Only when conditions in the stream or lake start to deteriorate do hydra reproduce sexually. Hydra then produce ovaries or testes in place of buds. Sponges and bryozoans are two other common freshwater animal groups that alternate periods of active growth and dormancy and utilize both asexual and sexual reproduction during their life cycles. Sponges are simple invertebrate animals whose cells are organized only into tissues and whose bodies have no organs like mouths or stomachs. Inside the sponge body, specialized cells along water channels create currents and engulf food particles.
Sponges also have great powers of regeneration from injury or predation.
There are only about two dozen different species of freshwater sponges in North America, but they can be both large and abundant in some ponds and lakes. Spongilla lacustris colonies measuring more than a cubic meter in size have been reported. They can also play an important part in keeping the water in which they live clean. For example, biologist T. M. Frost found that, at its peak density, the sponge population in one New Hampshire pond could filter the entire volume of the pond in about a week. During active summer growth sponges can also reproduce sexually.