Carol and I went to Pescadero Natural Preserve today. It was delightfully cool (about 70 degrees) with high fog blocking much of the sun. The tide was quite low when we arrived, so I wandered around looking at the variety of organisms in tide pools: sea anemones, crustaceans, molluscs, seaweeds, etc. Then we decided to walk to the mouth of Pescadero Creek and head up into the marsh.
As we followed the creek upstream under the Highway 1 overpass, Carol noticed that there were dozens of jellyfish washed up along the high tide line, translucent organisms looking a little like plastic bags filled with water. Most of the organisms were damaged; some had obviously been stepped on, some appeared to have broken in pieces, some no longer had a definable shape. I walked closer to the water, and began to find a few organisms that looked less damaged; one in particular, shown in the photograph below, retained a good deal of its structure:
When we got home, I did some online research to find out what kind of Cnidarian this was. I concluded it was a Moon Jellyfish, the common name of Aurelia species; most likely Aurelia labiata, which the authorities I consulted online agreed was the Aurelia species found along the San Mateo County coast. More specifically, what I saw was most likely the central morph of Aurelia labiata, the type specimen of which came from Monterey Bay (M. N. Dawson, Macro-morphological variation among cryptic species of the moon jellyfish, Aurelia [Cnidaria: Scyphozoa], Marine Biology  143:369-379).
Based on this tentative identification, I modified a drawing in Dawson (2003) to highlight some of the structures that might be seen in the photograph.
The gonads and manubrium are clearly visible in the photograph. I think at least one of the whitish (translucent) structures emanating from the manubrium is one of the oral arms. I did not see any tentacles when I looked at the organism, and none are visible in the photograph. Some of the radial canals are visible, enough to give the sense that the organism has some radial symmetry.
The organism I photographed, and most of the Moon Jellyfish I saw along the creek, were clear to translucent. One of them, though, was amber-colored; others had dark red or purple structures. The Encyclopedia of Life Web page on Moon Jellyfish, citing an article by the Monterey Bay Aquarium, states that Aurelia labiata are translucent when young, turning “milky white, sometimes with a pink, purple, peach, or blue tint” at maturity. So most of the individuals I saw were, apparently, young jellyfish.
Carol and I walked around the marsh for a couple of hours, and came back to the creek as the tide was beginning to flood again. I watched the tide pick up some of the stranded jellyfish, hoping to see some signs of life. A few of the jellyfish floated away, looking intact, with their bells nicely extended; perhaps they survived their exposure. But most of them sank beneath the water, or crumpled up, or seemed to be coming apart; it seems very unlikely these individuals survived.