My granddaughters, 7 and 11, were peering into my aquarium as I tickled jellyfish to make them swim. "Where did you get them, Grandma?" the younger one asked.
A good place to start. With, "I don't know."
I can guess. They came from Boundary Bay, but they didn't enter my tank as jellyfish. They were probably the fuzz on sand grains, or old eelgrass blades, or maybe even hermit crab shells. The fuzz would have been about 1/2 mm high, barely enough to see. It could be the brownish stuff that coats the lower edge of the glass walls, where I don't clean as thoroughly as I do the rest of the tank.
The jellies' parents were hydroids. These ones make a creeping mat crowned by stalks, each with four tentacles. At the tip, they form into bulbs that swell into tiny medusas. When they are about 1 mm. long, they break free and swim away. (Photos, Bodegahead)
|Freshly released medusa, Cladonema pacificum*|
These new baby jellies have 9 canals marking the bell, a tiny hanging stomach, and 9 tentacles around the lip. Each tentacle is branched; one branch is tipped with an adhesive pad, the other holds stinging cells for capturing food.
|An older jelly, in a plastic lid, showing adhesive tentacles. 4 per tentacle stem, now.|
With these sticky feet, they grab onto any surface; walls, sand, eelgrass, and even the undersurface of the water. There they hang, waving the longer tentacles around, catching dinner, mainly copepods.
As the jellyfish grow, the tentacles lengthen and branch out. The older adhesive pads change, becoming chains of stinging cells.
|Early development of stinging tentacles. Note the flower-like mouth at bottom left of central column.|
They grow to about 3.5 mm high. I measured a large one of mine: 3 mm., not counting the tentacles, of course.
And they have more eyes than a spider, 9 in total**, although they probably only distinguish light and shadow. The jellies in my tank like to congregate near the light source in the afternoon, although earlier in the day they are usually hiding down on the sand or the eelgrass.
|One red eye spot, or ocellus, at the base of each tentacle stem.|
The central column of the jellyfish contains its stomach, with the mouth at the bottom. When the jelly is feeding, copepods and other plankton are snared by the tentacles, which convulse suddenly, bringing the captive down to the lip of the jelly. The whole gastric column then swings around to eat.***
Since the jellyfish has no other opening, digested food is spit out through the mouth. And rather quickly, too, because otherwise, it would weigh the jelly down, making it sink.
|Backlit jellies. The rear one is stuck to the undersurface of the water.|
|In a mature jelly, the mouth hangs down (up, in this case) to just beyond the lip of the bell.|
When it is full grown, the male will release sperm into the water. The female jelly develops eggs. Once they are fertilized, she releases them as planulae, flat plankton with cilia for swimming. Since these have no mouth or digestive system, they hurry to find a good spot to settle down, so they can develop into hydroids. And the cycle is complete.
*There is no common name for this genus.
**The red-eye medusa, otherwise very similar to this one, has hundreds of tentacles and over 100 eyes.
***Tomorrow, if YouTube cooperates, I will post a video of these jellies feeding.