Why don't these snails stay underwater?
I keep tetras, otos and snails in a 100-litre, open-topped aquarium. My nerite snails keep coming out of the water at the top of the tank, sitting there most of the day and then eventually just going back into the water again. Why do they do this? All my water parameters are fine.
RAY D.
NEALE ADVISES: The answer to this very much depends on the species of nerite snail. The family Neritidae contains something like 300 different species. The majority live in shallow marine and brackish water habitats, but a fair few live in freshwater.
Although often quite hardy and adaptable animals, nerites tend to favour places with brisk currents and lots of oxygen. They are also well adapted to intertidal environments like rock pools and mangroves, hunkering down onto a solid substrate for a few hours when the water level drops and they’re left high and dry, much like limpets. When the tide comes back in, the snails spring back to life, scraping away at the microbial film that most species consume: algae, tiny invertebrates, and even cyanobacteria in some cases.
So, your nerites could be coming out of the water for a number of different reasons. If there isn’t enough oxygen, like most snails, they’ll come to the top where the oxygen level is highest and hang about there. The widely traded African, or zebra, nerite, Neritina natalensis, has a tendency to do this. It doesn’t necessarily mean there’s anything wrong with their tank by our standards, but just that the water temperature might be too high, or the oxygen level too low, for them to feel completely at ease deep down the water column.
If they’re an intertidal species, such as the Virginia nerite, Neritina virginea, they’re simply doing what comes naturally to them.
Although nerites usually have the good sense to return to the water without getting lost, I’d be a little leery of leaving them in an open topped tank in case they didn’t. Short of a cover mesh, some sort of inward-facing rim or flange should stop them escaping if you think there’s a risk of that.