Stomatella impertusa, copyright Marine Life Society of SA.

Belongs within: Trochidae.

A different kind of shell
Published 3 November 2008

Both of the readers of this page will doubtless be glad to know that this 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 varia, photographed by Scott Collins.

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.

Systematics of Stomatellinae
<==Stomatellinae [Macrostoma, Stomatellidae, Stomatidae, Stomatiidae, Stomatiinae]
|--Phaneta everettiF26, G88
|--Broderipia Gray 1847KC60
| `--*B. rosea (Broderip 1834) [=Scutella rosea]KC60
|--Mesogena Kutassy ex Cox in Knight, Cox et al. 1960KC60
| `--*M. arcta (Hoernes 1855) [=Inoceramus arctus]KC60
|--Praestomatia Kutassy ex Cox in Knight, Cox et al. 1060KC60
| `--*P. acutangula (Koken 1897) [=Stomatia acutangula]KC60
|--Roya Iredale 1912P61
| `--*R. kermadecensis Iredale 1912P61
|--Microtis Adams & Adams 1850 non Br. 1810 [ICBN]W93 [=Microtina Adams in Sowerby 1854KC60]
| |--*M. tuberculata Adams 1850 [=*Microtina tuberculata, Stomatia (*Microtis) tuberculata]KC60
| |--M. heckeliana (Crosse 1871)W93
| `--M. rubra (Deshayes 1843)W93
|--Stomatia Helbling 1779BR05 [=Stomax Montfort 1810KC60; incl. NiphoniaF26]
| | i. s.: S. angulata Adams 1850H09
| | S. australis Adams 1850H09
| | S. decussata Adams 1850H09
| | S. heckeliana (Crosse 1871)WBS-S15
| |--*S. (Stomatia) phymotis Helbling 1779 [=*Stomax phymotis]KC60
| `--S. (Miraconcha Bergh in Schepman 1908)KC60
| `--S. (*M.) obscura (Bergh in Schepman 1908) [=*Miraconcha obscura]KC60
|--Pseudostomatella Thiele 1921 [incl. Stomatolina Iredale 1937]W93
| |--*P. papyracea (Gmelin 1791)KC60 (see below for synonymy)
| |--P. decolorata (Gould 1848) [incl. P. mariei Crosse 1871]W93
| |--P. erythrocomaBC01
| | |--P. e. erythrocomaBC01
| | `--P. e. cycloradiata Usticke 1959BC01
| |--P. maculata (Quoy & Gaimard 1834)W93
| `--P. rufescens (Gray 1847) [=Stomatella rufescens, P. (*Stomatolina) rufescens]KC60
`--Stomatella Lamarck 1816BR17 [incl. Gena Gray 1850W93, Phymotis Rafinesque 1815 (n. n.)KC60, Plocamotis Fischer 1885W93]
|--S. impertusa Burrow 1815W93 (see below for synonymy)
|--S. biporcata Adams 1850H09
|--S. concinna Gould 1845H09
|--S. elegans Gray 1847H09
|--*Plocamotis’ laevisW93
|--S. maculata Quoy & Gaimard 1834H09
|--S. orbiculata Adams 1850H09
|--S. sulcifera Lamarck 1822H09
|--S. terminalis (Verco 1905)W93
`--S. ungula (Hedley 1907)W93 [=Gena ungulaH09]

*Pseudostomatella papyracea (Gmelin 1791)KC60 [=Turbo papyraceusKC60, Stomatella papyraceaKC60, Stomatia (*Pseudostomatella) papyraceaF26]

Stomatella impertusa Burrow 1815W93 [incl. *S. auricula Lamarck 1816W93, KC60, Gena auriculaM54, G. lentricula Adams 1850W93, Stomatella nigra Quoy & Gaimard 1834W93, KC60, S. (*Gena) nigraKC60, Gena striatula Adams 1850W93, S. strigosa Adams 1850W93, G. varia Adams 1850W93, S. variaBW09]

*Type species of generic name indicated


[BR05] Bouchet, P., & J.-P. Rocroi. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1–2): 1–397.

[BR17] Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

[BC01] Boyko, C. B., & J. R. Cordeiro. 2001. Catalog of Recent type specimens in the Division of Invertebrate Zoology, American Museum of Natural History. V. Mollusca, part 2 (class Gastropoda [exclusive of Opisthobranchia and Pulmonata], with supplements to Gastropoda [Opisthobranchia], and Bivalvia). Bulletin of the American Museum of Natural History 262: 1–170.

[BW09] Bryce, C., & C. Whisson. 2009. The macromolluscs of Mermaid (Rowley Shoals), Scott and Seringapatam Reefs, Western Australia. Records of the Western Australian Museum Supplement 77: 177–208.

[F26] Finlay, H. J. 1926. A further commentary on New Zealand molluscan systematics. Transactions and Proceedings of the New Zealand Institute 57: 320–485.

[G88] Gray, J. 1988. Evolution of the freshwater ecosystem: the fossil record. Palaeogeography, Palaeoclimatology, Palaeoecology 62: 1–214.

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

[H09] Hedley, C. 1909. The Marine Fauna of Queensland: Address by the President of Section D. Australasian Association for the Advancement of Science: Brisbane.

[KC60] Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I169–I331. Geological Society of America, and University of Kansas Press.

[M54] Macpherson, J. H. 1954. Molluscs (sea shells and snails). In: Willis, J. M. (ed.) The Archipelago of the Recherche pp. 55–63. Australian Geographical Society: Melbourne.

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.

[P61] Powell, A. W. B. 1961. Shells of New Zealand: An illustrated handbook 4th ed. Whitcombe and Tombs Limited: Christchurch.

[WBS-S15] Willan, R. C., C. Bryce & S. M. Slack-Smith. 2015. Kimberley marine biota. Historical data: molluscs. Records of the Western Australian Museum Supplement 84: 287–343.

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

[W93] Wilson, B. 1993. Australian Marine Shells vol. 1. Prosobranch Gastropods. Part One. Odyssey Publishing: Kallaroo (Western Australia).

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