Cassiosomes are cell clusters that contain symbiotic dinoflagellates and nematocysts and released in a suspension of mucus by the up-side down jellyfish Cassiopea xamachana into the water column. Divers passing Cassiopea colonies experience skin irritation without directly disturbing the jellyfish. Cassiosomes and the structures that release them (vesicular appendages) were examined via shotgun mass spectrometry to determine whether these structures contain toxins that could be responsible for skin irritation.