As long as the water is kept between 70-84 °F (from www.fishindex.com) they will be fine. So if you have your house heating on all the time, or you live somewhere it doesn't get too cold, they will be fine without a heater.
They can be kept without a heater but it's not something I would recommend because of fluctuations in tempature and just because they are alive, does not mean they are healthy IMO.