unfortunate - after all, it's not as if eggs are dead.

    I dunno, maybe, "released pre-hatched"?


  2. Considering that the mollusks are the 2nd most speciose phylum, one would think that there would be proportionately more information about them on the web. One problem may be that there aren't that many malacologists around.

    >they incubate their eggs internally until they hatch out and are released as live young

    How about "released as baby snails"?

  3. *Is there a better way of putting this? I just realised that the phrase "live young" is a bit unfortunate - after all, it's not as if eggs are dead.

    Now your sounding like one of those crazy pro-slug-lifers.

  4. I agree with Aydin, who forwarded me a link to your post. There are very few malacologists in total, and most of the professionals (except Aydin and a few outstanding others) are way too busy with career-building (think resume and publications list) to spend time putting info onto the 'net. Two other great on-line sources are Malacolog and the SeaSlugForum, both are for marine species though.

    I did want to point out that the free online encyclopedia Wikipedia has a project called WikiProject Gastropods, which is rapidly expanding its coverage of the class, and is currently stronger on terrestrial groups. There are already well over 5,000 gastropod-related articles in total (OK, the majority are stubs.) 2,000 articles were created in the last 2 years. The creation, maintenance and expansion of these articles is all done entirely on a volunteer basis.

    Over the next couple of years the coverage of the major groups will become much more fleshed-out. The article "love dart" is already pretty great I think, and the articles Clausiliidae and clausilium are showing some promise.

    In the interests of full disclosure: I volunteer a lot on this project. It would be excellent if you, or any other seriously interested person ever feels like contributing, even in the most minor way.

    To look at the project page (and its associated discussion or talk page) put "Wikipedia:WikiProjectGastropods" into the Wikipedia search slot.

    Very best to you,

    Susan J. Hewitt

  5. Thank you, Susan, for your work on the project and for telling us about it here.

  6. I signed up last week (I think ... I've lost track of time) for the Gastro WikiProject, so I should pull my finger out and contribute. It's a very worthwhile project!

  7. Potentially stupid question:

    Is there even a real taxonomic distinction between slugs and snails? Or are they all snails? Are "slugs" paraphyletic?

  8. A slug is a really just a shell-less snail. "Slugs" are hugely polyphyletic, and an entire continuum could be put together from snails with fully-developed shells to ones with reduced, tiny shells (like the one in the top photo on this post) to species with mostly-internalised shells to entirely shell-less species. Terrestrial gastropods with reduced shells are often referred to as "semi-slugs".

  9. Gastrodontidae also possess an internal duct between the male and female parts of the reproductive system

    So there's a vas deferens between them?

    bdum-tish :D

  10. Try the online biodiversity library. There is life outside Google:
    Warning: it is strongly addictive.


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