Lined Water Jelly, RHACOSTOMA ATLANTICUM

from the August 28, 2011 Newsletter issued from Mayan Beach Garden Inn 20 kms north of Mahahual; Caribbean coastal beach and mangroves, ~N18.89°, ~W87.64°, Quintana Roo state, MÉXICO
LINED WATER JELLY
The other day several gelatinous, transparent discs turned up on the beach. You can see one bove. The first thought is that it's a jellyfish. However, all jellyfish I've seen are shaped like upside-down bowls, with tentacles hanging below. This was flat, and nothing hung below it.

*UPDATE: In 2024, with many more identification resources avaiable on the Internet, I upload our image to iNaturalist, where the AI automatic identifier suggests the Lined Water Jelly, RHACOSTOMA ATLANTICUM, which usually shows low, radiating ridges more apparent than those vaguely apparent on our individual; however, no other species seems to match. It's the only species in its genus, thus a unique form. Mostly it's reported from the US eastern and Gulf coast, and southern Brazil, but here and there elsewhere in the world's waters. iNaturalists currently reports four other unconfirmed observations on Mexican coasts.

Still, volunteer identifier Bea in Ontario and I figure that it's the remains of a jellyfish, probably the Moon Jellyfish, AURELIA AURITA*.

I think that what we're finding here is dead Moon Jellyfish with their appendages and insides torn away by animals and/or wave action. Maybe what turns up on the beach is just the transparent cap of what once was a much handsomer, upside-down-bowl-shaped creature.