Clea helena
(assassin snail)

Pictures

Picture	Title	Caption	Copyright
Title Adults mating Caption Clea helena; aquarium specimens mating. Copyright Neale Monks	Adults mating	Clea helena; aquarium specimens mating.	Neale Monks
Title Shell variety Caption Clea helena; a variety of adult and juvenile shells. Note the loss of the apex commonly seen on the adult shells. Copyright Neale Monks	Shell variety	Clea helena; a variety of adult and juvenile shells. Note the loss of the apex commonly seen on the adult shells.	Neale Monks
Title Pathway Caption Advertisement for Clea helena at an aquarium retail-shop in the UK. Copyright Neale Monks	Pathway	Advertisement for Clea helena at an aquarium retail-shop in the UK.	Neale Monks

Identity

Preferred Scientific Name

• Clea helena (Philippi, 1847)

Preferred Common Name

• assassin snail

Other Scientific Names

• Anentome helena Philippi, 1847

International Common Names

• English: bumblebee snail; killer snail; snail-eating snail

Local Common Names

• Netherlands: slaketende slak

Summary of Invasiveness

Clea helena is a Southeast Asian freshwater snail closely related to the marine whelks (family Buccinidae). It has not been reported in the literature as an invasive species but is potentially a threat in warmer regions for a number of reasons. Firstly, it is widely traded as an aquarium animal and can be commonly found in aquarium shops in North America, Europe and Asia already, and will likely be regularly traded elsewhere in due course. Secondly, unlike some other freshwater invertebrates traded as aquarium animals, such as the shrimp Caridina multidentata and the snail Neritina coromandeliana, its life cycle does not include a marine stage, so it is able to reproduce under aquarium conditions. Finally, under aquarium conditions at least, it will readily consume small aquatic snail species that it would not encounter in its natural habitat, and there is no reason to suppose it would be any less predatory if introduced populations became established outside the species’ natural range.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Mollusca
•             Class: Gastropoda
•                 Subclass: Caenogastropoda
•                     Order: Neogastropoda
•                         Unknown: Buccinoidea
•                             Family: Buccinidae
•                                 Genus: Clea
•                                     Species: Clea helena

Notes on Taxonomy and Nomenclature

The higher classification of the Gastropoda of Bouchet and Rocroi (2005) places Clea in clade Neogastropoda of the Caenogastropoda. 

Clea helena was originaly described as Anentome helena and both names are widely used for this species; Anentome is frequently used as a subgenus, i.e., as Clea (Anentome) helena. This organism does not have any significant commercial value outside the aquarium industry, hence the prevalence of common names featured in the aquarium literature. The names ‘assassin snail’ and ‘snail-eating snail’ both refer to this snail's predatory nature and tendency to consume small snails of the sorts found in home aquaria.

Description

C. helena is a small predatory snail (feeding on both live prey, including other snails, and carrion), which belongs to the same family as the marine whelks but is confined to freshwater habitats of various kinds.

It conforms to its family in terms of shape, having a tightly coiled shell with moderately deep radial ribs and one, two, or three brown spiral bands. Maximum length is around 30 mm but usually smaller; in particular, the apical part of the shell is often worn away on older individuals. There is an obvious siphonal notch through which the siphon protrudes and the operculum is brown. Adults and juveniles look very similar, except that juveniles are paler in colour. For further details see Brandt (1974).

Distribution

C. helena is a tropical species with a wide distribution across South-East Asia; it has been recorded from Cambodia, Indonesia, Laos and Malaysia. Ng et al. (2016a) report occurrence in Singapore as the first non-native establishment of this species, further asserting the introduction was via the aquarium trade. Despite substantial holdings of Vietnamese molluscs in the collection of the Academy of Natural Sciences in Philadelphia, USA, there are no specimens of C. helena (DG Robinson, USDA-APHIS, USA, personal communication, 2012). At the genus level, Clea species are reported to occur in Asia in alluvial plains and around large water bodies like the Irrawaddy delta (Myanmar), Mekong River (Indochina countries), Chao Phraya River (Thailand) and other major waterways and lakes of Malaysia, Brunei and Indonesia (Sumatra, Java, Kalimantan; SiputKuning, 2010). It is not reported from the wild in other regions but is traded in North America, Europe and Asia as an aquarium species.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

Ng et al. (2016a) describe the discovery of the species in a reservoir in Singapore, and believe it to be the first report of Clea helena as an introduced species. While the species naturally occurs in adjacent Malaysia, the authors believe the species got into Kranji Reservoir via the aquarium trade.

Habitat

Little is known about the ecology of C. helena in the wild, but it is known to occur in a range of freshwater habitats including streams, rivers, lakes and reservoirs, typically in places with a soft substrate such as mud. In some locations it may be amongst the dominant gastropod species. Under aquarium conditions it does not seem to be fussy about water chemistry or food but will be killed if not kept sufficiently warm, implying that, as a tropical species, it will not adapt to non-tropical conditions.

Habitat List

Biology and Ecology

C. helena feeds predominantly on decaying protein, but has been observed to attack living snails and worms (Brandt, 1974). In aquaria it spends some of the time burrowing under the surface of the substrate but appears to forage on the surface. Unfortunately, little is known about its ecology in the wild.

It is dioecious. Under aquarium conditions snails mate above the surface of the substrate and then the females deposit their eggs one at a time in small, semi-transparent capsules (sacs) on solid surfaces such as stones and plant stems. Hatching and development are imperfectly known, but aquarium reports suggest that the juveniles hide in the substrate most of the time, and only become obvious once they are about six months old by which time they have a shell length of around 8 mm. Breeding under aquarium conditions is said to be easy.

Climate

Water Tolerances

Means of Movement and Dispersal

C. helena is transported widely as an aquarium species (Ng et al., 2016a).

Pathway Causes

Impact: Economic

C. helena is traded as an aquarium species.

Impact: Environmental

Impact on Habitats

None reported.

Impact on Biodiversity

None reported, but a potential predator on small aquatic snails.

Risk and Impact Factors

• Has a broad native range
• Abundant in its native range
• Capable of securing and ingesting a wide range of food
• Fast growing
• Has high reproductive potential

• Modification of natural benthic communities
• Reduced native biodiversity
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Predation
• Rapid growth

• Highly likely to be transported internationally deliberately
• Difficult to identify/detect as a commodity contaminant

Uses List

General

• Pet/aquarium trade

Detection and Inspection

Images of C. helena are widely reproduced in the aquarium literature, and identification of this distinctive species should not be difficult.

Similarities to Other Species/Conditions

While there are other Clea species known, none has so far been as widely traded within the aquarium industry. The distinctive shape of this species, its yellowy shell with brown spiral markings, and its predatory habits make it easily distinguished from all other aquarium snails. Nonetheless, Ng et al. (2016b) have Clea bockii as another species available in the aquarium trade. This species is similar in shape and size, but its shell is more or less uniformly brown.

Gaps in Knowledge/Research Needs

Very little is known about the ecology of this species beyond records of its presence/absence at particular locations within its range. Such data that does exist comes from aquarium observations that may or may not be relevant to understanding its natural history in the wild. For example, Ng et al. (2016a) comment on the fact that the feeding habits of this species are based on aquarium observations rather than their natural behaviour in the wild.

Trematodes pose a public health problem across large parts of the world, and many of these have a life cycle that involves freshwater snails. Although Anucherngchai et al. (2016) collected Clea helena from the Chao-Phraya Basin in Thailand and didn’t find any carrying trematodes, specimens from other parts of their range have not so far been examined.

References

Anucherngchai S; Tejangkura T; Chontananarth T, 2016. Epidemiological situation and molecular identification of cercarial stage in freshwater snails in Chao-Phraya Basin, Central Thailand. Asian Pacific Journal of Tropical Biomedicine, 6(6):539-545. http://dx.doi.org/10.1016/j.apjtb.2016.01.015

Aquaportail, 2008. [English title not available]. (Anentome helena: fiche pour maintenance en aquarium.) Anentome helena: fiche pour maintenance en aquarium. http://www.aquaportail.com/fiche-invertebre-736-anentome-helena.html

Boon-ngam P; Sriyarun J; Tanamai S; Dumrongrojwattana P, 2010. Preliminary taxonomic study of land snail and freshwater mollusk species in Sakaeo Province, Eastern Thailand. 10 pp. http://kucon.lib.ku.ac.th/dbstat/download_count.php?rec_no=KC4805019&mfn=012249&db=kucon

Bouchet P; Rocroi JP, 2005. Classification and nomenclator of gastropod families. Malacologia, 47(1/2):397 pp.

Brandt RAM, 1974. The non-marine aquatic Mollusca of Thailand. Archiv für Molluskenkunde, 105:1-405.

Monks N, 2009. Assassin Snails and Sulawesi Elephant Snails: Keeping Clea and Tylomelania in the aquarium. Conscientious Aquarist Magazine, 6(4). unpaginated. http://www.wetwebmedia.com/ca/volume_6/volume_6_4/clea.html

Monks N, 2010. Natural born killers. Practical Fishkeeping, 2010(13):14-15.

Ng TingHui; Foon JunnKitt; Tan SiongKiat; Chan MKK; Yeo DCJ, 2016. First non-native establishment of the carnivorous assassin snail, Anentome helena (von dem Busch in Philippi, 1847). BioInvasions Records, 5(3):143-148. http://www.reabic.net/journals/bir/2016/3/BIR_2016_Ng_etal2.pdf

Ng TingHui; Tan SiongKat; Wong WingHing; Meier R; Chan SowYan; Tan HeokHui; Yeo DCJ, 2016. Molluscs for sale: Assessment of freshwater gastropods and bivalves in the ornamental pet trade. PLoS ONE, 11(8):e0161130. http://dx.doi.org/10.1371/journal.pone.0161130

Philippi RA, 1847. Abbildungen und Beschreibungen neuer oder wenig gekannter Conchylien. Cassel. (Th. Fischer), 1-231.

SiputKuning, 2010. Freshwater whelks, anyone? SiputKuning (Yellow Snail) Blog, 2 January 2010. http://siputkuning.blogspot.com/2009_12_27_archive.html

Sripongpun G, 2003. Benthic Macroinvertebrates as a Biological Index of Water Quality in the Lower Thachin River. Silpakorn University International Journal - Science, 3(1-2):168-193.

Tesana S, 2002. Diversity of mollusks in the Lam Ta Khong reservoir, Nakhon Ratchasima, Thailand. The Southeast Asian Journal of Tropical Medicine and Public Health, 33(4):733-738.

Distribution References

Aquaportail, 2008. [English title not available]. (Anentome helena: fiche pour maintenance en aquarium). In: Anentome helena: fiche pour maintenance en aquarium, http://www.aquaportail.com/fiche-invertebre-736-anentome-helena.html

Boon-ngam P, Sriyarun J, Tanamai S, Dumrongrojwattana P, 2010. Preliminary taxonomic study of land snail and freshwater mollusk species in Sakaeo Province, Eastern Thailand., 10 pp. http://kucon.lib.ku.ac.th/dbstat/download_count.php?rec_no=KC4805019&mfn=012249&db=kucon

CABI, Undated. Compendium record. Wallingford, UK: CABI

Ng TingHui, Foon JunnKitt, Tan SiongKiat, Chan M K K, Yeo D C J, 2016. First non-native establishment of the carnivorous assassin snail, Anentome helena (von dem Busch in Philippi, 1847). BioInvasions Records. 5 (3), 143-148. http://www.reabic.net/journals/bir/2016/3/BIR_2016_Ng_etal2.pdf

SiputKuning, 2010. Freshwater whelks, anyone? In: SiputKuning (Yellow Snail) Blog, http://siputkuning.blogspot.com/2009_12_27_archive.html

Sripongpun G, 2003. Benthic Macroinvertebrates as a Biological Index of Water Quality in the Lower Thachin River. In: Silpakorn University International Journal - Science, 3 (1-2) 168-193.

Tesana S, 2002. Diversity of mollusks in the Lam Ta Khong reservoir, Nakhon Ratchasima, Thailand. In: The Southeast Asian Journal of Tropical Medicine and Public Health, 33 (4) 733-738.

Contributors

17/10/16 Updated by: 

Neale Monks, Consultant, UK

28/07/11 Original text by:

Neale Monks, Consultant, UK

Reviewers' names are available on request.

Distribution Maps