univalve shell

Recent weeks, found some small univalve shell in my aquarium. There water hyacinth in it. Perhaps, shells live in/around the roots of it and bocame visible recently.
Looked around the web, it might be "MONOARAIGAI". "TANISHI" or its kinds do not produce eggs out. They bear eggs within!
They eat green moss, which grows so quickly at low tempreture. So decided to take some of them and let them live in another tank.