a lion's mane jellyfish (Cyanea capillata) is not the most popular marine animal i can think of and it is not what i want to see while out swimming but they are quite common in these waters during the summer.
the long sticky tentacles, of which there are many, contain tiny capsules of a toxin, complete with a needle like injection system so the poison is actually injected into who or whatever happens to come in contact with them. it gives a real sting and while this is only extremely rarely fatal to humans, it is an effective predatory strategy
interestingly, these sea animals reproduce asexually at an early (polyp) stage in their life and sexually later on as mature medusas. now i admit i am curious as to how that actually occurs...
anyway, i took these photos a few days ago as this one propelled itself into clear and shallow waters in a shaded area where there was maximum visibility under the surface.