Huge swarms of these medusae, inhabitants of Jellyfish Lake, have symbiotic algae. The medusae swim back and forth across the lake to stay in the sun as long as possible. Their nematocysts have lost all their potency.
Stomolophus meleagrisSource: <http://commons.wikimedia.org/wiki/File:Stomolophus_meleagris.jpg> by DocTaxon; DocWatch, permission by Dauphin Island Sea Lab.