how they reproduce
The freshwater jellyfish exhibit a varied life cycle comprising three stages: egg, polyp and medusa. Two kinds of larvae and a cyst stage also form.The jellies start as eggs produced by female medusae. If fertilized, each egg hatches into a tiny, flat larva called a planula that is hard to see with the naked eye. It swims for a few days among zooplankton before settling down on an underwater plant, log, rock or piece of sediment. The planula then becomes a polyp that looks like a miniature hydra, sort of a bitty little bud without tentacles that has stinging cells to stun equally small prey. The tiny polyp soon forms a bud near its base that stays attached and develops into a second polyp. These two polyps are identical twins since they formed asexually. Soon, the twins form yet more fixed buds and expand into a polyp colony of perhaps 2-12 buds. don't expect some massive colony here. You'd still need a hand lens or microscope to study them.Now and then the jellyfish polyps form a second kind of detachable bud that develops into a tiny, cigar-shaped larva. This larva frees itself from its parent polyp and either crawls a few inches away or is carried off by flowing water before it settles down to start forming polyps of its own.
The picture below shows the reproduction cycle.
So far, our life cycle involves asexual reproduction: polyps forming buds that either attach or float before settling down to form new polyps. In some years, especially during hot summers, the polyp colony produces medusae buds. Each of these top buds becomes either a male or female medusa. After a week or two, and still quite small, the medusa leaves home to become free swimming. In another five weeks, the medusa matures into a nearly transparent body called a bell, that dangles with long, hairlike tentacles we all associate with jellyfish.Sometimes just female medusae form, sometimes just males form. Only rarely in North America and Europe do both male and female medusae appear together. Why jellyfish produce swarms of same-sex medusae, a seeming waste of energy, still remains a mystery. If female medusae produce eggs that are fertilized by a male, they hatch into planula larvae and the whole jellyfish life cycle starts over again. The medusae live but a few weeks, release eggs and die. The polyps can live from spring until fall, when they into cysts that are covered with a chitinous "skin" enclosing fairly dry cells. The cysts are able to survive drought and cold. In Wisconsin, the cysts survive on the bottoms of ice-covered ponds, lakes, and quiet river pools where the water is slightly above freezing. But the cysts are more than a winter resting stage. They are a vehicle for jellyfish to spread north of their home range and invade new waters.
The picture below shows the reproduction cycle.
So far, our life cycle involves asexual reproduction: polyps forming buds that either attach or float before settling down to form new polyps. In some years, especially during hot summers, the polyp colony produces medusae buds. Each of these top buds becomes either a male or female medusa. After a week or two, and still quite small, the medusa leaves home to become free swimming. In another five weeks, the medusa matures into a nearly transparent body called a bell, that dangles with long, hairlike tentacles we all associate with jellyfish.Sometimes just female medusae form, sometimes just males form. Only rarely in North America and Europe do both male and female medusae appear together. Why jellyfish produce swarms of same-sex medusae, a seeming waste of energy, still remains a mystery. If female medusae produce eggs that are fertilized by a male, they hatch into planula larvae and the whole jellyfish life cycle starts over again. The medusae live but a few weeks, release eggs and die. The polyps can live from spring until fall, when they into cysts that are covered with a chitinous "skin" enclosing fairly dry cells. The cysts are able to survive drought and cold. In Wisconsin, the cysts survive on the bottoms of ice-covered ponds, lakes, and quiet river pools where the water is slightly above freezing. But the cysts are more than a winter resting stage. They are a vehicle for jellyfish to spread north of their home range and invade new waters.