Mashomack Musings: Bioluminescent in the bay

Summer on the East End is a time that engages the senses like no other.

Whether it’s the sounds of cicadas or the fragrance of flowers, summer yields an endless trove of natural treasures. Perhaps the most remarkable is the eerie nighttime glow of a little-known sea creature, the comb jelly.

Comb jellies fall into a group of animals known as ctenophores and are named for their comb-like cilia, or hair-like appendages used for swimming. Despite their name, comb jellies are not “true jellyfish.”

True jellyfish, or cnidarians, have specialized stinging cells called nematocysts that look like tiny harpoons under a microscope. Comb jellies don’t have nematocysts, so they can’t sting or harm humans, and instead use sticky cells called colloblasts to trap their prey.

Throughout the world there are more than 100 species of ctenophores. Our local species is Mnemiopsis leidyi, or the sea walnut. They are sometimes called pearfish, based on the pear-like shape of their larvae.

These small creatures are almost completely transparent because their body is 97% water. This trait makes them almost impossible to observe during the day, so they are best seen at night, as their bioluminescent glow gives them away. 

The sea walnut produces a green-blue light upon disturbance in dark conditions. The exact reason for their bioluminescence is unclear, but it may deter predators and possibly attracts prey.

Whatever the true evolutionary function of this feature is, the result is a magnificent and otherworldly glow.

As the summer continues, look for ctenophores on your next nighttime trip to the water and be amazed.