Marisa cornuarietis is a very nice, lively and interesting snail. But definitely don’t get it in a planted aquarium – I strongly warn you against it. It is a huge eater and will eat anything… No plant can resist it, and within a few days it’s just gnawed stems floating on the surface, and in a few more days not even that. And then be prepared for that eater to ask you why you don’t give it some tasty herbs again.
Although it’s not obvious at first glance, Marisa cornuarietis (syn. M. rotula, M. chiquitensis, M. intermedia, Ceratodes cornuarietis, Ampullaris cornuarietis) belongs to the ampullariid gastropods (family Ampullariidae). Only one other species, Marisa planogyra, belongs to the same genus. The bladder snails resemble only smaller snails (up to 2 cm), the shell of older specimens is flattened. Often this species is assigned by laymen to the Giant Ramshorn Snail.
The shell can reach a size of up to 5.5 cm with a width of about 2 cm; in old specimens it consists of up to 4 whorls. Females tend to be larger than males. The colouration is typically striped, with 3-6 stripes largely on the ‘underside’ of the shell, i.e. the left side. The right side is yellow and the left side dark. Similar to the ampullaria, a yellow mutation without stripes or with very light stripes exists in aquaria.
Marisa cornuarietis is native to Central and South America (Costa Rica, Honduras, Panama, Venezuela, Colombia and Brazil). It inhabits mostly shallow, densely vegetated places: lakes and ponds, irrigation canals, rivers, swamps. It can even tolerate brackish water, but then stops breeding. It has been recorded spreading from one stream across a coastal brackish lagoon to another river 1 mile away. It can tolerate very polluted water with low oxygen content; it can even survive subtropical conditions (temperature 12 °C; however, the optimum range is 18-28 °C).
It has gradually been introduced to other tropical regions, either intentionally or unintentionally. First it was to Cuba and Puerto Rico, and later it appeared in Florida and Texas (very likely the ancestors of these populations were unwitting aquarium inhabitants, released into the wild). It is currently also found in Africa and Asia, where it was introduced with a special mission. More at the end of the article…
Marisa cornuarietis is, as I have already written, a very voracious creature. It feeds on plants, dead material, small animals and their eggs… it will eat just about anything, even paper. When it comes to plants, it has an interesting method of disposal – it doesn’t nibble leaves from the edge, but prefers to go right from the stem. I left a cryptocoryne planted in a basket in the maris aquarium, hoping to transplant it elsewhere before it could get drunk. Within minutes, only a few millimeters of petioles were peeking out of the basket, the rest of the plant floating on the surface. Marisa just hit something edible and started nibbling and nibbling… I’ve never seen anything like this in any other species.
But to write something positive: marisa eats all kinds of algae, even filamentous and bristle worms. But practically, it is not easy to use it in the aquarium… it prefers taller plants! But it is perfect for aquariums without plants, which it can keep completely algae-free. Beware, however, that Marisa cornuarietis has very long antennae, which are a great attraction for fish. So keeping maris is not at all trouble-free.
Various keepers have found their snails to have interesting preferences when it comes to plant species – some have remained intact in the aquarium for a long time. But this cannot be relied on, because in others, even supposedly “resistant” plant species took over very quickly…
All representatives of ampullariids are gonochorists, i.e. they have separate sexes unlike most other gastropods. However, it is often difficult to distinguish females and males by external features. This is not9 the case with this species; Marisa cornuarietis has a well-defined sex. The female tends to be larger and has a rather oval shell opening, whereas the male has an almost regularly circular opening.
There is another peculiarity in this family, and that is the laying of eggs out of the water. Again, however, this does not apply to maris. The whitish eggs in a jelly-like shell are laid on plants below the water’s surface. They measure 2-3 mm and grow to 4 mm during their development, which takes about 2-3 weeks. In the period before hatching, the egg wall is transparent. There are usually 30-50 eggs per clutch and the female, who stores sperm for longer after mating, can lay eggs about every 3 weeks.
Marisa cornuarietis has lungs and gills as well as a respiratory tube (siphon), although slightly shorter than that of the ampullaria. This allows the gastropod to breathe so that it remains underwater and does not become easy prey for predators, especially birds. This species may also occasionally make trips above the surface, especially when food is scarce, although this is not common. In such cases, it can survive for some time on dry land, as it has a cap (operculum) that can be retracted entirely into the shell. The body is thus well protected against drying out. However, this species is not resistant to prolonged drought; drying out irrigation canals is one way of getting rid of it where it is not wanted.
Marisa cornuarietis has both lungs and gills, as well as a respiratory tube (siphon), although slightly shorter than the ampullaria. This allows the gastropod to breathe so that it remains underwater and does not become easy prey for predators, especially birds. This species may also occasionally make trips above the surface, especially when food is scarce, although this is not common. In such cases, it can survive for some time on dry land, as it has a cap (operculum) that can be retracted entirely into the shell. The body is thus well protected against drying out. However, this species is not resistant to prolonged drought; drying out irrigation canals is one way of getting rid of it where it is not wanted.
Marisa cornuarietis is one of the most exploited snail species in various areas of human activity. Apart from aquaristics, which entails a negative phenomenon (spreading to new areas where the species can be invasive and decimate the local fauna and flora), it is also used for medicine and biological control of various pests.
In the Caribbean, Marisa cornuarietis is often planted for its appetite as an effective eradicator of aquatic weeds, especially floating Eichhornia crassipes, Pistia stratiotes or Hydrilla verticillata and similar species. It is also important in the fight against mosquitoes. Unfortunately, such an intervention was carried out without first determining how the introduced species would behave in the new environment, whether it would actually destroy weeds or prefer other food sources – it was not actually verified whether the snail itself would become an unwelcome problem. Marisa leaves behind areas completely devoid of vegetation. Nowadays, there is an effort to investigate these introductions and to study what their impact is.
Marisa was introduced to tropical areas of Asia and Africa to compete with the snails Bulinus sp. and Biomphalaria sp. These snails often host parasites that cause schistosomiasis, a disease that affects more than 200 million people in the tropics and subtropics and threatens about 600 million more. Marisa does not transmit the disease and, thanks to its voraciousness, competes with native snails, both by depriving them of food and by directly destroying their eggs and young. Where it has been introduced, it has been shown that vector populations have been rapidly reduced or eliminated within 2-3 years.
In Puerto Rico, in addition to killing Eichhornia crassipes and Biomphalaria snails, it is useful in another way – it is directly consumed by the local population.
Marisa cornuarietis is also used for bioindication as an organism sensitive to the presence of so-called endocrine disruptors, substances that act in minimal concentrations and affect living organisms, including humans, through hormonal pathways. In the body of organisms, substances are stored and passed on from mother to offspring, causing some serious disorders including infertility.
Marisa cornuarietis is an interesting gastropod in many ways, but its keeping in aquariums can only be recommended to enthusiasts. After all, there are aquarists amongst us who keep, for example, the magnificent large plant-eating tetras or African cichlids; in their aquariums the marisa would make an interesting addition.