Some years ago, a magazine advertisement that showed three individuals standing on the floor of the ocean looking upward through a picture frame at a group of Aurelia jellyfish, commonly known as moon jellies (translucent, foot in diameter, plate-shaped medusoid jellyfish, with thousands of inch-long tentacles around their margin), caught my attention.  It reminded me of a time when I, dressed in SCUBA gear, was in the middle of such a scene.

At that time, I lived in Miami Beach and spent my weekends sailing on Biscayne Bay.  One Sunday the sailboat became trapped in the afternoon doldrums.  I anchored, donned a facemask, swim fins and SCUBA tank, and jumped off the stern.

Without flotation, I sank like a rock, bounced off the soft sandy bottom fifteen feet below the boat’s keel, and came to rest.  There I sat and allowed the cool water to hug me, while I adjusted to the dim underwater world and, for a moment, listened to clam shells close and pistol-shrimp snap their claws.

Above me the sailboat’s hull poked through the surface of the bay and appeared as a vertiginous, whitish mass outlined, above the water’s surface, by a serpentine gunwale, bow and stern.  The chrome safety rails, attached to the sinuous gunwale, coruscated, signaling the boat’s restless tether, while thin squiggly black lines and shrouds converged toward the top of the mast.  Also, between me, and the bottom of the boat’s hull, hundreds of pinkish-colored moon jellies drifted, rolled, twisted and pumped.  I rolled onto my abdomen and glided away for a clearer view.

From my perspective the moon jellies appeared like a sky full of parachutes moving in a gigantic three-dimensional ballet.  Although mostly invisible the water’s surface, underwater, highlighted by a sunny background, every feature in the moon jellies’ gelatinous bodies emerged distinctly.  In each individual I saw the characteristic horseshoe-shaped gonads, pouched stomach, and the intricate circulatory system.

Outside a moon jelly’s body I could see its four oral-arms attached around the mouth, sweeping freely.  These function to remove plankton and other microscopic particles that stick to the umbrella.  If I squinted my eyes and looked along the margin of the umbrella, the thousands of tentacles appeared as a haze.

Although moon jelly tentacles, like all true jellyfish tentacles, are armed with stinging cells, their sting is harmless to humans.  The stingers or pneumatophores ejected from the cnidocytes (stinging cells) cannot penetrate human integument.

Moon jellies cannot swim.  They sink and drift with the current, and orient at odd angles to the surface.  To keep from sinking too deep they rhythmically expand and contract, creating jets of water that push them upward.  Light and equilibrium sensors, located around the margin of the umbrella, aid them in maintaining their proper orientation, mouth, oral arms, and tentacles downward.

I sat on the bottom of the bay and watched this moon jelly ballet.  Some pumped upward at an angle, other floated downward; their oral arms spread open and tentacles curling upward.  But, as I watched, I noticed that some individuals ejected a cloudy substance.

“Males!”  I gurgled into my mouthpiece.  “I’m in the middle of a moon jelly sexual jubilee.”

I scanned the moon jellies that floated downward.  These did not contract, but remained fully expanded, sinking and collecting whatever particles drifted in the water beneath them.  This stuff becomes the moon jelly’s smorgasbord.  But on this day, moon jelly sperm was mixed into this haute cuisine.

The clouds of sperm being emitted, explained why I was seeing these moon jellies aggregating and circulating in a magnificent ballet—these jellyfish were reproducing, starting a process that would take their offspring on a long and complex life cycle.

When the sperm becomes trapped on the female moon jelly’s underside, it is identified as something special and, instead of being eaten; it is channeled off to fertilize the eggs attached around the mouth.  A couple of days after fertilization, the eggs hatch into paramecium-like, multicellular larvae, which are called planula larvae.  These planula swim to the bottom, attach to solid objects and grow into inconspicuous, simple bodied, polyp type jellyfish called a scyphistoma.  The scyphistomae grow and divide asexually, forming colonies, and after several years individual scyphistomae metamorphose into a body form that looks like a stack of irregular coins topped with several tentacles.  These new polyps are identified as the strobliae, because the coins eventually pop free of the body, like light flashing from a strobe, and becomes free-swimming larval stages called ephyra.  After a few month of drifting about the ocean, the ephyra grow into adult moon jelly, medusa-shaped individuals.  Then, when the waters of the oceans warm and other environmental condition are proper, these mature males and females will again come together to perform their three-dimensional ballet.

Captivated, I remained engrossed by this dance of procreation until a pod of foam and bubbles imploded at the surface and disturbed my reverie.  Unaware of the moon jelly ballet, my friend dove in for a swim and I floated to the surface.

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