Diodora italica, copyright Nicolas Lagière.

Belongs within: Fissurellidae.

Key limpets
Published 20 September 2020

On two occasions before, I’ve presented you with members of the Fissurellidae, the keyhole and slit limpets. It’s time for a return visit to the fissurellids, in the form of the diverse keyhole limpet genus Diodora.

Various views of shell of Diodora italica, copyright H. Zell.

Species have been assigned to Diodora from coastal waters pretty much around the world except for the coolest regions. They are small to moderate-size limpets, the largest species growing about three centimetres in length and two centimetres in height. The shell opens through a ‘keyhole’ at the apex through which the animal ejects waste matter and water that has been passed over the gills. The internal margin of this keyhole is surrounded by a callus on the underside of the shell; a distinguishing feature of Diodora is that this callus is posteriorly truncate. The external ornament of the shell is cancellate (arranged in a criss-cross pattern) and the margin of the shell is internally crenulated (Knight et al. 1960). Knight et al. (1960) listed three subgenera of Diodora distinguished by features of the keyhole shape and position but Herbert (1989) notes that these subgenera are not clearly distinct. A phylogenetic analysis of the fissurellids by Cunha et al. (2019) did recognise a clade including the majority of Diodora species analysed. However, species from the eastern Pacific formed a disjunct clade that may prove to warrant recognition as a separate genus.

As far as is known, Diodora species have a long lifespan, surviving for some ten to twenty years. They do not have a planktonic larva; young Diodora hatch directly from the egg as benthic crawlers. For the most part, they are presumed to graze on algae in the manner of other fissurellids and limpets. However, the northeastern Australian species D. galeata has been found feeding on the soft tissues of coral (Stella 2012), a habit that went unrecognised until fairly recently owing to the animal’s cryptic nature, hiding deep among the branches of the host. Whether other Diodora species might exhibit similar lifestyles would require further investigation.

Systematics of Diodora
<==Diodora Gray 1821BR05 (see below for synonymy)
|--D. graeca (Linnaeus 1758)APB17, KC60 [=Patella graecaKC60]
`--+--D. giannispadai Aissaoui, Puillandre & Bouchet 2017APB17
`--+--D. demartiniorum Buzurro & Russo 2005APB17
`--+--D. italica (Defrance 1820) [=Fissurella italica; incl. F. neglecta Deshayes 1830]APB17
`--+--D. dorsata (Monterosato 1878)APB17
`--D. gibberula (Lamarck 1822)APB17

Diodora incertae sedis:
*D. apertura (Montagu 1803) [=Patella apertura]KC60
‘Glyphis’ altaC64
D. aspera (Eschscholtz 1833)O27 (see below for synonymy)
|--D. a. asperaO27
`--D. a. densiclathrata Reeve 1850O27 [=Fissurella densiclathrataC64, Glyphis densiclathrataC64]
D. barbadensis [incl. Fissurella huttoni Suter 1906]F27
D. cayenensisAPB17
D. corbicula (Sowerby 1862)W93 (see below for synonymy)
D. funiculata (Reeve 1850)APB17
D. galeata (Helbling 1779)W93 [=Patella galeataH09, Eligidion galeataC45, Fissuridea galeataH09]
D. granifera (Pease 1861)M84, K65 [=Fissurella graniferaK65, Diadora (l. c.) graniferaK65, Glyphis graniferaK65]
D. inaequalis (Sowerby 1834)O27 [=Fissurella inaequalisO27, Glyphis inaequalisC64]
‘Fissurella’ incii Reeve 1850H09 [=Eligidion inciiC45, Fissuridea inciiH09]
D. jukesii (Reeve 1850)W93 [=Fissurella jukesiiH09, Eligidion jukesiC45, Fissuridea jukesiiH09]
D. lincolnensis Cotton 1930 [=D. (*Austroglyphis) lincolnensis]KC60
D. lineata (Sowerby 1835)W93 [=D. audax Iredale 1924W93, D. (*Elegidion) audaxKC60]
D. murina (Carpenter in Dall 1885) [=Fissurella (Glyphis) murina, Diadora densiclathrata var. murina]O27
D. mus (Reeve 1850)WBS-S15
D. nigropunctata Thiele 1930W93 [=Eligidion nigropunctataC45]
D. occidua (Cotton 1930)W93 [=Eligidion occiduusC45]
D. octogona (Reeve 1850)W93
D. ovalis Thiele 1930W93 [=Eligidion ovalisC45]
D. plicifera Thiele 1930W93 [=Eligidion pliciferaC45]
D. producta (Monterosato 1880)APB17
‘Fissurella’ proxima Sowerby 1862H09 [=Eligidion proximaC45, Fissuridea proximaH09]
D. pusilla Berry 1959PP78
D. quadriradiata (Reeve 1850)W93 (see below for synonymy)
D. rugosa Thiele 1930W93 [=Austroglyphis rugosaC45]
D. rupellii (Sowerby 1834)APB16 [=Fissurella rueppelliiH09, Eligidion ruppeliiC45, Fissuridea rueppelliiH09]
D. saturnalis (Carpenter 1864)PP78 [=Glyphis saturnalisC64]
D. singaporensis (Reeve 1850)W93 (see below for synonymy)
D. ticaonica (Reeve 1850)W93 [=Fissurella ticaonicaH09, Eligidion ticaonicaC45, Fissuridea ticaonicaH09]

Diodora Gray 1821BR05 [incl. Austroglyphis Cotton & Godfrey 1934W93, Capiluna Gray 1857KC60, Elegidion Iredale 1924W93, Eligidion (l. c.)KC60, Fissuridea Swainson 1840KC60, Glyphis Carpenter 1857 nec Ach. 1814 (ICBN) nec Agassiz 1843KC60]

Diodora aspera (Eschscholtz 1833)O27 [=Fissuridea asperaO27, Fissurella asperaC64, Glyphis asperaC64; incl. G. cratitiaC64, Fissurella lincolniC64, G. lincolniC64]

Diodora corbicula (Sowerby 1862)W93 [=Fissurella corbiculaH09, Eligidion corbiculaC45, Fissuridea corbiculaH09; incl. Diodora lanceolata Sowerby 1862W93]

Diodora quadriradiata (Reeve 1850)W93 [=Fissurella quadriradiataH09, Eligidion quadriradiataC45, Fissuridea quadriradiataH09]

Diodora singaporensis (Reeve 1850)W93 [=Fissurella singaporensisH09, Eligidion singaporensisC45, Fissuridea singaporensisH09]

*Type species of generic name indicated


[APB16] Aissaoui, C., N. Puillandre & P. Bouchet. 2016. New insights in the taxonomy of Mediterranean Diodora (Mollusca, Gastropoda, Fissurellidae). Journal of the Marine Biological Association of the United Kingdom 97 (7): 1527–1536.

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

[C64] Carpenter, P. P. 1864. Supplementary report on the present state of our knowledge with regard to the Mollusca of the west coast of North America. Report of the British Association for the Advancement of Science 33: 517–686.

[C45] Cotton, B. C. 1945. Australian keyhole limpets. South Australian Naturalist 23 (2): 13–17.

Cunha, T. J., S. Lemer, P. Bouchet, Y. Kano & G. Giribet. 2019. Putting keyhole limpets on the map: phylogeny and biogeography of the globally distributed marine family Fissurellidae (Vetigastropoda, Mollusca). Molecular Phylogenetics and Evolution 135: 249–269.

[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.

Herbert, D. G. 1989. A remarkable new species of Diodora Gray, 1821 from south-east Africa (Mollusca: Gastropoda: Fissurellidae). Annals of the Natal Museum 30: 173–176.

[K65] Kay, E. A. 1965. Marine molluscs in the Cuming collection, British Museum (Natural History) described by William Harper Pease. Bulletin of the British Museum (Natural History): Zoology Supplement 1: 1–96, 14 pls.

[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.

[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.

Stella, J. S. 2012. Evidence of corallivory by the keyhole limpet Diodora galeata. Coral Reefs 31: 579.

[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.

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

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