Elephant snail Scutus ambiguus, copyright Leon Altoff.

Belongs within: Vetigastropoda.
Contains: Diodora, Fissurella, Puncturella, Tugali, Emarginula.

The Fissurellidae, keyhole and slit limpets, are a group of limpet-like gastropods in which the shell bears a slit or perforation for the exhalant current from the gills. Members of the Diodorinae and Fissurellinae have an apical aperture on the shell bordered by a callus whereas Hemitoma and Emarginulinae have a more-or-less developed longitudinal slit running back from the front of the shell. In some cases the slit is only developed on the shell ventrally with only an anterior emargination or notch visible dorsally. Members of the genus Scutus have a depressed shell that in life is largely covered by the expanded mantle. In Hemitoma, the shell may be moderately elevated but not conical, with a distinct internal groove and a short slit or notch anteriorly (Knight et al. 1960).

Diodorinae and Fissurellinae may be distinguished by the shape of the callus around the aperture which is rounded in Fissurellinae but posteriorly truncate in Diodorinae. The New World genus Lucapinella bears a shell with a subcentral apex and large perforation; the shell is internally crenulate at each end and its posterior margin is slightly elevated (Knight et al. 1960).

Marginal limpets
Published 2 January 2012
Shells of Emarginula solidula, photographed by Jan Delsing.

For today’s subject taxon, I’ve drawn the Emarginulini. This is a tribe within the gastropod family Fissurellidae, members of which are commonly known as keyhole or slit limpets. They get these names because of openings in their shells: keyhole limpets have a distinct hole at the apex of their shell, while slit limpets have a longitudinal slit running back from the front of their shell. In both groups, the slit or ‘keyhole’ functions in excretion. Gastropods undergo a process early in development known as torsion: the viscera of the embryo twists around so that it reverses its original direction (you can see a basic diagram of the process here). The reasons why this happens remain somewhat uncertain (one early suggestion was that it is what allowed the larval gastropod to retract into its shell and close the shell opening with an operculum) but a potentially negative side-effect of the process is that the anus comes to open directly above the mouth. Unless you want to go through your life with a bit of a funny taste in your mouth, this is not ideal. Therefore, many torted gastropods develop some sort of sinus or recess in their shell so the anal opening can be moved rearwards, away from the mouth.

Live individual of Tugali parmophoroidea, from here. Tugali has an expanded mantle, but is still able to retract it body underneath the shell for protection.

The members of the Emarginulini as recognised by Bouchet & Rocroi (2005) are slit limpets rather than keyhole limpets, though in some species the slit has become much reduced and may only be visible on the underside of the shell (i.e there is a ventral groove rather than a full slit). Bouchet & Rocroi (2005) divided the Fissurellidae between the Fissurellinae and Emarginulinae, a classification based on the structure of the radula and shell muscles (Aktipis et al. 2011). The Fissurellinae are all keyhole limpets, but Bouchet et al.‘s Emarginulinae included (in addition to the Emarginulini) the tribe Diodorini, whose members are keyhole limpets like the Fissurellinae but have emarginuline internal anatomy. Other authors have recognised this group as a third intermediate subfamily. Two further tribes of Emarginulinae, the Scutini and Fissurellideini, include species with expanded mantles and reduced shells that may be entirely concealed within the soft body of the slug-like animal.

An elephant snail or shield limpet Scutus sp., from here. This genus has a greatly enlarged mantle, which usually folds over to conceal the reduced shell.

However, this classification of the fissurellids was challenged by the molecular analysis of Aktipis et al. (2011). The results of these authors indicated that the Emarginulini of Bouchet & Rocroi (2005) is para- or polyphyletic. A clade of Fissurellinae with Diodorini indicates a single origin of keyhole limpets, with the emarginuline radula and muscle structure being ancestral for fissurellids as a whole. This result is also consistent with the fossil record: ’emarginulines’ are known as early as the Triassic, but fissurellines and diodorines have not been found earlier than the Caenozoic. Aktipis et al. (2011) therefore recognised a more restricted monophyletic Emarginulinae containing the genera Emarginula, Montfortula, Tugali and Scutus, while more basal forms (sister to all other fissurellids) were separated as the Hemitominae (Aktipis et al. did not analyse the position of the Fissurellideini). Of Aktipis et al.‘s Emarginulinae proper, only Emarginula has a well-developed slit (the others have ventral shell grooves; Scutus has a quite reduced shell), but this genus was also not monophyletic. Instead of aligning by morphology, the species analysed formed clusters corresponding more to their biogeography: a Mediterranean Emarginula clade, a Pacific clade of Emarginula and Montfortula species, and an Australian clade of Scutus and Tugali. The relationships between these three clades varied by analysis method. The Australian Scutus clade was not necessarily sister to the remaining Emarginulinae, so it may not be worthwhile at this point in time distinguishing the tribes Emarginulini and Scutini.

Systematics of Fissurellidae

Characters (from Knight et al. 1960): Shell conical, porcellaneous; protoconch spiral; with perforation, slit, notch or emargination for passage of exhalant current; muscle scar horseshoe-shaped, open anteriorly.

<==Fissurellidae [Fissonellidae, Fissurellacea, Fissurelladae, Fissurelloidea, Larvadae, Patelloideae]
    |  i. s.: Leurolepas roseola McLean 1970PP78
    |         Laevinesta atlanticaM84
    |--Hemitoma Swainson 1840BR05 (see below for synonymy)
    |    |  i. s.: H. aspersaW93
    |    |         H. bella (Gabb 1865) [=Emarginula bella]O27
    |    |         H. cumingii (Sowerby 1863)W93, H09 [=Emarginula cumingiiH09, Subemarginula cumingiiH09]
    |    |         H. golischae Dall 1916O27
    |    |         H. yatesii Dall 1901O27
    |    |--H. (Hemitoma) octoradiata (Gmelin 1791)KC60 (see below for synonymy)
    |    |--H. (Montfortia Récluz 1843)KC60
    |    |    `--H. (M.) subemarginata (Blainville 1819)W93 (see below for synonymy)
    |    `--H. (Montfortista Iredale 1929)KC60
    |         |--H. (*M.) excentrica (Iredale 1929) [=*Montfortista excentrica]KC60
    |         `--H. (M.) panhi (Quoy & Gaimard 1834)W93 (see below for synonymy)
    |--Diodorinae [Diodorini]BR17
    |    |--DiodoraBR05
    |    |--Megathura Pilsbry 1890 [=Macrochasma Dall 1915]KC60
    |    |    `--M. crenulata (Sowerby 1825)KC60 (see below for synonymy)
    |    |--Pseudofissurella Haber 1932KC60
    |    |    `--*P. corallensis (Buvignier 1852) [=Fissurella corallensis]KC60
    |    |--Stromboli Berry 1954KC60
    |    |    `--*S. beebei (Hertlein & Strong 1951) [=Fissurella beebei]KC60
    |    `--Monodilepas Finlay 1927KC60
    |         |--*M. monilifera (Hutton 1873)P61 [=Lucapina moniliferaP61, Fissuridea moniliferaF27]
    |         |    |--M. m. moniliferaP61
    |         |    `--M. m. cookiana Dell 1952P61
    |         |--M. carnleyensis Powell 1955P61
    |         |--M. diemenensis Finlay 1930P61
    |         |--M. otagoensis Finlay 1930P61
    |         `--M. skinneri Finlay 1928P61 [=M. monilifera skinneriD56]
    |    |--FissurellaS11
    |    |--Atractotrema Cossmann 1888KC60
    |    |    `--*A. grata (Deshayes 1861) [=Fissurella grata]KC60
    |    |--Cosmetalepas Iredale 1924 [incl. Profissurellidea Wenz 1938]KC60
    |    |    `--*C. concatenatus (Crosse & Fischer 1864)W93 [=Fissurella concatenataKC60, Fissuridea concatenataF27]
    |    |--Megatebennus Pilsbry 1890KC60
    |    |    `--*M. bimaculatus (Dall 1872) [=Fissurellidea bimaculata]KC60
    |    |--Lucapina Gray ex Sowerby 1835 [incl. Chlamydoglyphis Pilsbry 1890, Foraminella Guilding ex Sowerby 1835]KC60
    |    |    |--L. sowerbii (Guilding ex Sowerby 1835) (see below for synonymy)KC60
    |    |    `--L. elisae Costa & Simone 2006S11
    |    |--Amblychilepas Pilsbry 1890 [incl. Sophismalepas Iredale 1924]KC60
    |    |    |--*A. javanicensis (Lamarck 1822)W93 (see below for synonymy)
    |    |    |--A. compressa Thiele 1930W93
    |    |    |--A. crucis (Beddome 1882)W93
    |    |    |--A. nigrita (Sowerby 1834)W93 (see below for synonymy)
    |    |    |--A. oblonga (Menke 1843)W93 [=Sophismalepas oblongaM54]
    |    |    `--A. omicron (Crosse & Fischer 1864)W93
    |    |--Macroschisma Sowerby 1839W93 (see below for synonymy)
    |    |    |--M. hiatula Swainson 1840 (see below for synonymy)W93
    |    |    |--M. bakiei Adams in Sowerby 1862W93
    |    |    |--M. macroschisma (Humphrey ex Solander 1786) [=Patella macroschisma]KC60
    |    |    |--M. madreporaria Hedley 1907H09
    |    |    |--M. munita Iredale 1940W93
    |    |    |--M. producta Adams 1850W93 [=M. (*Dolichoschisma) productaKC60]
    |    |    |--M. tasmaniae Sowerby 1866W93 [=M. (*Forolepas) tasmaniaeKC60]
    |    |    `--M. tasmanica Tenison Woods 1876TW76
    |    `--Lucapinella Pilsbry 1890KC60
    |         |--*L. callomarginata (Carpenter ex Dall 1872) [=Clypidella callomarginata]KC60
    |         |--L. gigas Usticke 1959BC01
    |         |    |--L. g. gigasBC01
    |         |    `--L. g. oblonga Usticke 1959BC01
    |         |--L. limatulaBC01
    |         |    |--L. l. limatulaBC01
    |         |    |--L. l. delicata Usticke 1969BC01
    |         |    `--L. l. hassleri Pérez Farfante 1943BC01
    |         `--L. nigrita (Sowerby 1834) [=Fissurella nigrita]H09
    `--Emarginulinae [Emarginulidae, Emarginulini]BR05
         |--Emarginella Pilsbry 1891KC60
         |    `--*E. cuvieri (Audouin 1826) [=Emarginula cuvieri]KC60
         |--Emersonia Haber 1932KC60
         |    `--*E. costata (Emerson 1870) [=Cemoria costata]KC60
         |--Loxotoma Fischer 1885KC60
         |    `--*L. neocomiensis (d’Orbigny 1843) [=Emarginula neocomiensis]KC60
         |--Clypidina Gray 1847 [Clypidinidae, Clypidinoidei]BR05
         |    `--*C. notata (Linnaeus 1758) [=Patella notata]BR17
         |--Rixa Iredale 1924W93
         |    `--*R. watsoni (Henn & Brazier 1894)W93 [=Glyphis watsoniW93, Puncturella (*Rixa) watsoniKC60]
         |--Rimulopsis Haber 1932KC60
         |    |--*R. (Rimulopsis) goldfussi (Roemer 1836) [=Emarginula goldfussi]KC60
         |    `--R. (Koniakaua Haber ex Dacqué 1933)KC60
         |         `--R. (*K.) multistriata (Zittel 1873) [=Rimula multistriata]KC60
         |--Fissurellidea d’Orbigny 1839 [Fissurellideinae, Fissurellideini, Fissurellidinae]BR05
         |    |--F. (Fissurellidea) hiantula (Lamarck 1822) (see below for synonymy)KC60
         |    `--F. (Pupillaea Sowerby 1835) [=Papillaea (l. c.), Pupilia (l. c.)]KC60
         |         `--F. (*P.) aperta (Sowerby 1825) [=Fissurella aperta]KC60
         |--Rimula Defrance 1827BR05 [=Rimularia Waldheim 1834KC60; Rimulacea, Rimulidae]
         |    |--*R. blainvillii (Defrance 1827)BR17 [=Emarginula blainvilliiBR17, *Rimularia blainvilliKC60]
         |    |--R. exquisita Adams 1853H09
         |    |--R. mariei Crosse 1866W93
         |    |--R. mazatlanicaC64
         |    `--R. mexicana Berry 1969PP78
         |--Austriacopsis Haber 1932KC60
         |    |--*A. (Austriacopsis) austriaca (Hörnes 1853) [=Rimula austriaca]KC60
         |    |--A. (Balinula Haber ex Dacqué 1933)KC60
         |    |    `--A. (*B.) triontina (Greco 1899) [=Emarginula triontina]KC60
         |    `--A. (Puncturellopsis Haber 1932)KC60
         |         `--A. (*P.) acuta (Eudes-Deslongchamps 1842) [=Fissurella acuta]KC60
         |--Montfortula Iredale 1915 [incl. Plagiorhytis Fischer 1885 non Chaudoir 1848]KC60
         |    |--*M. rugosa (Quoy & Gaimard 1834)KC60 (see below for synonymy)
         |    |--M. chathamensis Finlay 1928P61
         |    |--M. kaawaensis (Bartrum 1919) [=Tugalia kaawaensis]F27
         |    |--M. lyallensis Mestayer 1928P61
         |    |--M. pulchra (Adams 1851)W93
         |    `--M. variegata Adams 1852W78
         |--Zeidora Adams 1860BR05 (see below for synonymy)
         |    |  i. s.: Z. legrandi Tate 1894W93
         |    |         Z. lodderae Tate & May 1900H09
         |    |         Z. maoria Powell 1937P61
         |    |         Z. tasmanica Beddome 1883W93
         |    |--*Z. (Zeidora) calceolina Adams 1860P61
         |    `--Z. (Nesta Adams 1870)KC60
         |         `--Z. (*N.) candida (Adams 1870) [=*Nesta candida]KC60
         `--Scutus Montfort 1810BR05 (see below for synonymy)
              |  i. s.: S. breviculus (Blainville 1817)P61 [=Parmophorus breviculusS79, Emarginula breviculaS79]
              |         S. elongatus [=Patella elongata,; incl. Parmophorus laevis]S79
              |         ‘Parmophorus’ fragilisS79
              |         ‘Parmophorus’ fissurellaS79
              |         ‘Parmophorus’ gibbosusS79
              |         ‘Parmophorus’ sinensisS79
              |--S. (Scutus)W93
              |    |--*S. (S.) antipodes Montfort 1810W93 (see below for synonymy)
              |    |--S. (S.) ambiguus (Chemnitz 1795)F27 [=Patella ambiguaS79]
              |    |--S. (S.) olonguis Iredale 1940 [=*Aviscutum olonguis]W93
              |    `--S. (S.) unguis (Linnaeus 1758)W93 (see below for synonymy)
              |--S. (Nannoscutum Iredale 1937)KC60
              |    `--S. (*N.) forsythi (Iredale 1937) [=*Nannoscutum forsythi]KC60
              `--S. (Proscutum Fischer 1885)KC60
                   `--S. (*P.) compressus (Deshayes 1861) [=Parmophorus compressus]KC60

*Amblychilepas javanicensis (Lamarck 1822)W93 [=Fissurella javanicensisKC60, Fissuridea javanicensisF27, Megatebennus javanicensisH09; incl. Fissurella trapezina Sowerby 1835KC60, Amblychilepas trapezinaKC60]

Amblychilepas nigrita (Sowerby 1834)W93 [=Fissuridea nigritaF27, *Sophismalepas nigritaF27; incl. A. pritchardi Hedley 1895W93]

Fissurellidea (Fissurellidea) hiantula (Lamarck 1822) [=Fissurella hiantula; incl. *Fissurellidea megatrema d’Orbigny 1841]KC60

Hemitoma Swainson 1840BR05 [=Subemarginula Gray 1847KC60; incl. Siphonella Issel 1869 non Hagenow 1851KC60; Hemitomidae, HemitominaeBR17]

Hemitoma (Hemitoma) octoradiata (Gmelin 1791)KC60 [=Patella octoradiataKC60, Emarginula (Hemitoma) octoradiataO27; incl. E. tricostata Sowerby 1823BR17, *Hemitoma tricostataKC60, Patella tricostata non Gmelin 1791KC60, *Subemarginula tricostataKC60]

Hemitoma (Montfortia) subemarginata (Blainville 1819)W93 [incl. Emarginula australis Quoy & Gaimard 1834KC60, W93, KC60, H. (*M.) australisKC60, H. emarginata Blainville 1825W93]

Hemitoma (Montfortista) panhi (Quoy & Gaimard 1834)W93 [incl. Emarginula clathrata Adams & Reeve 1850H09, W93, Subemarginula clathrataH09, Hemitoma panihensis Adams 1852W93, H. tricarinata Born 1778 non Linnaeus 1767W93]

Lucapina sowerbii (Guilding ex Sowerby 1835) [=*Foraminella sowerbii; incl. Fissurella cancellata Sowerby 1835 non Gray 1825, *Lucapina cancellata]KC60

Macroschisma Sowerby 1839W93 [=Machrochisma Swainson 1840KC60, Macrochisma Gray 1840KC60; incl. Dolichoschisma Iredale 1940W93, Forolepas Iredale 1940W93]

Macroschisma hiatula Swainson 1840 [incl. *M. macroschisma Sowerby 1839 non Patella macroschisma Humphrey ex Solander 1786]W93

Megathura crenulata (Sowerby 1825)KC60 [=Fissurella crenulataKC60, Lucapina crenulataO27; incl. *Megathura californica Pilsbry 1890KC60, *Macrochasma californicumKC60]

*Montfortula rugosa (Quoy & Gaimard 1834)KC60 [=Emarginula rugosaP61, Clypidina (*Montfortula) rugosaKC60, Hemitoma rugosaF27, Subemarginula rugosaH09; incl. H. conoidea Reeve 1842F27, W93, Montfortula conoideaF27]

Scutus Montfort 1810BR05 [=Parmaphora Bowdich 1822KC60, Parmophora Desmarest 1859KC60, Parmophorus deBlainville 1817KC60, Scutum Sowerby 1842 non Schumacher 1817KC60; incl. Aviscutum Iredale 1940KC60; Scutinae, Scutini]

*Scutus (Scutus) antipodes Montfort 1810W93 [incl. Patella anatina Donovan 1820S79, S. anatinusW93, Parmophorus australis Lamarck 1822S79, Par. convexusS79, Par. elongatus Blainville 1817 non Patella elongataS79, Emarginula elongataS79, Scutus elongatusS79, S. tumidus Ad. 1851S79, S. veitchi Cotton 1953W93]

Scutus (Scutus) unguis (Linnaeus 1758)H09 [=Patella unguisH09, Mytilus lingua Lightfoot 1786K65; incl. Scutus angustatus Adams 1851S79, Parmophorus corrugatus Reeve 1842S79, R42, S. corrugatusS79, Par. elegans Gray 1825S79, Par. emarginatus Philippi 1851S79, Par. granulatus Blainville 1819S79, W93, Scutus granulatusS79, Par. imbricatusS79, S. imbricatusS79, Par. rueppeli Philippi 1851S79]

Zeidora Adams 1860BR05 [=Zidora Fischer 1885BR05; incl. Crepiemarginula Seguenza 1880KC60, Legrandia Beddome 1883 non Hanley 1872KC60; Zeidoridae, Zidoridae]

*Type species of generic name indicated


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[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.

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[F27] Finlay, H. J. 1927. A further commentary on New Zealand molluscan systematics. Transactions and Proceedings of the New Zealand Institute 57: 320–485.

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

[K65] Kay, E. A. 1965. The Reverend John Lightfoot, Daniel Solander, and the Portland Catalogue. Nautilus 79 (1): 10–19.

[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.

[M84] McLean, J. H. 1984. A case for derivation of the Fissurellidae from the Bellerophontacea. Malacologia 25 (1): 3–20.

[O27] Oldroyd, I. S. 1927. The Marine Shells of the West Coast of North America vol. 2 pt 3. Stanford University Press: Stanford University (California).

[PP78] Poorman, F. L., & L. H. Poorman. 1978. Additional molluscan records from Bahía de Los Angeles, Baja California Norte. Veliger 20 (4): 369–374.

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

[R42] Reeve, L. 1842. Descriptions of new species of shells, principally from the collection of Hugh Cuming, Esq. Proceedings of the Zoological Society of London 10: 49–50.

[S11] Simone, L. R. L. 2011. Phylogeny of the Caenogastropoda (Mollusca), based on comparative morphology. Arquivos de Zoologia 42 (4): 161–323.

[S79] Smith, E. A. 1879. Notes on the species of the genus Scutus. Journal of Conchology 2: 252–264.

[TW76] Tenison Woods, J. E. 1876. Description of new Tasmanian shells. Papers and Proceedings of the Royal Society of Tasmania 1875: 134–162.

[W78] Wells, F. E. 1978. Zonation of marine gastropods on a rocky intertidal shore in the Admiralty Gulf, Western Australia, with emphasis on the genus Nerita. Veliger 20 (3): 279–287.

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


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