Once the planula has found a good, solid surface to attach to, it will metamorphose into a polyp. This stage is fairly stable, as the polyp doesn't do much, nor does it move.
A polyp is essentially a small bud and stem, very much like an anemone. It has a large mouth compared to the body. When capturing food, it will sting the food material then retract its tentacle so the food comes flying through its mouth into the polyps oral cavity. When the polyp is hungry it is very easy to see its tentacles mouth an oral cavity under a microscope.
Polyps have to complete several tasks. Polyps have to eat, so they will extend their tentacles and open their mouths wide open. Polyps can also produce more polyps by budding off asexually, leaving you with two polyps. The third thing polyps will do is transform into a strobila. When favorable conditions occur, such as temperature spikes or drops or perhaps higher iodine levels, the polyps will begin to elongate. This means that they literally begin to get long and skinny. Then, the polyp begins to look bumping with ridges going up and down their upper region. These ridges get deeper until the polyp looks like a stack of discs. Each disc will eventually be a jellyfish. The discs begin to pulse as the near the end of their maturation. Then they will pulse off and swim away. This stage is known as the ephyra, and is the next stage in the life cycle.
|Two small polyps and a strobila are shown here in a petri dish, feeding on brine shrimp.|
This whole process of polyps making jellyfish is known as strobilation. The interesting thing here is that the polyps will go back to being regular polyps after the strobilate. They can produce hundreds to thousands of jellyfish in a lifetime under perfect conditions. So polyps can produce tons of ephyra and also make asexual copies of themselves. This means that if you have a few polyps you could theoretically have an infinite source of jellyfish as long as you don't let the polyp colony die. This is why I feel the polyp stage is the most important.
|A strobila under 10x magnification. Notice the clear stacks of disc shaped ephyra.|