Clea helena
(assassin snail)

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.

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.

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.

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.

General

• Pet/aquarium trade

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

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.

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.

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.