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Anentome 'helena' (Philippi, 1847)

Diagnostic features

The shell is conical with a distinct anterior notch and posterior indentation in the aperture. The sculpture consists of strong widely-spaced regular axial ribs.  Colour dark brown, with yellowish bands. Animal with narrow tentacles, with eyes situated on the outer side of each tentacle at a short distance from the base. Operculum with terminal nucleus. Possesses a retractile proboscis and a long anterior siphon that extends well beyond the shell.

Classification

Anentome helena (Philippi, 1847)

Common name: Assassin snail

Class Gastropoda

Infraclass Caenogastropoda

Order Neogastropoda

Superfamily Buccinoidea

Family Nassariidae

Subfamily Anentominae

Genus Anentome Cossmann, 1901 (Type species: Melania helena von dem Busch (in Philippi), 1847) (Synonym of genus:  Canidia H. Adams, 1862, preoccupied) 

Original name: Melania helena von dem Busch (in Philippi), 1847. Philippi, R. A. 1847. Abbildungen und Beschreibungen neuer oder wenig gekannter Conchylien. Cassel. (Th. Fischer), 1-231.

Type locality: Java.

State of taxonomy

This species has been recently transferred from the Buccinidae to the Nassariidae (Galindo et al. 2016), and Strong et al. (2017), the latter making a separate subfamily (Anentominae) for this taxon.

Strong et al. (2017) suggested that the common taxon found in aquaria is unlikely to be this taxon (originally described from Java) but did not provide an alternative name.

Biology and ecology

Found in the lower reaches of freshwater areas of rivers on sand and mud. Predator and scavenger, feeding on worms, gastropods and carrion. Animal with separate sexes. The female lays several clear egg capsules which are roughly square in shape. Each egg capsule contains a single egg. The egg capsules are generally laid on solid surfaces and commonly at the base of plants.

Distribution

Native to Southeast Asia, but introduced into aquaria into many parts of the world.

Notes

This is the only known freshwater nassariid. It could become a pest by preying on native molluscs if introduced into natural environments in Australia. There are no native carnivorous freshwater gastropods present in Australia, and such an introduction would therefore be of major concern.

The name Clea is sometimes used for this genus, but is now regarded as a separate genus. These taxa represent a radiation of freshwater nassariids found in SE Asia and India, and comprise the genera Anentome, Clea and Nassodonta (Strong et al. 2017).

An unrelated species that superficially resembles Anentome is Melanopsis praemorsa (Linnaeus, 1758) from Europe. It is a member of the Melanopsidae and is included in the Cerithioidea along with the Thiaridae. It is, however, somewhat similar to Anentome in outline, sometimes has axial ribs and has an anterior canal in the aperture. This species has been intercepted by Australian Biosecurity.

Further reading

Beesley, P. L., Ross, G. J. B. & Wells, A., Eds. (1998). Mollusca: The Southern Synthesis. Parts A & B. Melbourne, CSIRO Publishing.

Brandt, R. A. M. (1974). The non-marine aquatic Mollusca of Thailand. Archiv Für Molluskenkunde 105: 1-423.

Galindo, L. A, Puillandre, N., Utge, Lozouet, J. P. & Bouchet, P. (2016). The phylogeny and systematics of the Nassariidae revisited (Gastropoda, Buccinoidea). Molecular Phylogenetics and Evolution 99: 337-353.

Mienis, K. H. (2011). Will the uncontrolled sale of the snail-eating gastropod Anentome helena in aquarium shops in Israel result in another disaster for Israel's native freshwater mollusc fauna? Ellipsaria 13: 10-13.

Monks, N. (2009). Assassin Snails and Sulawesi Elephant Snails: Keeping Clea and Tylomelania in the aquarium Conscientious Aquarist Magazine 6: 4.

Newel, M. S. & Bourne, G. B. (2013). The assassin snail, Clea (Anentome) helena (Gastropoda: Buccinidae), as a model for developmental and environmental physiology. Integrative and Comparative Developmental Biology 2013 Annual Meeting, Oxford University Press.

Ng, T.H., Tan, S.K., Wong, W.H., Meier, R., Chan, S-Y., Tan, H.H. and Yeo, D.C.J. (2016). Molluscs for sale: assessment of freshwater gastropods and bivalves in the ornamental pet trade. PLOS One. DOI:10.1371/journal.pone.0161130.

Strong, E. E., Gargominy, O., Ponder, W. F. & Bouchet, P. (2008). Global diversity of gastropods (Gastropoda: Mollusca) in freshwater. Hydrobiologia 595: 149-166.

Strong, E. E., Galindo, L. A. & Kantor, Y. I. (2017). Quid est Clea helena? Evidence for a previously unrecognized radiation of assassin snails (Gastropoda: Buccinoidea: Nassariidae). PeerJ 5: e3638.