A Different Kind of Shell

Stomatella varia, photographed by Scott Collins.

Both of the readers of this page will doubtless be glad to know that the Taxon of the Week post is bringing a respite from my temporary obsession with Palaeozoic cephalopods. Instead, I'm going to shift my focus wildly through the world's biodiversity and introduce you to... an entirely different type of mollusc. Prepare to meet the marine gastropod genus Stomatella.

Stomatella is a genus of the Trochidae, one of the largest families of marine snails. I'll refrain from giving any actual numbers - there are a number of 'trochid'-type subfamilies, and every single treatment I've seen seems to have a different idea about which subfamilies should be included in the Trochidae proper and which should be placed in separate families (Bouchet et al., 2005; Williams et al., 2008). So I really should say that Stomatella is usually a genus of Trochidae - the two references just cited both include Stomatellinae in Trochidae, but Hedegaard (1997), for instance, indicated that it should be a separate family based on shell microstructure.

Most trochids are conical shells commonly referred to as "top shells". Stomatella, in contrast, is an example of what might be called the "shield-slug" morphology - the shell is flattened and ear-shaped (the technical term is "auriform") and relatively very small compared to the rest of the animal. Not surprisingly in light of this form, Stomatella also differs from other trochids in lacking the otherwise usually well-developed operculum. Being unable to retreat into its shell like other trochids, Stomatella has developed a different defense - it is able to autotomise the back part of the foot, which then wriggles in the manner of a lizard's tail while the animal makes good its escape. Stomatella are broadcast spawners, and are apparently regularly known to spawn and breed in captivity.

Stomatella are well-known animals in the marine aquarium hobby, though a scan of various noticeboards and such suggests that they are not so much something deliberately trafficked in as something that tends to turn up of its own accord, brought in as hitch-hikers on rocks and such, and is then tolerated because of their usefulness in controlling algae. Despite this, there seems to be a spectacularly frustrating lack of research in these animals. A Google Scholar search on "Stomatella" brings up a little over seventy results, including only a single 117-year-old two-page paper specifically focusing on Stomatella (Pilsbry, 1891), and a bit of searching reveals that the species described in that paper isn't even included in Stomatella any more. There isn't even a Wikipedia page for Stomatella. So let this be a lesson to you all - just because something is familiar, doesn't necessarily mean that it's well-known.


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.

Hedegaard, C. 1997. Shell structures of the Recent Vetigastropoda. Journal of Molluscan Studies 63 (3): 369-377.

Pilsbry, H. A. 1891. Note on the soft parts and dentition of Stomatella. Proceedings of the Academy of Natural Sciences of Philadelphia 43: 71-72.

Williams, S. T., S. Karube & T. Ozawa. 2008. Molecular systematics of Vetigastropoda: Trochidae, Turbinidae and Trochoidea redefined. Zoologica Scripta 37: 483-506.


  1. The flux in family vs. subfamily status for the various trochid-type taxa is similar to that going on in many family-group taxa of beetles (and other insect orders as well, I presume). Notable examples are scarabs and weevils - more often referred to now as "scarabaoids" and "curculionoids" (in reference to their superfamilial monikers). The superfamily seems to be is the new family in terms of recognizable clades.

  2. Does this mean "beetles in the bush" and I comprise "both readers of this page?" I'm sure there are many more out there. Very cool critter; where I come from, the most common trochids are Tegula spp, rather non-descript little guys, so your post has expanded my view of this family.


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