Comb Jellies Bring Bioluminescence in Late Summer

Look for bioluminescent flashes after sundown from the ferry (Photo Reggie)

One gorgeous night back in August, Captain Al pointed out some brilliant flashes of light in the wake of the Aggie Gray.  Comb Jellies, small non-stinging “jellyfish”, flash in the water to startle predators.  It may be that the wave action of the ferry’s wake triggers this response– nonetheless, it’s a treat to see the glowing flashes as the ferry churns along in the dark.  We saw a few this week, so it’s still possible to catch the phenomenon.

Comb jellies are from a different phylum altogether from stinging jellyfish.  They don’t even have stinging cells, or nematocysts.  They are named for the combs that run in radial symmetry, with cilia that aids in both locomotion and catching prey. They are planktonic, drifting with the tides and currents, although they can move short distances up and down through the water. They feed on plankton, fish larvae, and other comb jellies, and are eaten by jellyfish, fish, and sea turtles.  Their bodies are made up of at least 90% water.

I think the ones we have in our waters are also known as “Sea Walnuts,” Mnemiopsis leidyii.  We find them most commonly on Dewees when we catch them in a cast net at the rice trunk or crabbing dock.  Try to return them gently and quickly to the water; they are very fragile.  These jellies sometimes wash up dead on the beach as well.

you can faintly see the combs in this jelly that washed up on the beach

Another interesting thing about these jellyfish is that, while they are a natural part of the Western Atlantic ecosystems, they have become a problematic invasive species in other places where there are no predators.  They were first introduced into the Black Sea in the early ’80’s and rapidly became a huge problem there. They competed with and ate larval fish populations to the point where another ctenophore was introduced in order to prey on the Mnemiopsis leidyi.

Back to the pretty flashes of light:  We also noticed (thanks to Connor Mashman) that the wet sand on the front beach lit up with tiny flashes of bioluminescent light when disturbed by a finger scratching the surface.  I wonder if it is possible that small pieces or cells from comb jellies or other jellies are in the sand, and our disturbing them makes them flash just the same.  Bioluminescence (like what fireflies do when they light up) is basically the energy released from a chemical reaction.  The website at the University of California Santa Barbara estimates that 50% of all jellyfish exhibit some sort of bioluminescence.  It is usually used as a means of communication– to startle predators, to attract mates, or to attract prey.  If you are interested in learning more, they have a whole website about this phenomena, and there will be an exhibit at the Pacific Grove Museum of Natural History, opening in October.

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