|Craspedacusta sowerbii - Wikipedia CC|
C. sowerbii jellyfish are small, reaching the diameter of a quarter. They have typical jellyfish features, an umbrella with stringy tentacles attached to the edges. These contain cnidocytes, specialized stinging cells which they use to paralyze their prey, macroinvertebrates and small fish.
"A tiny, stalked form of the jellyfish (the polyp) lives as colonies attached to stable underwater surfaces such as rooted plants, rocks, or tree stumps. The microscopic polyp colonies feed and reproduce during the spring and summer months. The polyps reproduce asexually. Some of their offspring are the jellyfish that can be seen at the surface. The "jellyfish" or medusa reproduce sexually. Fertilized eggs develop into planula larvae which eventually settle to the bottom of the pond or lake and develop into polyps. However, in the United States, most populations of jellyfish are either all male or all female, so sexual reproduction may be rare." Freshwaterjellyfish.orgThe polyps contract in winter forming podocysts or resting bodies which may explain how they have spread to isolated ponds and water bodies, possibly transported by aquatic animals and birds.* They may be suddenly found in large numbers one year with none the following years. In some cases they have totally disappeared from a lake.