What are these 'double snails'.

[Joe31415]

In my head I call these 'double snails' since they look like two snails attached to each other. I only have them in one of my tanks, but I do have quite a lot of them. These are very small, maybe 2mms across and always. Interestingly, while I have a lot of them, I've never noticed them getting bigger than this, so I can only assume they die off before they get much bigger, but at the same time, I've not noticed piles of shells gathering anywhere (bare bottom tank, they'd be easy to spot), nor any eggs on the glass.
The only non-coral livestock in this tank bigger than them is an urchin and one or two astrea snails. I don't think either of those, even if they were a natural predator, would be fast enough to keep their numbers in check. They seem to be keeping their population in check on their own. Which, I suppose, is a good thing.



More pictures.

 

[Reefkeepers Archive]

In my head I call these 'double snails' since they look like two snails attached to each other. I only have them in one of my tanks, but I do have quite a lot of them. These are very small, maybe 2mms across and always. Interestingly, while I have a lot of them, I've never noticed them getting bigger than this, so I can only assume they die off before they get much bigger, but at the same time, I've not noticed piles of shells gathering anywhere (bare bottom tank, they'd be easy to spot), nor any eggs on the glass.
The only non-coral livestock in this tank bigger than them is an urchin and one or two astrea snails. I don't think either of those, even if they were a natural predator, would be fast enough to keep their numbers in check. They seem to be keeping their population in check on their own. Which, I suppose, is a good thing.



More pictures.

I can't remember the name of these (@ISpeakForTheSeas?) But I have them around my tank. Looks like those are two snails in the process of reproducing.

 

[IKD]

Collonista snail?

 

[ISpeakForTheSeas]

Collonista or relatives would be my first thoughts too, but if you're only seeing them like this, that would strike me as odd. So, a quick question: Are you sure these are single specimens and not two specimens actually joined together?

If it is Collonista snails, then this would be the first confirmation I've seen that they breed using internal fertilization instead (which would make sense, given that they reproduce without issues in our tanks).

So, assuming that these are Collonista snails, if you can, I would keep track of these snails (probably with a HOB, clear, plastic container with little pinholes in it for flow and to allow algae growth in it from the DT) and see if you can't catch them reproducing (either spawning, laying eggs, or giving birth to live young).