Key Limpets

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 (Moore 1960). Moore (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.

REFERENCES

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.

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

Moore, R. C. (ed.) 1960. 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. Geological Society of America, Inc. and University of Kansas Press.

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

The Crossostomatinae of the Mesozoic

The hard shell of many molluscs has left them with an excellent fossil record, one with few rivals among other groups of organisms. As a result, we are aware of a great many molluscan lineages that have inhabited this planet in the past, only to fade away long before the present day. One such group is the gastropod subfamily Crossostomatinae.

Crossostoma specimen, from Szabó et al. (1993).

The Crossostomatinae were Mesozoic representatives of the vetigastropods, one of the major subdivisions of gastropods corresponding to what used to be referred to as the archaeogastropods. Vetigastropods are primarily marine (off the top of my head, I can't think of any that are found in freshwater or terrestrial habitats, though I'm happy to be corrected) and crossostomatines were no exception. The classification of vetigastropods has tended to be rather unsettled but crossostomatines were definitely part of the lineage that includes the modern top shells (Trochidae) and cat's-eyes (Turbinidae), recognised as the superfamily Trochoidea in the recent synoptic classification of Bouchet et al. (2017). Within this lineage, the crossostomatines belong to the group of families possessing a calcareous operculum (sometimes treated as a separate superfamily Turbinoidea, but the significance of the calcareous vs horny operculum division in the trochoids seems to be the subject of debate). In recent treatments, the Crossostomatinae have been included within the family Colloniidae, characterised by the lack of a nacreous layer on the inside of the shell (Monari et al. 1996).

In general, crossostomatines were small shells with a smooth outer surface and broadly rounded whorls. They varied in shape from forms resembling modern cat's-eyes to lower-coiling, almost planispiral forms. A notable feature of the group is a tendency for the top of the aperture to be filled by a callus so the aperture appears almost perfectly circular. Other modifications of the mature shell opening are also common: Crossostoma, for instance, has the outer lip strongly thickened (Knight et al. 1960) whereas the final whorl of Adeorbisina turns away slightly from the regular coiling axis so that in top-down view the shell appears to bulge outwards before the terminus (Szabó et al. 1993).

Though they persisted through most of the Mesozoic, the number of known crossostomatine genera does not appear to be large. They seem to be associated with hard-ground deposits (Conti & Szabó 1987) so it is possible the group was more diverse in high-energy environments (organisms living in such environments, for instance along rocky shores, tend not to get preserved in the fossil record because their remains are broken up by wave action). It is possible that their lineage did not truly go extinct in the Mesozoic: Szabó et al. (1993) allude to the possibility of crossostomatines being ancestral to the subfamily Colloniinae, members of which may have survived to the Pliocene. Nevertheless, the Colloniidae as a whole did not survive to the present day, and it seems the line of the crossostomatines may have entirely passed from this Earth.

REFERENCES

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.

Conti, M. A., & J. Szabó. 1987. Comparison of Bajocian gastropod faunas from the Bakony Mts. (Hungary) and Umbria (Italy). Annales Historico-Naturales Musei Nationalis Hungarici 79: 43–59.

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.

Monari, S., M. A. Conti & J. Szabó. 1996. Evolutionary systematics of Jurassic Trochoidea: the family Colloniidae and the subfamily Proconulidae. In: Taylor, J. D. (ed.) Origin and Evolutionary Radiation of the Mollusca pp. 199–204. Oxford University Press: Oxford.

Szabó, J., M. A. Conti & S. Monari. 1993. Jurassic gastropods from Sicily; new data to the classification of Ataphridae (Trochoidea). Scripta Geologica, Special Issue 2: 407–416.