Assassin snail

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: sand.

Behavior

Plant interaction: plant safe.

Feeding

Obligate carnivore that specializes in other snails. The species takes ramshorn, bladder, Malaysian trumpet, and similar pests, and also scavenges meaty foods such as bloodworm, sinking protein pellets, and dead fish. Algae and plants are ignored entirely. Once the pest population in a tank is gone, the snails need supplemental feeding or the population will decline.

Compatibility

• Used as a biological control for pest-snail outbreaks (bladder, ramshorn, Malaysian trumpet). A handful in a heavily infested tank will work through it over a span of weeks to months. The pace depends on tank size, how dense the prey population is, and which species each individual assassin happens to prefer
• Do not put them in with nerites or any small ornamental snail meant to be kept. Adult mystery snails are usually left alone, but very small or weakened individuals can be at risk
• Reports on shrimp safety are mixed. Adult cherry and amano shrimp generally get ignored, but sick, slow, or freshly molted shrimp are sometimes taken, and shrimplets are at higher risk
• Each egg capsule produces only one juvenile, so the population grows slowly and cannot run away the way pest snails do
• Use sand or fine gravel. The snails spend most of their time half-buried, and juveniles stay below the surface for months. Activity peaks around dawn and dusk and whenever a food cue is in the water

Origin and habitat

A snail-eating freshwater gastropod from Southeast Asia, with reported populations in lakes, rivers, reservoirs, and streams across Malaysia and Thailand, plus Lake Toba on the Indonesian island of Sumatra. The species was first described in 1847 by von dem Busch as Melania helena and has been moved through several genera since (Canidia, Hemisinus, Clea); the aquarium trade still labels the same animal Clea helena or Anentome helena interchangeably. The current placement is Anentome (Cossmann, 1901) within the subfamily Anentominae, which Strong, Galindo and Kantor set up in their 2017 revision (PeerJ 5:e3638), under the otherwise marine family Nassariidae. That study used shell, operculum, radula, and molecular characters to separate the old genus Clea into two genera (a narrower Clea and the new Anentome), and the authors pointed out that the group is one of only two predatory neogastropod lineages, out of an otherwise marine radiation, to have made it into freshwater. The same paper also flagged that the snail being sold in the aquarium hobby may not actually be true A. helena; no replacement name has been published yet. The shell is conical, usually banded brown and yellowish-tan (the pattern that gets it the 'bumblebee' nickname, which also belongs to the unrelated marine snail Engina mendicaria), and sometimes plain brown. Wild adults run about 18 to 32 mm tall; aquarium specimens are usually a bit smaller because food limits growth. The sexes are separate but look the same externally. Most of the day the snail sits half-buried in soft substrate with a long, snorkel-like siphon extended above the surface to draw water across its gill; when it hunts, it crawls toward other snails, holds them down with its foot, and feeds through the shell aperture with an elongated proboscis. It also scavenges carrion. Aquarium releases have established the species outside its native range in Singapore and elsewhere, where it is considered a potential threat to native gastropods.

Breeding

Reproduces slowly in aquaria. Unlike most aquarium snails, which are hermaphrodites, this species has separate sexes, although males and females cannot be told apart from the outside. Mating goes on for hours and tends to happen at night. After mating, the female lays small, more-or-less square translucent egg capsules on hard surfaces such as driftwood, rocks, glass, or plant stems, generally one to a few capsules in a short row at a time. Each capsule produces a single juvenile, not a cluster. Reported incubation times range from about three to eight weeks; one controlled study (Coelho et al., 2013) put hatching at around 52 days at 25 C. Newborns are roughly 3 mm long, look like tiny adults, and stay buried in the substrate for months, so a sand or fine-gravel bed substantially improves the odds of seeing any of them survive to visible size. The whole reproductive output is low enough that the species cannot become a pest itself, which is part of why it is used as a control for the snails that can.

Common problems

They will not eat algae or detritus. Buying them expecting a general cleaning crew is the most common mistake; on a tank with no snail prey and no extra feeding, an assassin snail starves. The diet has to be either live snails or a regular drop of meaty food (bloodworm, sinking carnivore pellets, fragments of dead fish). A 2022 paper (Berkhout & Morozov, PLOS One 17:e0264996) added an interesting wrinkle to how they hunt: at the population level the species is a generalist, but individual snails fixate on a particular prey type and stick to that preference even when other prey is more abundant. In a tank, the practical consequence is uneven results, where one snail might clear out every ramshorn while a tankmate ignores them and goes after something else. They go after prey smaller than themselves or anything that cannot fully withdraw and seal its shell with the operculum; Malaysian trumpet snails do have an operculum but are still reportedly within reach. Healthy adult mystery snails are usually safe, but small or weakened individuals are taken occasionally, so they are not a reliable pairing. Nerites are more at risk because of their size. Several assassins together can attack larger prey collectively.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size